INTRODUCTION — Pyruvate kinase (PK) deficiency is an inherited (autosomal recessive) red blood cell (RBC) enzyme disorder that causes chronic hemolysis. It is the second most common RBC enzyme defect but is the commonest cause of chronic hemolytic anemia from an RBC enzyme deficiency.
This topic reviews the pathogenesis, clinical presentation, diagnosis, and treatment of PK deficiency. General approaches to evaluating the cause of hemolytic anemia are presented separately:
●Children – (See "Overview of hemolytic anemias in children".)
●Adults – (See "Diagnosis of hemolytic anemia in adults".)
Genetics — PK deficiency is an autosomal recessive disorder; affected individuals are either homozygous for a single pathogenic mutation or compound heterozygous for two different pathogenic variants affecting the function of the PK enzyme in red blood cells (RBCs) and liver [1,2]. Individuals who are heterozygous for PK deficiency have intermediate enzyme levels and are not affected clinically.
PK enzymes consist of several isoforms. They are products of two distinct genes, PKLR and PKM, both encoding enzymes that catalyze the transphosphorylation of phosphoenolpyruvate (PEP) into pyruvate and ATP during the terminal part of the glycolysis pathway. Clinical PK deficiency with hemolytic anemia is limited to mutations of the PKLR gene:
●PKLR – The PKLR gene encodes the L (liver) and R (RBC) isoenzymes. The R isoform, unique to RBCs, is 33 amino acids larger than the L isoform, which is unique to hepatocytes. Expression in RBCs versus liver is due to differential use of tissue-specific promoters, which drive expression as well as tissue-specific exon usage (use of exon 1 but not exon 2 in RBCs and exon 2 but not exon 1 in liver). The PKLR gene is located on chromosome 1q21.
●PKM – The PKM gene encodes the M (muscle) enzyme. This form is expressed in muscle, brain, white blood cells (WBCs), and platelets. There are two isoforms, M1 and M2, which result from differential processing of a single transcript. The M2 isoform is dominant during fetal development and is overexpressed in many tumors. After birth, the M2 isoform persists in WBCs and platelets. In RBC progenitor cells, the M2 isoform is progressively replaced with the R form during fetal development. The PKM gene is located on chromosome 15q22.
Well over 260 pathogenic variants have been reported on the PKLR gene . The variants include single nucleotide substitutions as well as intronic and exonic deletions and insertions (figure 1) [4,5]. Some variants are relatively common; as an example, the R510Q PKLR (also referred to as PKLR 1529A) mutation is found in approximately 40 percent of Northern European patients with PK deficiency . The R486W PKLR mutation is found in approximately 25 percent of patients with PK deficiency in southern Europe . There is relatively little predictive value with respect to the severity of the clinical course, and the phenotypic expression of identical mutations can be strikingly different .
The frequency of pathogenic PKLR gene mutations is far lower than that of the mutations affecting glucose-6-phosphate dehydrogenase (G6PD), which cause G6PD deficiency. However, unlike G6PD deficiency, which may only manifest hemolysis under certain circumstances such as exposure to oxidant drugs, in PK deficiency, the hemolysis is chronic. Thus, PK deficiency is the most common RBC enzyme defect causing chronic congenital non-spherocytic hemolytic anemia.
Mutations of genes other than PKLR have been shown to reduce PK enzymatic activity, although these are rare. As an example, compound heterozygous mutations in the gene that encodes Kruppel-like factor 1 (KLF1), the hematopoietic-specific transcription factor essential for induction of expression of adult beta globin and other erythroid genes, have been associated with severe transfusion-dependent hemolytic anemia and PK enzyme deficiency . (See "Fetal hemoglobin (hemoglobin F) in health and disease", section on 'Kruppel-like factor 1 (KLF1)'.)
Genes that increase PK activity have also been postulated, although no specific genes causing PK hyperactivity have been described .
PK enzymatic function — RBCs use several enzyme systems to maintain their viability and function. Two of the major processes under enzymatic control are the production of energy, in the form of ATP, and protection from oxidative injury by compounds that act as reducing agents. The PK enzyme functions in the energy-producing glycolytic pathway, which metabolizes glucose to generate ATP for the cell. However, as PK is the terminal enzyme in glycolysis, the proximal glycolytic intermediates are increased [2,12]. This includes increases in 2,3-diphosphoglycerate (2,3-DPG, also called 2,3-BPG), with resultant three- to fourfold increase in 2,3-DPG:ATP ratios . Levels of 2,3-DPG may be elevated up to two times normal in PK-deficient individuals, resulting in decreased hemoglobin oxygen affinity and improved oxygen delivery per unit of hemoglobin, with better tolerance of anemia than would be otherwise expected .
As shown in the figure, PK catalyzes the conversion of phosphoenolpyruvate (PEP) to pyruvate by removal of a phosphate group (figure 2). The phosphate group from PEP is transferred to ADP to create one molecule of ATP.
The active PK enzyme is a homotetramer that contains four molecules of the PK protein. Thus, in an individual who is heterozygous for two different PKLR mutations, five different tetramer compositions will be present (eg, tetramers containing 0, 25, 50, 75, and 100 percent of each of the two abnormal protein subunits). The common R510Q and R486W PKLR mutations as well as some but not all other PKLR mutations, destabilize formation of PK R enzyme tetramers .
PK is an allosteric enzyme (an enzyme for which binding of an effector to one region of the enzyme results in altered conformation and/or altered enzymatic activity towards a substrate that binds to another region of the enzyme). The substrate for PK is PEP and the allosteric regulator is fructose 1,6-diphosphate (FDP); binding to FDP changes the conformational structure of PK and its enzymatic activity towards PEP.
As noted above, there are two different genes that encode different PK isoforms (see 'Genetics' above). Individuals with PK deficiency have mutations in the PKLR gene that cause reduced PK activity in RBCs. However, the PK activity in other cell types such as WBCs, platelets, and other tissues is normal because the enzyme expressed in these cells is encoded on a separate gene.
Pathogenic mutations in the PKLR gene may affect any of the following properties of the PK enzyme [6,15]:
●Altered affinity for PEP (its substrate)
●Altered affinity for FDP (its allosteric activator)
●Altered protein stability
●Altered stability of PK homo- or heterotetramers
Genotype-phenotype correlations have been examined for several of the common mutations . The most severe clinical phenotypes are generally associated with mutations that cause premature stop codons, frameshifts, or large deletions; however, the genotype/phenotype association and predictions of their severity are not clearly foreseeable [1,16].
Mechanism of hemolysis — The mechanism for hemolysis in PK deficiency is not clear. Although the defect in ATP generation contributes to hemolysis, it is not a sufficient explanation; this conclusion is based on observations that ATP deficiency is difficult to demonstrate in some of the affected patients . In addition, other disorders with more severe degrees of ATP deficiency are not associated with significant hemolysis .
Hemolysis in PK deficiency is mainly extravascular (ie, due to phagocytosis of cells by reticuloendothelial macrophages); however, if the hemolysis is severe, there may be spillover to intravascular hemolysis. Thus, many affected patients have normal lactate dehydrogenase (LDH) levels, but most have elevated bilirubin and clinical jaundice .
It has been proposed that the mechanism of hemolysis in PK deficiency is similar to the as yet unexplained destruction of young RBCs described in individuals descending from high altitude, a process termed "neocytolysis." It is possible that the process may involve impaired mitochondrial autophagy, resulting in increased mitochondrial mass, in turn leading to increased production of reactive oxygen species (ROS) and prolongation of mitochondrial clearance, along with prolonged reticulocyte survival; however, this remains speculative [19-21].
Patients with hemolytic anemia who undergo splenectomy, with a resultant decrease in the hemolytic process and improvement of anemia, have a higher number of reticulocytes than they did before the splenectomy. This perplexing observation indicates that knowledge of the regulation of erythropoiesis and reticulocyte kinetics remains incomplete. A significant interaction may occur between the spleen and PK-deficient, young RBCs, through an as yet unknown mechanism that influences premature splenic destruction of reticulocytes and young RBCs, especially in patients with more severe PK deficiency [22-26].
The metabolic disturbances in PK deficiency affect the survival of RBCs as well as the maturation of erythroid progenitor cells in the spleen, which results in their apoptosis (referred to as ineffective erythropoiesis). Ineffective erythropoiesis in the spleen has been demonstrated in a four-year-old PK-deficient patient who underwent splenectomy, as well as in a mouse model of PK deficiency [26,27]. However, it remains to be established whether apoptosis of erythroid progenitor cells in the bone marrow plays a role in the anemia of PK deficiency, as well as whether PK activity has a more general role in apoptotic pathways [8,28].
Increased oxygen delivery — PK-deficient RBCs show enhanced oxygen delivery for a given partial pressure of oxygen in the bloodstream. This is because the block in glycolysis in PK deficiency is downstream of the Rappaport-Luebering shunt, which results in formation of the metabolic intermediate 2,3 diphosphoglycerate (2,3 DPG, also called 2,3 bisphosphoglycerate [2,3 BPG]). An accumulation of 2,3-DPG has been noted in PK deficiency, in contrast to a low concentration in hexokinase deficiency, which is upstream from the shunt. This increased 2,3-DPG leads to a "rightward" shift of the oxygen dissociation curve for hemoglobin in patients with PK deficiency (figure 2), resulting in better oxygen delivery to the tissues. As a result, individuals with PK deficiency may tolerate anemia better than those with defects upstream to the Rappaport-Luebering shunt that do not cause a rightward shift in the hemoglobin oxygen curve, such as hexokinase deficiency [8,22]. (See "Structure and function of normal hemoglobins", section on '2,3-bisphosphoglycerate'.)
Mechanism of iron overload — As in any patient with ineffective erythropoiesis and hyperactive production of early RBC precursors, iron absorption is increased, and iron retention in macrophages is decreased, due to erythroferrone-mediated decreases in the levels of hepcidin. As expected, the iron overload is even more common in those patients requiring transfusions . This pathway is discussed in more detail separately. (See "Regulation of iron balance".)
Possible protection from malaria — It is not clear whether PK deficiency protects against any form of malaria.
●Evidence supporting a protective role – In vitro, RBCs from individuals with PK deficiency show reduced invasion by Plasmodium falciparum, and RBCs from individuals with PK deficiency as well as heterozygous carriers show a preferential macrophage clearance of ring-stage-infected RBCs . In a mouse model, PK deficiency confers protection against malaria .
●Evidence against a protective role – The geographic distribution of PK deficiency does not show any indication of a positive selection pressure from malaria as there is for other inherited RBC gene variants such as the sickle mutation, thalassemia mutations, the Duffy blood group system, and certain forms of hereditary elliptocytosis. (See "Protection against malaria in the hemoglobinopathies" and "Protection against malaria by abnormalities in red cell surface antigens and cytoskeletal proteins".)
EPIDEMIOLOGY — PK deficiency is extremely rare; its true prevalence is unknown. Based on the gene frequency of the PKLR 1529A variant (encoding PK R510Q) in White people and its relative abundance in people with hemolytic anemia, a prevalence of approximately 51 cases per million has been estimated in White Europeans . However, in the experience of this author, the true prevalence of PK-deficiency encountered by hematologists to whom rare RBC disorders are referred is much lower. This coincides with the published prevalence of diagnosed PK deficiency, suggesting that most people with PK deficiency remain undiagnosed [32,33]. Over 200 individuals with PK deficiency are enrolled in an ongoing natural history study (NCT02053480).
PK deficiency has a worldwide distribution, but it is more common among people of northern European and perhaps Chinese ancestry .
As with any autosomal recessive disorder, PK deficiency can be more common in groups with a history of consanguinity. As an example, a high frequency of PK deficiency has been documented in Pennsylvanian Amish communities and in a fundamentalist branch of the Church of Jesus Christ of the Latter-day Saints (FLDS Church) at the Utah/Arizona border [35,36]. In such isolated populations, a "founder" effect can be implicated. In these populations, the frequency of heterozygosity may exceed 1 percent .
PK deficiency has also been described in an inbred mouse strain and in the Basenji dog [38,39].
TYPICAL PRESENTATION AND CLINICAL FEATURES
Overview of presenting findings — PK deficiency is a lifelong condition, but the age of presentation is not predictable and may vary widely due to significant heterogeneity in the severity of hemolysis and anemia, even among individuals who share the same genotype (figure 3). In some cases, the burden of disease can be high, especially in early childhood .
In most cases, members of a family with the same gene variant tend to have a similar degree of disease severity. The disease has been reported to be particularly severe among the Amish of Pennsylvania, with occasional lethal outcomes in children unless splenectomy is performed . (See 'Genetics' above.)
The rarity of the condition and variability of presentation make diagnostic delays common, and affected individuals may carry a diagnosis of unexplained anemia or may be misdiagnosed as having other causes of anemia. In a report from 2018 that included 254 individuals in a PK registry, the median age at diagnosis was approximately five months, but the range was large (from birth to 60 years of age) . Approximately one-third of the individuals in the registry were genetically related to other participants, which may explain the early age at diagnosis for some. In contrast, in a registry study from 2005 that included 61 individuals referred to a single center, the median age at diagnosis was 16 years (range, 1 day to 65 years) .
The frequency of findings from the larger cohort was as follows :
●Anemia – Most of the individuals; however, some have compensated hemolysis
●Neonatal hyperbilirubinemia – 90 percent
●Transfusions – 84 percent
●Iron overload – 47 percent (including some individuals who never received a transfusion)
●Gallstones – 45 percent
●Perinatal complications (preterm birth, intrauterine transfusion, growth retardation, preterm labor) – 28 percent
●Bone fractures – 17 percent
Other complications included leg ulcers, cirrhosis, and endocrine dysfunction; these may have been related to iron overload. The high rate of neonatal jaundice has been observed in other studies, such as a series of 124 children, in whom 88 percent had neonatal hyperbilirubinemia .
The majority of patients (59 percent) had a splenectomy, usually to treat anemia and/or to decrease the transfusion burden (at a median age of 4.1 years); of those who did not undergo splenectomy, 35 percent had splenomegaly . A smaller proportion (40 percent) had undergone cholecystectomy (median age, 15.1 years).
The severity of presenting findings is highly variable, from death in utero or shortly after birth due to hydrops fetalis, to transfusion-dependent anemia, to a mild, compensated hemolysis that does not require transfusions . Some children have severe, transfusion-dependent anemia at birth . Some adults may present with liver failure, which is almost always associated with iron overload; however, a contribution of the PK L isoform remains a hypothetical possible contributing factor.
The three major types of presentations are:
●Kernicterus/neonatal jaundice soon after birth (see 'Chronic hemolytic anemia from birth' below)
●Anemia (during childhood or adulthood), which may be discovered incidentally, based on a positive family history or due to typical anemia symptoms (see 'Complications of chronic hemolysis' below)
●Complications such as iron overload caused by ineffective erythropoiesis and increased iron absorption, pigment gallstones, or transient anemia due to a bone marrow insult such as parvovirus infection (see 'Iron overload' below)
The findings on the complete blood count (CBC) include normocytic anemia, an increased reticulocyte count, and an absence of specific red blood cell (RBC) morphologic abnormalities on the peripheral blood smear. Other laboratory testing is consistent with a Coombs-negative hemolytic anemia (table 1). (See 'Laboratory findings' below.)
Unlike glucose-6-phosphate dehydrogenase (G6PD) deficiency, PK deficiency is not associated with increased susceptibility to hemolysis after exposure to oxidant agents.
Chronic hemolytic anemia from birth — An illustrative unpublished case involves a 19-year-old female who presented with anemia (hemoglobin 7.5 g/dL), profound reticulocytosis (46 percent), and progressive liver failure. She had kernicterus as a neonate and was found to have hemolytic anemia without any specific morphologic RBC abnormalities on the peripheral blood smear. She required weekly blood transfusions. A diagnosis of PK deficiency was made based on the typical presentation, absence of positive findings on testing for other inherited hemolytic anemias, and positive testing for PK deficiency on enzymatic testing performed by a reference laboratory. (See 'Biochemical testing' below.)
Management included splenectomy at the age of five months. No transfusions were administered after splenectomy; however, she was moderately anemic (hemoglobin 7.5 to 9 g/dL) and had laboratory evidence of iron overload (ferritin >3000 ng/mL, transferrin saturation [TSAT] close to 100 percent). She was treated with iron chelation and judicious phlebotomies (starting with volumes of approximately 50 mL and progressively increasing volumes up to 250 mL), and after two years of therapy, her liver function improved and normalized. During this time, her hemoglobin ranged from 8.2 to 9.4 g/dL and her iron markers improved (ferritin decreased to 420 ng/mL; transferrin saturation [TSAT] decreased to 62 percent).
This case illustrates the combination of clinical features that can develop, their contribution to overall health, and the importance of treating iron overload. (See 'Iron overload' below.)
Complications of chronic hemolysis — Some individuals with mild hemolysis due to PK deficiency may not be symptomatic. Those with more severe hemolysis may present with (or develop) one or more of the following:
●Pallor from severe anemia
●Icterus from hemolysis
●Splenomegaly of varying degree
●Pigment (bilirubin) gallstones
●Folate deficiency secondary to increased requirements
Hemolysis may worsen during pregnancy and after use of oral contraceptives [43,44]; the mechanism is unknown.
Anemia may worsen in the setting of transient bone marrow aplasia caused by infections such as parvovirus, which might not cause significant anemia in individuals without chronic hemolysis.
Individuals with gallstones who require surgery should be evaluated for the need for splenectomy, as it may be possible and/or advantageous to perform both procedures at the same time in selected cases.
Iron overload — Iron overload is less common than neonatal jaundice or chronic anemia as a presenting finding that brings the patient to medical attention, but it becomes clinically more apparent during adulthood. Children can also develop iron overload; in a series of 124 children with PK deficiency, iron overload was present in nearly half of those who were evaluated by serum ferritin and/or liver magnetic resonance imaging (MRI) . This high prevalence of iron overload is the rationale for routine surveillance and preventive strategies. (See 'Typical monitoring schedule' below and 'Prevention/treatment of iron overload' below.)
The causes include ineffective erythropoiesis, leading to increased erythroferrone and decreased hepcidin, which in turn leads to increased intestinal iron absorption, as well as transfusional iron overload. Similar to thalassemia, severe iron overload may occur in non-transfused patients as well.
Iron overload may present in a number of ways depending on the organ most affected. Examples include heart failure, liver disease, and endocrine dysfunction; these may be similar to the findings in individuals with other forms of iron overload. (See "Approach to the patient with suspected iron overload", section on 'Typical clinical findings'.)
Laboratory findings — PK deficiency produces a chronic, Coombs-negative, non-spherocytic hemolytic anemia (table 1). The severity of hemolysis and degree of anemia in PK-deficient patients is highly variable [8,45]. In a series of 61 PK-deficient, non-splenectomized patients, the median hemoglobin was 9.8 g/dL (range 2.2 to 14.4 g/dL) [8,41]. (See 'Typical presentation and clinical features' above.)
Unlike G6PD deficiency, the hemolysis is present at all times and is not precipitated by exposure to drugs (figure 3).
The following findings are typically seen:
●Complete blood count (CBC)
•Low hemoglobin and hematocrit
•Normal to increased mean corpuscular volume (MCV; increases due to reticulocytosis)
•Normal white blood cell (WBC) count, WBC differential
•Platelet count may be increased (as reactive thrombocytosis) or normal
•Increased reticulocyte count (eg, >50 percent), especially if postsplenectomy; this degree of extreme reticulocytosis has not been observed in any other hemolytic anemia
●Peripheral blood smear
•Normal RBC morphology or nonspecific changes such as echinocytes (burr cells), anisocytes, or poikilocytes (table 2)
•Possibly polychromasia related to reticulocytosis (picture 1)
•Normal WBCs and platelet morphology and abundance
●Serum chemistries (findings consistent with non-immune hemolysis)
•Increased indirect bilirubin
•Decreased haptoglobin (variable, may be normal)
•Increased lactate dehydrogenase (LDH; variable and may be normal)
•Negative direct antiglobulin (Coombs) test (DAT)
•Negative indirect antiglobulin test
A rapid non-invasive method to assess the degree of hemolysis based on measurements of exhaled carbon monoxide (eg, end-tidal breath carbon monoxide [ETCO]) has been described and is clinically available. An assay of RBC survival using radioactive chromium is no longer available in the United States . (See "Diagnosis of hemolytic anemia in adults", section on 'Diagnostic approach'.)
Echinocytes are variably present and are neither sensitive nor specific for PK deficiency [47,48]. Acanthocytes (spiculated RBCs) have also been reported on the peripheral blood smears of Basenji dogs with PK deficiency . (See "Burr cells, acanthocytes, and target cells: Disorders of red blood cell membrane", section on 'Pyruvate kinase deficiency'.)
Unlike congenital spherocytic hemolytic anemias, RBC osmotic fragility is normal in PK deficiency (figure 3). The autohemolysis test, in which hemolysis is evaluated in vitro in the presence or absence of added glucose, lacks physiologic relevance and should not be used .
Indications for testing — As noted above, delayed diagnosis of PK deficiency is common (see 'Overview of presenting findings' above). Testing for PK deficiency is appropriate in the following settings:
●Any sibling of a patient with PK deficiency who has unexplained anemia or unexplained hemolysis.
●Any individual with suspected congenital hemolytic anemia who has a negative direct antiglobulin (Coombs) test (DAT) and absence of morphologic abnormalities suggestive of other conditions (eg, absence of profound microcytosis, sickle cells, spherocytes, basophilic stippling, Heinz bodies). These findings and the conditions they suggest are described briefly below (see 'Differential diagnosis' below) and in separate topic reviews.
●Prenatal testing should be offered to parents who have had a child with hydrops fetalis or severe transfusion-dependent anemia not abrogated by splenectomy.
Initial evaluation — The first step in the evaluation of a person with possible PK deficiency is to establish if hemolysis is present. This is done by measuring the reticulocyte count and serum markers of hemolysis such as indirect bilirubin or haptoglobin. Hemolytic anemia is characterized by an increased reticulocyte count, increased indirect bilirubin, and possibly by increased LDH and decreased haptoglobin. (See 'Laboratory findings' above.)
Not all affected individuals have anemia, however, as some may have compensated hemolysis without anemia. In some cases, these individuals come to medical attention when they develop an aplastic crisis (eg, in the setting of parvovirus infection), when they present with complications of hemolysis such as pigment gallstones, or when they develop iron overload.
The next step is to determine whether the hemolysis is chronic and present since birth versus acquired, following a period of normal findings (absence of anemia and hemolysis). If hemolysis has been present throughout life, or if this information is inferred (eg, due to family history) or not verifiable, the peripheral blood smear is examined, with a focus on red blood cell (RBC) morphology (eg, schistocytes, elliptocytes, echinocytes, spherocytes, acanthocytes, sickle cells, RBC inclusions). This may allow elimination of other possible causes of inherited hemolytic anemia with highly characteristic findings on the blood smear (table 3), such as hemoglobinopathies (eg, sickle cell disease, unstable hemoglobin, or autoimmune hemolytic anemia) and membrane disorders (eg, hereditary spherocytosis, hereditary elliptocytosis). PK deficiency should be considered after these obvious alternative diagnoses have been eliminated. (See 'Differential diagnosis' below.)
In many cases, the presence of chronic lifelong anemia and characteristic RBC morphologies are strongly suggestive of specific inherited disorders of the RBC membrane or hemoglobinopathies, and specific diagnostic testing can be obtained. The pace of the evaluation and whether testing is ordered sequentially or simultaneously depends on the severity of the anemia, patient symptoms, and other considerations such as the ease of additional blood draws and follow-up testing.
If PK deficiency is suspected based on the family history and/or initial evaluation, subsequent testing can be done using a biochemical assay and/or genetic testing, as discussed below. (See 'PK-specific testing: Where and how to test' below.)
Additional details of routine diagnostic testing that may be appropriate prior to testing for PK activity or PKLR gene mutations are presented in separate topic reviews. (See "Diagnosis of hemolytic anemia in adults" and "Approach to the child with anemia".)
PK-specific testing: Where and how to test — Testing for PK deficiency can be done by measuring PK activity in RBCs (biochemical testing) and/or by identifying a pathogenic PKLR gene mutation (genetic testing) .
We prefer biochemical testing if possible because it is the most direct evidence of functional PK deficiency. However, genetic testing may be appropriate in selected cases (eg, family member of a known affected individual for whom the pathogenic PKLR variant[s] have been identified). Another exception may be a patient who has had a recent transfusion; the PK in the transfused RBCs will have normal activity and can make the patient's results appear normal. In the author's experience, two infants with PK deficiency who were receiving monthly blood transfusions had PK enzyme activity only slightly below the normal range; however, after splenectomy and no transfusions for five months, PK activity was markedly deficient.
Thus, biochemical testing should optimally be deferred for two to three months following the last transfusion; unfortunately, this may not always be possible in severely anemic, transfusion-requiring individuals. In a patient who requires chronic transfusions, DNA testing (ie, PKLR gene sequencing) may be preferable. (See 'Genetic testing' below.)
Biochemical testing — The gold standard test for diagnosing PK deficiency is testing of PK activity and estimation of enzyme kinetics from RBCs free of white blood cells (WBCs) and platelets and the allosteric activator of PK activity, fructose 1,6-diphosphate (FDP).
This gold standard testing can be obtained through the laboratory of Richard van Wijk, PhD, at University Medical Center Utrecht, Netherlands (contact email: R.vanWijk@umcutrecht.nl) . Most of the research laboratories that perform this testing have closed, and the gold standard assay in which WBCs, platelets, and FDP are removed is not available at commercial laboratories.
●Removal of WBCs and platelets is important because these cells express PK from a different PK gene (PKM) that is unaffected by the causative PKLR gene mutations
●Removal of FDP is important to detect mutants with altered allosteric interaction with FDP
WBCs and platelets are removed by filtration with cellulose chromatography. FDP is removed by dialyzing the hemolysate; the enzyme activity is measured in filtered, dialyzed hemolysate tested at different concentrations of the substrate phosphoenolpyruvate (PEP), with and without FDP [50,52]. As noted above, transfused RBCs cannot be removed, and biochemical testing may be affected (eg, falsely normalized) by recent transfusion. In a series of 61 patients with PK deficiency, the median PK activity was 35 percent of normal .
The mutant PK enzyme may also be analyzed via kinetic and electrophoretic studies of the partially purified enzyme, although these are also not widely available .
An alternative to the gold standard biochemical assay is to use a rapid screening assay that is widely available in many commercial laboratories. In this assay, the activity of PK is measured in an RBC hemolysate, without the initial steps for removing platelets, WBCs, and FDP. This rapid assay identifies most but not all PK-deficient patients, and, if positive for PK deficiency, it is sufficient to confirm the diagnosis (see 'Diagnostic confirmation' below). However, a negative assay cannot be used to eliminate the possibility of PK deficiency, because the contribution of WBC and platelet PK may make the RBC PK activity appear normal when in fact it is not. A similar issue arises in a patient who has recently received an RBC transfusion because PK activity in the transfused RBCs may cause the assay to appear negative.
In addition to performing a potentially less sensitive assay, routine commercial laboratories typically cannot perform the appropriate quantitative analyses with varying concentrations of the PEP substrate, which is helpful for screening for high Km (low affinity) mutants or to remove FDP, to detect mutations with altered FDP interaction .
Thus, if a screening assay is negative and the suspicion for PK deficiency is high (eg, due to Coombs-negative hemolytic anemia, negative testing for other common intrinsic RBC disorders, and/or a family history of PK deficiency), additional testing is required. Options are to use testing from a specialized laboratory such as the one in Utrecht or to perform genetic testing. (See 'Genetic testing' below.)
Other biochemical testing such as measuring the level of the glycolytic intermediate 2,3-DPG is not used. Increased 2,3-DPG is a common occurrence in PK deficiency but is nonspecific and highly variable. It has been reported that the measurements of RBC 2,3-DPG:ATP ratios would be more specific, but this is not done in routine practice .
Genetic testing — Genetic testing for PK deficiency is increasingly available, including testing performed by commercial laboratories and advanced genomic sequencing methods (See "Next-generation DNA sequencing (NGS): Principles and clinical applications".)
This testing is appropriate in circumstances where PK deficiency is suspected but initial biochemical testing is negative or borderline, and/or in a family with a known genetic defect [50,54]. If the familial genetic variant is not known, it may be necessary to sequence the entire gene (figure 1), including all exons, flanking regions, and the PKLR promotor [41,52].
Genetic testing is not typically used as the initial test because there are many genetic variants in PKLR, and their pathogenicity may be unclear (ie, a variant may be identified that has no clinical significance). Additionally, some patients have large deletions and intronic mutations at cryptic splice sites that may not be detected by routine sequencing methods . It is important that the RBC PK gene (PKLR) and not the PKM gene is analyzed. In individuals with more than one PKLR mutation (possible compound heterozygotes), it is also important that parent samples be obtained, to determine whether the two mutations are present in cis (both from the same parent) or in trans (one from each parent) . (See 'Genetics' above.)
Another benefit of genetic testing is that it can be used to identify a familial variant, which in turn can facilitate testing of potentially affected family members, prenatal testing, and genetic counseling . The determination of the causative mutation(s) and their detection in parents is essential for determining the specific method of prenatal diagnosis that is used.
However, as noted above, sequencing of the PKLR gene will not diagnose the rare mutations of genes other than PKLR that have been shown to reduce PK enzymatic activity. (See 'Genetics' above.)
Diagnostic confirmation — The diagnosis of PK deficiency is confirmed in a patient with hemolytic anemia (or compensated hemolysis) who has laboratory evidence of reduced RBC PK enzymatic activity and/or genetic evidence of pathogenic PKLR mutations .
This may include one or more of the following laboratory findings:
●Low levels of RBC PK enzymatic activity, either on a rapid screening test or on a more sophisticated laboratory test
●Homozygosity or compound heterozygosity for missense or deletional PKLR mutations or other PKLR variants that would be expected to impair PK enzyme activity
These assays are described in more detail above. (See 'PK-specific testing: Where and how to test' above.)
As the severity of hemolysis may be variable even among relatives with the same PKLR mutation, it is likely that some affected yet relatively asymptomatic patients are never diagnosed.
DIFFERENTIAL DIAGNOSIS — The differential diagnosis of PK deficiency includes other intrinsic red blood cell (RBC) defects that present as congenital non-spherocytic hemolytic anemias (table 3), as well as certain other inherited anemias (figure 3) and acquired causes of hemolysis.
●Other inherited RBC enzyme disorders – Other RBC enzyme disorders may present similarly to PK deficiency. Examples include glucose-6-phospate dehydrogenase (G6PD) deficiency, which is relatively common but typically manifests with isolated episodes of hemolysis rather than chronic hemolytic anemia, and deficiencies of enzymes involved in anaerobic glycolysis, glutathione metabolism, and nucleotide salvage, which are rare (eg, glutathione synthase deficiency, associated with Heinz bodies; pyrimidine-5'-nucleotidase-1 deficiency, associated with basophilic stippling of RBCs; glucose-6-phosphate isomerase deficiency). The evaluation and diagnosis for these other conditions is discussed separately. (See "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency" and "Disorders of the hexose monophosphate shunt and glutathione metabolism other than glucose-6-phosphate dehydrogenase deficiency" and "Overview of hemolytic anemias in children", section on 'Intrinsic hemolytic anemias'.)
•Like PK deficiency, these are associated with chronic or intermittent non-immune hemolytic anemia (and related symptoms and laboratory findings). In G6PD deficiency and some of the other disorders, hemolysis may be intermittent and/or exacerbated by exposure to oxidant medications or other substances.
•Unlike PK deficiency, these other conditions have normal RBC PK activity.
●Hemoglobinopathies or RBC membrane disorders – Other inherited anemias include hemoglobinopathies (disorders due to hemoglobin mutations such as sickle cell disease [SCD]), unstable hemoglobins and rare hemoglobin H disease, or RBC membrane defects (eg, disorders due to mutations in membrane components, such as hereditary spherocytosis [HS] or hereditary elliptocytosis [HE]). (See "Hemoglobin variants including Hb C, Hb D, and Hb E" and "Methods for hemoglobin analysis and hemoglobinopathy testing" and "Hereditary spherocytosis" and "Hereditary elliptocytosis and related disorders".)
•Like PK deficiency, these cause chronic hemolytic anemia, and in some cases, iron overload may develop.
•Unlike PK deficiency, in these disorders, the RBCs have classic morphologic features on the peripheral blood smear that suggest the diagnosis (eg, sickle cells in SCD, microcytosis in thalassemia, spherocytes in HS, elliptocytes in HE) (table 2). In these disorders, there are diagnostic findings on hemoglobin electrophoresis or other testing such as osmotic fragility, and PK activity is normal.
●Congenital dyserythropoietic anemia – The congenital dyserythropoietic anemias (CDAs) are a group of rare inherited RBC disorders characterized by mutations affecting RBC development in the bone marrow . (See "Overview of causes of anemia in children due to decreased red blood cell production", section on 'Congenital dyserythropoietic anemia'.)
•Like PK deficiency, these disorders cause chronic anemia, jaundice and splenomegaly, and, later in life, iron overload may develop. Similar to PK deficiency, the RBC morphology may be normal or may show nonspecific abnormalities.
•Unlike PK deficiency, in the CDAs, the reticulocyte count is low and the bone marrow shows various abnormalities in developing RBC precursor cells .
●Acquired hemolysis – The causes of acquired hemolysis or hemolytic anemia are numerous and include both intrinsic (intracorpuscular) RBC defects and extrinsic (extracorpuscular) defects. (See "Overview of hemolytic anemias in children" and "Diagnosis of hemolytic anemia in adults".)
•Like PK deficiency, the age of presentation and severity of anemia are variable, the reticulocyte count is increased, and the peripheral blood smear may be unrevealing (eg, with normal RBC morphology or nonspecific changes); occasional spherocytes may be present.
•Unlike PK deficiency, in acquired hemolysis and hemolytic anemia, a causative condition or medication can usually be identified from the medical history, medication list, or laboratory testing such as Coombs testing or flow cytometry, which may reveal antibodies to RBCs or other defects such as absence of glycosylphosphatidylinositol (GPI)-anchored proteins.
Some of these conditions and their site of RBC destruction (ie, intravascular or extravascular) are listed in the table (table 3).
Overview of management — Treatment of PK deficiency depends on the age when the disorder becomes evident.
●Before birth – Fetal hydrops due to severe anemia may require intrauterine transfusion. (See "Intrauterine fetal transfusion of red cells".)
●Neonatal period – Hyperbilirubinemia during the neonatal period may necessitate phototherapy or exchange transfusion. (See "Unconjugated hyperbilirubinemia in term and late preterm infants: Epidemiology and clinical manifestations" and "Unconjugated hyperbilirubinemia in term and late preterm infants: Management".)
●Infancy through adulthood – Severe anemia in infants, children, and adults may require one or more of the following:
•Red blood cell (RBC) transfusions (see "Red blood cell transfusion in infants and children: Indications" and "Indications and hemoglobin thresholds for red blood cell transfusion in the adult")
•Splenectomy (see 'Splenectomy' below)
•Iron chelation (see 'Prevention/treatment of iron overload' below)
•Investigational therapies such as hematopoietic stem cell transplant, gene therapy, or gene editing (see 'Hematopoietic stem cell transplant and gene therapy (investigational)' below)
As with any hereditary hemolytic anemia, it is important to thoroughly evaluate the possibility of other causes of anemia for individuals who have a change in symptoms, hemoglobin level, or reticulocyte count, rather than attributing these changes to the underlying disorder. As examples, a decline in hemoglobin and reticulocytes may be a sign of parvovirus infection, new macrocytosis may be a sign of vitamin B12 or folate deficiency, and new microcytosis may be a sign of iron deficiency. At a minimum, these individuals warrant increased monitoring, and if the changes do not resolve in a reasonable period of time, additional testing may be needed, as discussed in separate topic reviews. (See "Approach to the child with anemia" and "Diagnostic approach to anemia in adults".)
As with any genetic condition, individuals of childbearing potential may benefit from prenatal genetic counseling. (See 'Genetic counseling, prenatal testing, and pregnancy' below.)
Glucocorticoids are of no value in PK deficiency. Drugs with oxidant potential appear to be safe. (See "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency", section on 'Inciting drugs, chemicals, foods, illnesses'.)
Typical monitoring schedule — Patients with PK deficiency are monitored during routine medical care for symptoms of anemia and require an evaluation for any changes in symptoms. The minimal evaluation includes any history of new medications, changes in symptoms, or new medical conditions; examination for appropriate growth and development, signs of anemia, and findings associated with other causes of anemia; and laboratory testing with complete blood count (CBC) and reticulocyte count. Additional testing may be indicated depending on the details of the presentation.
Iron studies (or ferritin levels) are monitored periodically with additional testing as appropriate (interval ranges from annually to more frequently for those with increasing ferritin). (See 'Prevention/treatment of iron overload' below.)
Individuals who have a dramatic decline in reticulocyte count may have a bone marrow insult such as parvovirus infection. In most such cases, detection of viral infection is sufficient without the need for bone marrow examination. If bone marrow examination is done, testing for parvovirus should be included. (See "Clinical manifestations and diagnosis of parvovirus B19 infection".)
Transfusions and phototherapy — Phototherapy with or without exchange transfusion is indicated for severe hyperbilirubinemia during the neonatal period. (See "Unconjugated hyperbilirubinemia in term and late preterm infants: Management", section on 'Assessment of risk severity'.)
Transfusions are generally given for symptoms. The level of hemoglobin should not be the sole criterion, and the need for transfusions should be individualized. As noted above, RBCs in individuals with PK deficiency have a right-shifted hemoglobin oxygen dissociation curve, which means that they may be less symptomatic than other individuals with a similar degree of anemia; this is the reason that the hemoglobin level alone should not be used to guide transfusions . In a series of patients with PK deficiency referred to a single center, 38 of 59 (64 percent) received blood transfusions (median number, 15; range, 1 to >100), and 19 (32 percent) were transfusion-dependent during childhood or until they underwent splenectomy .
For those receiving chronic transfusions, chelation therapy should be instituted early, before iron overload develops. (See 'Prevention/treatment of iron overload' below.)
Mitapivat for symptomatic anemia — Mitapivat (previously called AG-348) is an oral small molecule that activates PK enzymatic function in RBCs. It was approved by the US Food and Drug Administration (FDA) in February 2022 for adults with PK deficiency .
●Indications – It would be reasonable to try mitapivat in any individual with PK deficiency who requires transfusions. While some individuals with specific pathogenic variants in the PLKR gene may have a suboptimal response or no response, even a borderline response would be of significance in an individual who is transfusion-dependent. Mitapivat is very unlikely to be effective in individuals who are compound heterozygotes for two non-missense mutations, and these individuals may reasonably choose not to take mitapivat.
Mitapivat may also be reasonable in individuals with symptomatic anemia who do not require transfusions, and even in individuals with compensated hemolysis who do not have overt anemia, as they may have mild symptoms that only become apparent when hemolysis is reduced (eg, fatigue of which they were unaware until it resolved). Further, amelioration of the severity of hemolysis is likely to reduce the accumulation of excess iron leading to iron overload.
Safety data for pregnancy are not available, and we would not use mitapivat during pregnancy or in individuals who may become pregnant. Safety in children is under study.
●Dosing – The starting dose is 5 mg orally twice per day. If the hemoglobin level does not increase after four weeks, it can be titrated by increasing the dose to 20 mg twice daily, and again after four weeks to a maximum of 50 mg twice daily if needed.
●Adverse effects – Abrupt discontinuation could cause dramatic hemolysis; the dose should be tapered before discontinuation. Product information states that concomitant use with strong CYP3A inducers or inhibitors should be avoided.
●Supporting evidence – The efficacy and safety of mitapivat has been demonstrated in two randomized trials.
•A 2022 trial randomly assigned 80 adults with anemia due to PK deficiency who were not receiving regular transfusions to receive mitapivat or placebo for 24 weeks, with a dose escalation period during the first 12 weeks . At least one of their PKLR pathogenic variants had to be a missense mutation; most had been previously treated with splenectomy and/or cholecystectomy. An increase in hemoglobin of 1.5 g/dL occurred in 16 of 40 in the mitapivat arm (response rate, 40 percent; mean hemoglobin increase, 3.5 g/dL); none of the 40 patients in the placebo arm had a response. Patient-reported outcomes were improved with mitapivat. Missing data precluded assessment of response in three patients in the mitapivat arm and five in the placebo arm. Improvement in the PKDD (pyruvate kinase daily diary) score was greater in the mitapivat arm. Therapy was well-tolerated, with similar rates of adverse events in both arms.
•A 2019 open-label trial randomly assigned 52 adults with anemia due to PK deficiency who were not receiving regular transfusions to treatment for 24 weeks with one of two doses of mitapivat (50 or 300 mg twice daily) . The dose could be increased if the hemoglobin remained low or decreased for adverse events or excessive hemoglobin increases. Half of the patients had an increase in hemoglobin of >1 g/dL, with a mean increase of 3.4 g/dL at a median of 10 days. The higher dose did not result in greater response rates, suggesting that the lower dose was sufficient. Responses were best in individuals with missense mutations in PKLR; responses did not occur (or were lower) in individuals who were homozygous for R479H or for two non-missense mutations. Of the 20 individuals with a good response who continued therapy in an extension phase, 19 had a maintained increase in hemoglobin. Therapy was well-tolerated. One individual who had the dose held for a rapid increase in hemoglobin level developed acute hemolysis; following that event, the remaining individuals who required a dose reduction had their dose tapered rather than stopped abruptly without incident.
In vitro testing studies using RBCs from patients with PK deficiency have demonstrated that PK activity can be increased by more than 10-fold over baseline .
Mitapivat is a quinolone sulfonamide that was developed using biochemical assays for compounds that could increase PK activity in RBCs. The mechanism of action involves allosteric activation of the PK enzyme, similar to the mechanism of fructose 1,6-diphosphate (FDP) but with greater efficacy . By making the dysfunctional PK enzyme function better, treatment with the drug reduces hemolysis. (See 'PK enzymatic function' above.)
Folic acid — Increased RBC turnover in PK deficiency may lead to folate deficiency in those with inadequate fruit and vegetable intake, but routine folate administration is not needed in those with adequate intake of fresh fruits and vegetables or a diet that includes folic-acid-supplemented grains.
For individuals who place a high value on avoiding folate deficiency, which could cause worsening anemia, taking daily folic acid (typical dose, 1 to 2 mg daily) is safe and inexpensive, and there are essentially no side effects or contraindications.
Splenectomy — Splenectomy improves hemolytic anemia, but there are several risks, including surgical complications, the possibility of sepsis due to encapsulated organisms, and the increased risk of venous thromboembolism (VTE).
Thus, splenectomy is typically reserved for individuals with severe transfusion-dependent anemia not responsive to mitapivat; the decision of whether to pursue splenectomy must be made on a case-by-case basis. We generally raise the possibility of splenectomy in all individuals who require chronic transfusions despite mitapivat; we also evaluate the possibility of splenectomy in individuals who do not require chronic transfusions but who have a significant decrease in daily activities due to anemia despite mitapivat.
In a series of patients with PK deficiency, 18 of 61 (30 percent) underwent splenectomy (11 before diagnosis and seven after diagnosis) . The median hemoglobin level was 7.3 g/dL in those who underwent splenectomy; this increased by approximately 1.8 g/dL (range, 0.4 to 3.4 g/dL increase) after the procedure. In comparison, those who were not treated with splenectomy had a median hemoglobin level of 9.8 g/dL. Another study in 124 children with PK deficiency reported that most had increased hemoglobin and reduced transfusion burden, but complication rates were high (sepsis or infection in 12 percent and thrombosis in 1.3 percent) .
For additional decision support, it is reasonable to assume that an individual is likely to have a similar benefit as other affected family members and to follow guidelines for splenectomy used in more common congenital hemolytic anemias such as hereditary spherocytosis. (See "Hereditary spherocytosis", section on 'Splenectomy'.)
For individuals who require cholecystectomy for pigment gallstone disease, the option of concomitant splenectomy should be discussed, and if the patient is considering splenectomy, the option of combining the procedures should be strongly encouraged because these procedures can at times be performed simultaneously using laparoscopic techniques. Likewise, any splenectomy candidate should be evaluated for gallstones to consider cholecystectomy at the same time.
For those who elect to pursue splenectomy, especially children, we try to delay the procedure as long as possible (eg, until after the age of three years or after age six years if possible). Young children who undergo splenectomy should be treated with penicillin until they reach the age of three years. Evidence to guide the optimal age of splenectomy comes mainly from observational data in other conditions. (See "Hereditary spherocytosis", section on 'Splenectomy'.)
We also make sure to provide pre-splenectomy vaccinations against encapsulated organisms, similar to individuals undergoing splenectomy for other hematologic conditions such as immune thrombocytopenia (ITP). (See "Second-line and subsequent therapies for immune thrombocytopenia (ITP) in adults", section on 'Pre-splenectomy considerations'.)
Patients must be educated about the potential risks of serious/life-threatening infections and VTE and the need to seek immediate medical attention for fever or symptoms of VTE, as discussed in detail separately. (See "Prevention of infection in patients with impaired splenic function" and "Clinical features, evaluation, and management of fever in patients with impaired splenic function" and "Second-line and subsequent therapies for immune thrombocytopenia (ITP) in adults", section on 'Splenectomy risks'.)
For those undergoing splenectomy, the skill of the surgeon is important in preventing surgical complications and allowing a laparoscopic technique, which appears to have lower rates of morbidity and mortality in other conditions such as ITP. In some individuals, it may be possible to perform partial splenectomy. A single report indicates failure of partial splenectomy (80 percent) to reduce the transfusion requirement of a four-year-old patient with PK deficiency . Six months later, she successfully underwent total splenectomy and became transfusion independent. This subject is discussed in more detail separately. (See "Elective (diagnostic or therapeutic) splenectomy", section on 'Partial splenectomy'.)
The beneficial effect of splenectomy on hemolysis has been well documented. Typically, hemolysis and anemia are ameliorated but not entirely abated. In severe cases, the transfusion requirement is generally, but not invariably, abolished. Results of splenectomy in other family members may also be of major guidance in this matter. However, there is no reliable way to predict success of splenectomy.
Prevention/treatment of iron overload — Individuals with PK deficiency are at risk of iron overload from frequent blood transfusions as well as from increased iron absorption due to ineffective erythropoiesis. Iron overload in turn can cause serious organ toxicity to the liver, heart, and endocrine organs.
●Rarely, symptoms of these organ toxicities may be responsible for bringing the patient to medical attention and ultimately leading to the diagnosis of PK deficiency. (See 'Iron overload' above.)
●In some cases, clinically significant iron overload can develop even if no transfusions have been administered; some of these individuals were found to have concomitant hereditary hemochromatosis [62-64].
Thus, it is important to monitor for iron overload and to institute an iron chelation program in those with early signs of iron overload or in those at greatest risk of iron overload (eg, those on a chronic transfusion program). (See 'Typical monitoring schedule' above.)
The details of iron chelation therapy including choice of chelating agent, dosing, monitoring of iron burden, and monitoring for adverse effects of chelating agents are discussed in detail separately. (See "Iron chelators: Choice of agent, dosing, and adverse effects".)
Hematopoietic stem cell transplant and gene therapy (investigational)
●Hematopoietic stem cell transplant – Allogeneic hematopoietic stem cell transplant is a potential option for patients with extremely severe disease who continue to require chronic transfusions after splenectomy; a case report described a five-year-old boy with severe PK deficiency who was treated with allogeneic transplant from a human leukocyte antigen (HLA)-identical sibling donor .
●Gene therapy and gene editing – Monogenic disorders such as PK deficiency are amenable to gene therapy and potentially gene editing approaches. In hematopoietic disorders, this is likely to require gene transduction or CRISPR-mediated correction of PKLR mutations using autologous hematopoietic stem cells and followed by autologous transplant, similar to approaches that are being tested in sickle cell disease. Ex vivo lentiviral-mediated gene therapy has been initiated, and follow up of two patients for over 6 months documented sustained reduction of hemolysis and increase of hemoglobin concentration . (See "Overview of gene therapy, gene editing, and gene silencing", section on 'Inherited single gene disorders'.)
Genetic counseling, prenatal testing, and pregnancy — As with all inherited conditions, individuals with PK deficiency may benefit from genetic counseling regarding the risk of a child being affected. This counseling can be provided by a specialist who understands the risks and can obtain a thorough family history and assess possible consanguinity (eg, hematologist) or a genetic counselor. Individuals with heterozygosity for a PKLR mutation (eg, children of an affected individual) who are of childbearing potential may benefit from a discussion of testing their partner. (See "Genetic counseling: Family history interpretation and risk assessment".)
Individuals who have had a severely affected child may wish to pursue in vitro fertilization with preimplantation genetic testing or to use prenatal testing to determine whether a subsequent pregnancy is affected. The type of genetic analysis will depend on the specific PK mutation. If desired, intrauterine transfusions of a severely affected (eg, hydropic) fetus may be possible. (See "Preimplantation genetic testing".)
Management of pregnancy in an individual with PK deficiency is similar to other inherited hemolytic anemias, with close monitoring of hematologic status and transfusion when appropriate . Pregnancy has been thought to precipitate hemolysis in patients with PK deficiency. In-utero blood transfusion may be required, as some fetuses and neonates with severe PK deficiency present with hydrops fetalis . Guidance on fetal blood sampling and intrauterine transfusion is provided separately . (See "Intrauterine fetal transfusion of red cells".)
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Anemia in adults".)
SUMMARY AND RECOMMENDATIONS
●Genetics and mechanisms of hemolysis – Pyruvate kinase (PK) deficiency is an autosomal recessive hemolytic anemia characterized by reduced activity of the red blood cell (RBC) isoform of the PK enzyme, which is encoded by the PKLR gene (PK in liver and RBCs). Affected individuals are either homozygous for a single pathogenic variant or compound heterozygous for two different pathogenic variants (figure 1). PK generates a molecule of ATP during glycolysis, and reduced ATP may contribute to hemolysis, although the exact mechanism for hemolysis remains unclear. PK-deficient RBCs show enhanced oxygen delivery for a given partial pressure of oxygen in the bloodstream; thus, anemia may be better tolerated than in other inherited hemolytic anemias. (See 'Pathophysiology' above.)
●Prevalence – PK deficiency is rare, with a prevalence estimated at <51 cases per million population. It has a worldwide distribution, but it is more common among people of northern European and perhaps Chinese ancestry. (See 'Epidemiology' above.)
●Clinical features – PK deficiency is a lifelong condition. Symptoms are variable and there is a large range in the age of presentation; diagnostic delays are common. The three major presentations are with neonatal jaundice, chronic non-immune, non-spherocytic hemolytic anemia (table 1), gallstones, and, rarely, symptoms of iron overload. The severity of the anemia is variable, and the findings on the peripheral blood smear are nonspecific. (See 'Typical presentation and clinical features' above.)
●Evaluation (initial) – Testing for PK deficiency is appropriate in any sibling of a patient with PK deficiency who has unexplained anemia, or any individual with Coombs-negative hemolytic anemia and absence of morphologic abnormalities suggestive of other conditions (eg, absence of profound microcytosis, sickle cells, spherocytes, basophilic stippling, Heinz bodies) (table 2). Prenatal testing should be offered to parents who have had a child with hydrops fetalis or severe transfusion-dependent anemia not abrogated by splenectomy. (See 'Indications for testing' above.)
●Evaluation (specialized testing) – PK deficiency can be diagnosed by measuring reduced PK activity in RBCs (biochemical testing) and/or by identifying homozygous or compound heterozygous pathogenic PKLR gene mutations (genetic testing). We prefer biochemical testing if possible because it is the most direct evidence of functional PK deficiency; genetic testing may be appropriate in selected cases. The gold standard biochemical test measures PK activity and estimates enzyme kinetics from RBCs free of contaminating white blood cells (WBCs), platelets, and the allosteric activator of PK activity, fructose 1,6-diphosphate (FDP); however, availability of this testing is very limited. An alternative is to use a rapid screening assay that is widely available in many commercial laboratories; this testing is reasonable with certain caveats. (See 'PK-specific testing: Where and how to test' above and 'Diagnostic confirmation' above.)
●Differential diagnosis – The differential diagnosis of PK deficiency includes other intrinsic RBC defects that present as congenital non-spherocytic hemolytic anemias (figure 3), as well as certain other inherited anemias (eg, congenital dyserythropoietic anemia [CDA]) and acquired causes of hemolysis (table 3). (See 'Differential diagnosis' above.)
•Transfusions for severe anemia – Individuals with severe anemia, including fetuses, infants, children, and adults, may require RBC transfusions (intermittent or chronic, based on symptoms and hemoglobin level). Phototherapy may be indicated for neonatal hyperbilirubinemia. (See 'Transfusions and phototherapy' above.)
•Mitapivat for symptomatic anemia – For individuals with symptomatic anemia (transfusion-dependent or not requiring transfusions), we suggest mitapivat (Grade 2B). Mitapivat is also reasonable in individuals with mild anemia without symptoms or compensated hemolysis without anemia. Individuals with two non-missense mutations in the PKLR gene may reasonably choose not to take mitapivat as it is unlikely to be effective. (See 'Mitapivat for symptomatic anemia' above.)
•Folic acid for hemolysis – For individuals with signs of ongoing hemolysis, we suggest folic acid (Grade 2C). A typical dose is 1 mg daily. Folic acid may reasonably be omitted for individuals with a varied diet or whose hemolysis resolves with mitapivat. (See 'Folic acid' above.)
•Splenectomy for severe, refractory disease – Splenectomy may be indicated in selected individuals with transfusion-dependent anemia not responsive to mitapivat (ideally, deferred until later childhood and preceded by indicated vaccinations). (See 'Splenectomy' above.)
•Other interventions for selected individuals – Cholecystectomy may be required for pigment gallstones. Iron chelation may be needed to prevent or treat iron overload. Genetic counseling may be appropriate; preimplantation genetic testing or prenatal testing may be offered. (See 'Prevention/treatment of iron overload' above and 'Genetic counseling, prenatal testing, and pregnancy' above.)
ACKNOWLEDGMENTS — We are saddened by the death of Stanley L Schrier, MD, who passed away in August 2019. The editors at UpToDate gratefully acknowledge Dr. Schrier's role as Section Editor on this topic, his tenure as the founding Editor-in-Chief for UpToDate in Hematology, and his dedicated and longstanding involvement with the UpToDate program.
The UpToDate editorial staff also acknowledges extensive contributions of Donald H Mahoney, Jr, MD to earlier versions of this topic review.