Hemoglobin variants that alter hemoglobin-oxygen affinity

INTRODUCTION — Normal adult hemoglobin (HbA) binds oxygen cooperatively, as illustrated by the sigmoidally shaped oxygen-hemoglobin dissociation curve (figure 1). (See 'Regulation of hemoglobin oxygen affinity' below.)

Rarely, genetic mutations (variants) affecting the alpha or beta globin chains can change the affinity of the hemoglobin molecule for oxygen, thereby disturbing the normal loading of oxygen in the lungs and delivery of oxygen to the tissues. (See "Structure and function of normal hemoglobins".)

●Variants that increase the affinity of hemoglobin for oxygen cause isolated erythrocytosis.

●Variants that decrease the affinity of hemoglobin for oxygen cause cyanosis and/or anemia.

Usually, individuals with disorders of hemoglobin oxygen affinity do not require treatment. It is important to make the correct diagnosis to allow the individual to avoid costly and potentially dangerous invasive diagnostic testing or interventions and to provide the diagnostic information to the involved families.

This topic discusses genetic disorders that alter the affinity of hemoglobin for oxygen, their clinical phenotypes, and an approach to the evaluation and management.

Separate topic reviews discuss general approaches to evaluating polycythemia and cyanosis. (See "Diagnostic approach to the patient with erythrocytosis/polycythemia" and "Approach to cyanosis in the newborn" and "Approach to cyanosis in children" and "Methemoglobinemia".)

PATHOPHYSIOLOGY

Regulation of hemoglobin oxygen affinity — Adult hemoglobin (HbA) is a tetramer consisting of two alpha globin chains and two beta globin chains; each chain can bind one molecule of oxygen gas. The binding of oxygen to hemoglobin is cooperative, which means that binding of the first molecule(s) of oxygen increases the affinity for binding of subsequent molecule(s). (See "Structure and function of normal hemoglobins", section on 'Cooperativity'.)

Deoxygenated hemoglobin (referred to as the "tense" [T] form) has a lower affinity for ligands such as oxygen gas than oxygenated hemoglobin (referred to as the "relaxed" [R] form). Cooperative binding and oxygen affinity are discussed in more detail separately. (See "Structure and function of normal hemoglobins", section on 'Oxygen affinity'.)

Cooperative binding enables the hemoglobin molecule to bind oxygen tightly in the high-oxygen environment of the lungs and release it in the tissues, where the oxygen content is lower. This is illustrated by the sigmoidal curve of the oxygen-hemoglobin dissociation curve (figure 1).

The p50 is the partial pressure of oxygen at which the hemoglobin molecule is half-saturated with oxygen. The p50 is often used as a marker of changes to the oxygen-hemoglobin dissociation curve.

●When a hemoglobin's affinity for oxygen is high (low p50, left-shifted curve), oxygen delivery to the tissues is impaired, stimulating erythropoietin production and increasing the red cell mass, resulting in erythrocytosis.

●Conversely, when a hemoglobin has a low oxygen affinity (high p50), these variants deliver more oxygen to the tissues per gram of hemoglobin; mild anemia can result, and hemoglobin desaturation can be accompanied by cyanosis.

A number of factors other than hemoglobin variants can modulate hemoglobin-oxygen affinity and shift the oxygen-hemoglobin dissociation curve to the left or the right.

●Left – Left-shift (increased oxygen affinity; reduced delivery to the tissues) can be caused by:

•High pH (more basic)

•Low temperature

•Low 2,3-bisphosphoglycerate (2,3-BPG)

•Methemoglobinemia

•Fetal hemoglobin (HbF)

•Bisphosphoglycerate (BPGM) mutase deficiency

●Right – Right-shift (reduced oxygen affinity; increased delivery to the tissues) can be caused by:

•Low pH (more acidic)

•High temperature

•High 2,3-BPG

The genetic variants discussed below alter the affinity of hemoglobin for oxygen and shift the oxygen-hemoglobin dissociation curve. (See 'Hemoglobin variants that can affect oxygen affinity' below.)

Pharmacologic regulators of hemoglobin oxygen affinity — Voxelotor is a drug used in sickle cell disease that increases hemoglobin affinity for oxygen by binding alpha globin chains. In doing so it inhibits the polymerization of deoxy sickle hemoglobin because sickle hemoglobin polymerization requires hemoglobin to be in the deoxy form. (See "Investigational therapies for sickle cell disease", section on 'Reducing Hb S polymerization'.)

Activators of pyruvate kinase in RBCs (mitapivat, etavopivat) that decrease levels of 2,3 BGG also reduce the p50 and inhibit sickle hemoglobin polymerization. These are used in PK deficiency and are also being studied as therapeutics for thalassemia and sickle cell disease therapeutics [1]. (See "Management of thalassemia", section on 'Mitapivat' and "Investigational therapies for sickle cell disease", section on 'Pyruvate kinase activation (mitapivat, etavopivat)'.)

Hemoglobin variants that can affect oxygen affinity — In addition to modulators of hemoglobin-oxygen affinity listed above, mutations (variants) affecting the alpha or beta globin chains of hemoglobin can also affect the affinity of hemoglobin for oxygen and be clinically apparent. (See 'High oxygen affinity hemoglobins: Erythrocytosis' below and 'Low oxygen affinity hemoglobin variants: Cyanosis' below.)

Variants in the genes that encode alpha globin (HBA) and beta globin (HBB) can alter the following aspects of the hemoglobin tetramer that raise or lower the p50:

●The R to T transition

●The affinity of the hemoglobin for 2,3-BPG

●Length of the globin chains

●The heme binding pocket

●The alpha1-beta1 interface, which is not normally dissociable and does not participate in the R to T transition

2,3-BPG is a polyanion that binds strongly to deoxyhemoglobin, stabilizing the T conformation and reducing oxygen affinity. Binding to hemoglobin takes place in the central cavity between the two beta chains where the negative charges of 2,3-BPG are neutralized by the beta globin chain amino terminus, beta2 histidine, beta82 lysine, and beta143 histidine. (See "Structure and function of normal hemoglobins".)

High oxygen affinity variants are more common (by approximately twofold) than low oxygen affinity variants.

●High oxygen affinity – A low p50 is consistent with an increased oxygen affinity variant; these individuals typically have erythrocytosis. (See 'High oxygen affinity hemoglobins: Erythrocytosis' below.)

Single amino acid substitutions, substitutions of two amino acids, deletions and insertions of amino acids, reading frameshift mutations, and fusion genes can all result in hemoglobins with increased oxygen affinity [2]. Most (but not all) of these high affinity hemoglobins, can be understood based on knowledge of the structure-function relationships in hemoglobin. (See "Structure and function of normal hemoglobins", section on 'Hemoglobin structure'.)

Underlying all known high ligand affinity hemoglobin variants is the furtherance of the R state (the high oxygen affinity conformer of the hemoglobin molecule). This can occur by stabilizing the R state or destabilizing the T state (low oxygen affinity conformer). (See 'Regulation of hemoglobin oxygen affinity' above.)

The magnitude of erythrocytosis varies in different high oxygen affinity variants, even if the p50 is the same. As an example, individuals with Hb Osler have the same p50 as individuals with Hb McKees Rocks, but the hemoglobin level in Hb Osler is more than 4 g/dL higher. Specific high oxygen affinity variants are discussed below. (See 'Selected high oxygen affinity Hbs' below.)

●Low oxygen affinity – A high p50 is consistent with a low oxygen affinity variant; these individuals typically have cyanosis. (See 'Low oxygen affinity hemoglobin variants: Cyanosis' below.)

Variants affecting the gamma globin or delta globin chains are unlikely to cause clinically significant effects. Gamma globin chains (in Hb F) are replaced by beta globin chains shortly after birth, while delta globin chains (in Hb A2) only comprise 2 percent of total hemoglobin. Selected variants and their mechanisms are discussed below. (See 'Selected variants' below.)

HIGH OXYGEN AFFINITY HEMOGLOBINS: ERYTHROCYTOSIS

Clinical presentation (high oxygen affinity Hbs) — Salient clinical features and presentations of patients with high oxygen affinity hemoglobin (Hb) variants are as follows:

●Asymptomatic with isolated erythrocytosis on the complete blood count (CBC); typical hemoglobin 15 to 19 g/dL in adults. However, the definition of elevated hemoglobin depends on the normal range, which differs in pediatric age groups and by sex. Age-specific and sex-specific values are discussed separately. (See "Diagnostic approach to the patient with erythrocytosis/polycythemia", section on 'CBC/blood smear'.)

●No history of (or clinical evidence of) cardiac or pulmonary disease

●Normal oxygen saturation on pulse oximetry

●No evidence of a myeloproliferative neoplasm (no splenomegaly; normal white count and platelet count)

●Family history of erythrocytosis

●No suspicion of a kidney or liver tumor or other erythropoietin-producing cancer

●Abnormal newborn hemoglobin screening by high-performance liquid chromatography (HPLC) or other protein-based diagnostic assay

Concomitant thalassemia may make the phenotype more severe, as it decreases the amount of normal hemoglobin and in turn increases the proportion of the high oxygen affinity hemoglobin. This phenomenon has been described in several case reports, such as the first description of an individual with Hb Crete and an individual with Hb Luton. (See 'Management (high oxygen affinity Hbs)' below and 'Selected high oxygen affinity Hbs' below.)

The thrombotic risk is not well characterized, but there are cases of thrombotic events described with other risk factors [3]. (See 'Management (high oxygen affinity Hbs)' below.)

Evaluation (high oxygen affinity Hbs) — The evaluation for a high oxygen affinity hemoglobin variant typically starts with identification (and confirmation) of erythrocytosis on the complete blood count (CBC). (See 'Clinical presentation (high oxygen affinity Hbs)' above.)

Other individuals may come to medical attention due to a positive family history of a high oxygen affinity hemoglobin or unexplained erythrocytosis and/or when newborn screening identifies an abnormal hemoglobin. (See 'Specific testing for high oxygen affinity Hbs' below.)

The greatest value of early diagnosis is the opportunity to avoid unnecessary invasive diagnostic procedures and inappropriate (and often dangerous) therapeutic interventions. Some patients, for example, have undergone cardiac catheterization, while others have received aggressive treatment based upon a mistaken diagnosis of polycythemia vera. Establishing the proper diagnosis is also critical for appropriate family screening.

Exclude other causes of erythrocytosis — The first step is to exclude other causes of erythrocytosis, such as cardiopulmonary disease, carbon monoxide poisoning, a myeloproliferative neoplasm (MPN), or an erythropoietin-producing tumor (table 1). This evaluation is discussed in detail separately and summarized briefly below. (See "Diagnostic approach to the patient with erythrocytosis/polycythemia", section on 'Causes of absolute polycythemia'.)

●History – The history focuses on:

•Familial disorders

•Carbon monoxide exposure

•Cardiopulmonary symptoms

•Symptoms of liver or kidney disease

•Symptoms related to a tumor (abdominal fullness, central nervous system abnormalities)

Inheritance patterns of specific disorders are discussed below; the majority are autosomal dominant. (See 'Selected high oxygen affinity Hbs' below.)

●Pulse oximetry – Pulse oximetry is used to determine whether erythrocytosis is occurring in response to hypoxemia. Low oxygen saturation suggests a cardiac or pulmonary cause of secondary erythrocytosis.

●Erythropoietin – A serum erythropoietin level is used to determine whether erythrocytosis is primary (due to an MPN, in which erythropoietin will be low) or secondary (due to increased erythropoietin production); an overview of causes is presented in the table (table 1).

•The normal stimulus for erythropoietin production is anemia; this is due to sensing by the kidney of low oxygen delivery. The figure illustrates the normal increase in erythropoietin as hematocrit declines (figure 2).

•With high oxygen affinity hemoglobins, the kidney senses low oxygen delivery despite a normal or increased hemoglobin level and a normal oxygen saturation. The erythropoietin level is high or normal, either of which is inappropriate in an individual with erythrocytosis or polycythemia and normally functioning hemoglobin.

•High erythropoietin may also be seen in chronic pulmonary disease or heart disease, but in these cases, oxygen saturation will be low. (See "Diagnostic approach to the patient with erythrocytosis/polycythemia", section on 'Hypoxia-associated polycythemia'.)

•Markedly increased levels of erythropoietin can also be caused by exogenous sources of erythropoietin production including kidney and liver tumors and hemangioblastoma with autonomous erythropoietin production. (See "Diagnostic approach to the patient with erythrocytosis/polycythemia", section on 'Tumor-associated polycythemia'.)

•High erythropoietin levels can also be seen with exogenous use as an athletic performance-enhancing hormones. (See "Use of androgens and other hormones by athletes".)

•Congenital erythrocytosis due to pathogenic variants in the oxygen sensing pathways (such as Chuvash syndrome due to VHL gene mutations) can also be associated with increased erythropoietin levels. (See "Molecular pathogenesis of congenital erythrocytoses and polycythemia vera", section on 'Chuvash erythrocytosis'.)

•In polycythemia vera and other MPNs, the erythropoietin level is usually low or absent. Splenomegaly and elevation of the white blood cell (WBC) count or platelet count may also be present. Genetic testing includes JAK2 V617F and other MPN variants in JAK2, MPL, and CALR. (See "Diagnostic approach to the patient with erythrocytosis/polycythemia", section on 'Primary polycythemia'.)

Specific testing for high oxygen affinity Hbs — Once other causes of erythrocytosis have been excluded, individuals with normal pulse oximetry and erythrocytosis should have specific testing for abnormal hemoglobins:

●Low p50 – A low p50 is the "gold standard" for diagnosis of a high oxygen affinity hemoglobin variant. An estimate of the p50 can be obtained on a venous blood gas from most blood gas analyzers. This approach may be relatively specific (77 percent) but not sensitive (5 percent). P50 is best measured directly, especially if a high oxygen affinity variant is suspected but the p50 on blood gas analyzer is normal and a variant hemoglobin cannot be detected using protein-based diagnostics [4]. Assays for direct measurements and resources for obtaining them are discussed below. (See 'Resources (p50, genetic testing)' below.)

●Hemoglobin HPLC or electrophoresis – Variants in globin genes that result in amino acid substitutions with electrostatic charge changes can be identified using protein-based detection techniques including high-performance liquid chromatography (HPLC), isoelectric focusing, or capillary electrophoresis. However, an isoelectric substitution may not be detected by these methods, and protein analytics will only identify approximately one-half of the cases of a high oxygen affinity variant. Thus, an analysis that reveals an abnormal hemoglobin is helpful, but a normal evaluation does not exclude a high affinity variant.

Hemoglobinopathy evaluation is performed as part of newborn screening in the United States, primarily to identify sickle cell disease. However, other hemoglobin variants are also commonly incidentally identified, including high oxygen affinity variants. The age at which symptoms occur depends on which globin chain is affected:

•Gamma chain variants are present in utero and at birth and will resolve within 12 months as HbF production wanes.

•Beta chain mutants are often asymptomatic in the newborn period, as expression does not reach adult levels until 9 to 12 months.

•Alpha chain variants are present in utero and at birth and persist throughout life.

Parental studies can help to predict the phenotype, based on findings in an affected parent.

●Genetic testing – If the clinical evaluation suggests a high or low oxygen affinity hemoglobin variant, DNA sequencing can identify the specific variant, which can confirm the diagnosis and facilitate testing of family members when appropriate. If a familial syndrome is already known, testing can focus on the specific familial variant.

Resources for obtaining genetic testing are discussed below. (See 'Resources (p50, genetic testing)' below.)

If the p50 is normal and/or protein-based methods or genetic testing does not reveal a hemoglobin variant, evaluation for other rare variants is appropriate, in consultation with a hematologist. (See 'Differential diagnosis' below.)

Diagnostic confirmation (high oxygen affinity Hb) — We consider the diagnosis of a high oxygen affinity hemoglobin to be confirmed in the appropriate clinical setting (erythrocytosis with normal oxygen saturation and other causes of secondary polycythemia have been excluded) by documenting the presence of the following (see 'Specific testing for high oxygen affinity Hbs' above):

●A low p50 and Documentation of a hemoglobin variant by HPLC or another protein-based analytic or genetic testing that documents a known or novel hemoglobin variant likely to cause high oxygen affinity. (See 'Resources (p50, genetic testing)' below.)

-or-

●Genetic testing that documents a known high oxygen affinity variant such as those described below. (See 'Selected high oxygen affinity Hbs' below.)

Resources for p50 and genetic testing are discussed below. (See 'Resources (p50, genetic testing)' below.)

Differential diagnosis — The differential diagnosis of high oxygen affinity hemoglobins includes other heritable and acquired disorders that cause erythrocytosis.

Like high oxygen affinity hemoglobins, these other disorders can cause erythrocytosis; some of them are associated with normal oxygen saturation on pulse oximetry and some are associated with normal to increased erythropoietin. Unlike high oxygen affinity hemoglobins, these disorders do not have a relevant hemoglobin variant on genetic testing.

●Congenital enzyme deficiencies – Erythrocytes in 2,3-bisphosphoglycerate (2,3-BPG) mutase deficiency, hexokinase deficiency, and other rare enzyme deficiencies can have high oxygen affinity [5]. In comparison with high oxygen affinity hemoglobins, the affinity of the hemolysate with 2,3-BPG mutase deficiency is normal and the level of 2,3-BPG is very low.

●Congenital methemoglobinemia and carboxyhemoglobinemia – Several gas measuring apparatuses can give an accurate reading of methemoglobinemia (with the exception of some M hemoglobins) and carboxyhemoglobinemia in whole blood.

Evaluations of erythrocytosis and other heritable disorders of hemoglobin oxygen affinity (2,3-BPG mutase deficiency, congenital methemoglobinemia, variants affecting the erythropoietin receptor, and others) are discussed in separate topic reviews. (See "Diagnostic approach to the patient with erythrocytosis/polycythemia" and "Molecular pathogenesis of congenital erythrocytoses and polycythemia vera".)

Management (high oxygen affinity Hbs) — Patients with high oxygen affinity hemoglobins accompanied by erythrocytosis usually have a benign clinical course without complications, except when a confounding disorder is present. Low ambient oxygen tension (pO₂), as in unpressurized airplanes and ascent to altitude, do not represent a risk since high affinity hemoglobins are avid for oxygen.

Often, one of the most important aspects of management is education about the disorder and helping the patient provide information to other clinicians, to allow them to avoid inappropriate diagnostic evaluations and therapeutic interventions. This may be provided in the form of a letter, a clear description in the individual's medical record, and/or this UpToDate topic can be printed or emailed to the individual's other clinicians.

Most centers have little clinical experience managing patients with high oxygen affinity variants. In a review of 41 patients with high oxygen affinity variants, 34 (83 percent) affecting the beta globin chain, hemoglobin concentration was 16 to 21.9 g/dL and p50 was 12 to 25 mm Hg [6]. Thrombosis occurred in 10 (24 percent). Phlebotomy was used in 23 and anti-platelet therapy was used in 21 individuals. Hemoglobin level was not correlated with either thrombotic or non-thrombotic symptoms. Of 23 pregnancies, 18 (78 percent) resulted in live births and no fetal loss was attributed to erythrocytosis.

High affinity hemoglobin variants identified on newborn screen are often asymptomatic in the newborn period. Parental studies are helpful to assess the expected phenotype of the variants. Testing and counseling of first-degree relatives can be helpful for similar reasons. (See "Genetic counseling: Family history interpretation and risk assessment".)

The lack of complications and the lack of necessity for treatment reflect the typically moderate increases in hemoglobin concentration. Exceptions may occur in rare cases such as Hb Malmö. (See 'Selected high oxygen affinity Hbs' below.)

Specific management considerations include the following:

●Phlebotomy – Prudence dictates that before embarking on a regimen of chronic phlebotomy, one should be conservative and review the hematologic and physiologic findings at frequent intervals during the first few years after diagnosis. In older patients, special attention should be directed to blood flow and oxygen delivery to the heart and central nervous system.

However, rare individuals appear to have benefited from phlebotomy, and thus other unknown factors may be interfering with their normal compensation for high hemoglobin oxygen affinity, and increased blood viscosity may have become a burden. If symptoms are vague and possibly caused by erythrocytosis, the minimal phlebotomy needed to bring some relief and not induce severe iron deficiency would seem to be the best approach.

●Thrombosis risk – There is a potential concern for microcirculatory and thrombotic complications with high oxygen affinity hemoglobins due to hyperviscosity, although the actual risk is unknown. Erythrocytosis has been shown to be associated with increased risk of arterial and venous thrombotic complications in specific conditions, such as polycythemia vera and Chuvash erythrocytosis [7]. The risk is less clear in other conditions such as cyanotic heart disease [8]. The reasons for lower risk in some conditions compared with others is not well understood.

There are no data to guide thrombosis management specifically in individuals with high oxygen affinity hemoglobin variants that increase thrombosis risk. Management similar to the general population would be prudent.

•Treatment such as phlebotomy should be individualized based on elevation of hemoglobin, clinical symptoms, and associated risks.

•Treatment low-dose aspirin is also individualized. While there are no data demonstrating efficacy in patients with high oxygen affinity variants, low-dose aspirin may be considered based on the degree of erythrocytosis and other thrombotic or cardiovascular risks, and if there are no bleeding contraindications. As an example, low-dose aspirin may be appropriate for an adult with polycythemia and other thrombosis risk factors or an individual with familial polycythemia and a positive family history for thrombosis. (See "Polycythemia vera and secondary polycythemia: Treatment and prognosis", section on 'Low-dose aspirin'.)

●Pregnancy – Increased morbidity or mortality has not been observed during pregnancy (or in the fetus or child). A study of a family, including their newborns, carrying a low affinity hemoglobin demonstrated that the reversal of the physiologic fetomaternal oxygen affinity had no effects on fetal development [9].

LOW OXYGEN AFFINITY HEMOGLOBIN VARIANTS: CYANOSIS

Clinical presentation (low oxygen affinity Hbs) — The main clinical finding in individuals with low oxygen affinity hemoglobin variants (low oxygen affinity Hbs) is cyanosis, with slate gray color of skin and mucous membranes. Anemia can also be present. (See "Unstable hemoglobin variants".)

The age of onset of cyanosis depends on whether the variant affects alpha globin or gamma globin, both produced in utero, or beta globin after the switch from gamma globin to beta globin during the first year of life. Other salient clinical features and presentations include the following:

●Central cyanosis affecting the mucous membranes, when there is >5 g/dL of desaturated hemoglobin.

●No evidence or history of cardiac or pulmonary disease.

●Low oxygen saturation on pulse oximetry.

●Cyanosis at birth that persists during the first year of life suggests an alpha chain variant.

●Cyanosis at birth that disappears during the first year of life suggests a gamma globin variant [10].

●Cyanosis in later infancy (middle to the end of the first year of life) suggests a beta chain variant.

●Family history of cyanosis (with or without anemia) or a low oxygen affinity hemoglobin.

Evaluation (low oxygen affinity Hbs) — The evaluation for a low oxygen affinity hemoglobin variant (low oxygen affinity Hb) typically starts with identification (and confirmation) of cyanosis. (See 'Clinical presentation (low oxygen affinity Hbs)' above.)

In some cases, hemoglobin analysis done as part of routine newborn screening will identify an abnormal hemoglobin. (See 'Specific testing for low oxygen Hb variants' below.)

One of the greatest values of early diagnosis is the opportunity to avoid unnecessary invasive diagnostic procedures and inappropriate therapeutic interventions.

Exclude cardiopulmonary disease and other causes of cyanosis — The evaluation starts by excluding a cardiopulmonary disorder or another cause of cyanosis due to significant right-to-left cardiac shunting or inadequate oxygenation of the blood.

Other causes of cyanosis include:

●Methemoglobinemia or a heritable M hemoglobin

●Sulfhemoglobinemia

Cardiopulmonary disease is the most common cause of cyanosis and the most important etiology to exclude. One of the major differences between cardiopulmonary disease and a low oxygen affinity Hb is that individuals with cardiopulmonary disease are often symptomatic, whereas those with a low oxygen affinity Hb are asymptomatic.

Another major distinguishing factor is the ability to oxygenate blood using 100 percent oxygen. This can be tested by measuring pulse oximetry and/or observing changes in the color of the patient's blood.

●Pulse oximetry – Pulse oximetry can be performed on room air and while the patient is breathing pure (100 percent) oxygen. Individuals with low oxygen affinity hemoglobins will have low oxygen saturation when breathing room air that will increase to normal when breathing 100 percent oxygen. In contrast, individuals with other conditions (cardiac disease, right-to-left shunting, methemoglobinemia, or sulfhemoglobinemia) will have low oxygen saturation on room air that will remain low when they are breathing 100 percent oxygen.

●Blood color change – Blood can be exposed to pure (100 percent) oxygen. Purple-greenish color of the blood that turns bright red with 100 percent oxygen suggests a low oxygen affinity variant, whereas blood that does not turn red suggests methemoglobinemia or sulfhemoglobinemia.

Additional details of the evaluation for cyanosis are discussed separately. (See "Approach to cyanosis in the newborn" and "Diagnosis and initial management of cyanotic heart disease in the newborn" and "Approach to cyanosis in children".)

Testing for methemoglobinemia and heritable M hemoglobins is also discussed separately. (See "Methemoglobinemia", section on 'Initial evaluation'.)

Specific testing for low oxygen Hb variants — Once other causes of cyanosis have been excluded, specific testing for abnormal hemoglobins should be performed.

●Arterial blood gas measurement – Patients with low oxygen affinity hemoglobin variants typically have a normal PaO₂ while breathing room air but a low arterial oxygen saturation (SaO₂).

A normal PaO₂ with a low oxygen saturation by pulse oximetry might also be consistent with a low oxygen affinity hemoglobin variant; however, low oxygen affinity variants can spuriously reduce pulse oximetry readings [11]. Discordance between the PaO₂ on the arterial blood gas (ABG) and the oxygen saturation on pulse oximetry can also be seen with methemoglobinemia (acquired or heritable), although methemoglobin can cause both the PaO₂ and the oxygen saturation to be normal or falsely elevated.

●High p50 – Demonstration of a high p50 suggests a low oxygen affinity hemoglobin variant.

●Hemoglobin analysis – As mentioned above, some individuals come to medical attention when high-performance liquid chromatography (HPLC) or other newborn screening method demonstrates an abnormal hemoglobin, or this testing can be performed once a hemoglobin variant is suspected. (See 'Evaluation (low oxygen affinity Hbs)' above.)

●Genetic testing – If the clinical evaluation suggests a low oxygen affinity hemoglobin variant, DNA sequencing can identify the specific variant, which can confirm the diagnosis and facilitate testing of family members when appropriate. If a familial variant is already known, testing can focus on the specific familial variant.

Resources for obtaining a p50 and genetic testing are discussed below. (See 'Resources (p50, genetic testing)' below.)

If the p50 is normal and/or protein-based methods or genetic testing does not reveal a hemoglobin variant, evaluation for other rare variants is appropriate, in consultation with a hematologist. (See 'Differential diagnosis' below.)

Diagnostic confirmation (low oxygen affinity Hb) — After exclusion of cardiopulmonary disease, methemoglobinemia, and sulfhemoglobinemia, we consider the diagnosis of a low oxygen affinity hemoglobin to be confirmed in the appropriate clinical setting (cyanosis with a normal PaO₂ and low oxygen saturation that corrects with 100 percent oxygen) by documenting:

●A high p50 and documentation of a hemoglobin variant by HPLC or another protein-based analytic or genetic testing that documents a known or novel hemoglobin variant likely to cause low oxygen affinity. (See 'Resources (p50, genetic testing)' below.)

-or-

●Genetic testing that documents a known low oxygen affinity variant such as those described below. (See 'Selected low oxygen affinity Hbs' below.)

Differential diagnosis — The differential diagnosis of low oxygen affinity hemoglobin variants includes other causes of cyanosis and unstable hemoglobins that cause anemia or abnormalities on hemoglobin analysis.

●Cardiopulmonary disease – Like low oxygen affinity hemoglobins, individuals with heart or lung disease can have cyanosis. Unlike low oxygen affinity hemoglobins, heart and lung disease with hypoxia typically cause significant symptoms whereas low oxygen affinity hemoglobins are typically asymptomatic and are associated with a normal hemoglobin level or anemia. Additional details of the evaluation for cyanosis are discussed separately. (See "Approach to cyanosis in the newborn" and "Diagnosis and initial management of cyanotic heart disease in the newborn" and "Approach to cyanosis in children".)

●Congenital disorders – Like low oxygen affinity hemoglobins, methemoglobinemia and pyruvate kinase deficiency can be heritable. Heritable M hemoglobins typically cause asymptomatic cyanosis, and pyruvate kinase deficiency due to PKLR variants can cause hemolysis. Unlike low oxygen affinity hemoglobins, acquired methemoglobinemia or sulfhemoglobinemia may cause significant symptoms along with cyanosis. Unlike low affinity hemoglobins, individuals with methemoglobinemia and sulfhemoglobinemia do not have improvement in oxygenation when breathing 100 percent oxygen. (See "Methemoglobinemia", section on 'Initial evaluation' and "Pulse oximetry", section on 'Sulfhemoglobin' and "Pyruvate kinase deficiency".)

Management (low oxygen affinity Hbs) — Individuals with low oxygen affinity hemoglobins have adequate oxygen delivery to the tissues. Thus, they do not require medical intervention for hypoxia.

Often, one of the most important aspects of management is education about the disorder and helping the patient provide information to other clinicians, to allow them to avoid inappropriate diagnostic evaluations and therapeutic interventions. This may be provided in the form of a letter, a clear description in the individual's medical record, and/or this UpToDate topic can be printed or emailed to the individual's other clinicians.

Testing and counseling of first-degree relatives can be helpful for similar reasons. (See "Genetic counseling: Family history interpretation and risk assessment".)

Increased tissue oxygen delivery accounts for the mild anemia that can accompany these disorders. Some individuals with unstable hemoglobin variants can also have cyanosis. For these individuals, it is important to counsel the patient (or family) about medications that may precipitate hemolysis and anemia. Hemolytic anemia rather than cyanosis is the more common presentation of unstable hemoglobin variants. (See "Unstable hemoglobin variants", section on 'Avoidance of oxidant drugs' and "Unstable hemoglobin variants", section on 'Prevention and prompt treatment of infections'.)

RESOURCES (P50, GENETIC TESTING)

●p50 – The p50 should be measured, if possible, if a high or low oxygen affinity variant is suspected; this is considered the gold standard test. Two definitive assays are available if a variant is suspected but the p50 estimate on blood gas analyzer is normal and/or a variant hemoglobin cannot be detected using protein-based diagnostics:

•HEMOX-ANALYSER

•HEM-O-SCAN

A problem with the availability of these assays was illustrated in a survey that reported that fewer than one-half of the laboratories surveyed had these instrumentations [12]. Thus, if genetic testing is easier to obtain and documents a pathogenic variant, the p50 testing can be omitted. If the clinical phenotype and genetic testing are confirmatory, p50 adds little to the individual's treatment but might be useful for research to fully characterize the variant and link structure to function.

●Genetic testing – Genetic testing may be easier to obtain than p50 testing and can be sufficient for diagnosis and treatment if it reveals a pathogenic variant in a globin gene. Several reference laboratories are available for gene sequencing to identify the pathogenic variant causing abnormal hemoglobin oxygen affinity.

The decision concerning which reference laboratory to send the sample to depends upon several factors. Many hospitals and medical centers have contractual agreements with one of the national reference laboratories (LabCorp, Quest, ARUP, Mayo Clinic) and will forward the samples to these laboratories. Some institutions prefer to forward samples to one of the few laboratories that specialize in globin abnormalities. This choice may depend upon the familiarity of the referring clinician and possibly insurance coverage.

There are three laboratories in the United States that specialize in globin abnormalities and are based at academic institutions:

•Titus HJ Huisman Hemoglobinopathy Laboratory at Augusta University, Augusta, GA (https://www.augusta.edu/centers/blood-disorders/hemoglobinopathy/index.php)

•Boston Medical Center Hemoglobin Diagnostic Reference Laboratory, Boston, MA (https://www.bmc.org/medical-professionals/hemoglobin-diagnostic-reference-laboratory)

•Reference Laboratory at University of California San Francisco Children's Hospital of Oakland Research Institute, Oakland, CA (https://hemoglobinlab.ucsf.edu/)

SELECTED VARIANTS — Selected variants that affect hemoglobin oxygen affinity, with their mechanisms, are discussed below.

A listing of the known high and low oxygen affinity hemoglobin variants affecting the genes for alpha globin (HBA) and beta globin (HBB) can be found on the Globin Gene Server (https://globin.bx.psu.edu/).

Alpha versus beta globin variants — Variants in HBB (affecting beta globin chains) account for the majority of both high oxygen affinity and low oxygen affinity variants (approximately 75 to 80 percent in both cases).

Variants affecting HBA may be less severe, as there are four alpha globin genes and each one only produces approximately one-fourth of total hemoglobin, whereas there are only two beta globin genes, each producing approximately one-half of total hemoglobin. Concomitant thalassemia reduces the number of normal globin genes and may make the phenotype more severe.

Selected high oxygen affinity Hbs — As noted above, there are several potential alterations of the hemoglobin molecule that can increase hemoglobin affinity for oxygen. (See 'Hemoglobin variants that can affect oxygen affinity' above.)

Variants that affect critical molecular regions involved in the R—>T state transition may result in R state stabilization or T state destabilization, either of which increase oxygen affinity. Examples include:

●HBA variants

•Hb Chesapeake – Hb Chesapeake (HBA arg92leu), an alpha globin variant, was the first high oxygen affinity hemoglobin variant described, from an 81 year old with erythrocytosis [13]. Family studies revealed 15 additional affected family members.

Arg92 is a conserved amino acid that stabilizes the R state (relaxed, high affinity state) at the alpha1beta2 area of contact, making T (tense state) contacts less favored. Oxygen affinity studies revealed a whole blood p50 of 19 mmHg (normal: 26 mmHg) with a normal Bohr effect (increase in oxygen affinity with elevations in pH) and normal 2,3-bisphosphoglycerate (2,3-BPG) binding. The variant produces mild erythrocytosis. On alkaline hemoglobin electrophoresis, it represents approximately 20 percent of the hemolysate and migrates rapidly on the gel. (See 'Alpha versus beta globin variants' above.)

•Hb Montefiore – Hb Montefiore (HBA asp126tyr) is an alpha globin variant that produces a mild erythrocytosis [14]. Oxygen binding studies have revealed a decrease in p50, normal binding to 2,3-BPG, and decreased cooperativity. Functional studies suggest a destabilized T state that switches to R upon ligand binding. The hemoglobin represents 20 percent of the hemolysate and migrates close to HbF (at pH 8.6) on alkaline electrophoresis.

•Hb Tarrant and Hb Fukutomi – Two other high oxygen affinity alpha globin variants, Hb Tarrant (HBA asp126asn) and Hb Fukutomi (HBA asp126val), affect the same amino acid residue. Hb Tarrant was described in a homozygote from a family in which both parents were heterozygotes. The homozygote had 50 percent Hb Tarrant, a p50 of 9 mmHg, and a hemoglobin concentration of almost 19 g/dL. The heterozygous father with 25 percent Hb Tarrant and a p50 of 14 mmHg had a similar hemoglobin level, whereas other heterozygotes have had more modest erythrocytosis.

●HBB variants

•Hb Crete – Hb Crete (HBB ala129pro) alters the alpha1beta1 area of contact and disrupts the overall conformation of hemoglobin, favoring the R state. Proline, the new amino acid formed in this mutation, is not accommodated in the helix and disturbs the H helix while also having a longer range effect on nearby residues in the alpha1beta1 interface, perturbing this area of subunit contact.

The initial case report described an individual with a syndrome of erythrocytosis, hemolysis, splenomegaly, abnormal red cell morphology and marked erythroid hyperplasia [15]. The individual was found to be a compound heterozygote for Hb Crete and delta-beta-thalassemia, with 67 percent Hb Crete and 30 percent fetal hemoglobin (HbF; the delta-beta thalassemia deletion prevents the synthesis of any normal adult hemoglobin [HbA]). The red cells had a low p50, moderately decreased cooperativity, and a normal Bohr effect. Hb Crete is also mildly unstable, which likely accounts for hemolysis. (See "Unstable hemoglobin variants", section on 'Hemolytic anemia'.)

Coincident thalassemia and the resulting increase in HbF likely accentuated erythrocytosis; a family member who was heterozygous for Hb Crete and did not have thalassemia had a relatively more normal p50 and did not have significant erythrocytosis. Coincident thalassemia also appears to have caused splenomegaly; splenomegaly is not a usual feature of high oxygen affinity hemoglobins.

•Hemoglobin Malmö – Hemoglobin Malmö (HBB his97gln) decreases interactions between the alpha1 and beta2 subunits of hemoglobin [16]. Individuals with Hb Malmö can have symptomatic erythrocytosis with headaches and other symptoms, which may require phlebotomy [17]. (See 'Management (high oxygen affinity Hbs)' above.)

•Others

-Hb Kempsey (HBB asp99asn)

-Hb Yakima (HBB asp99his)

-Hb Radcliffe (HBB asp99ala)

-Hb Ypsilanti (HBB asp99tyr)

-Hb Hotel Dieu (HBB asp99gly)

-Hb Chemilly (HBB asp99val)

-Hb Coimbra (HBB asp99glu)

●2,3-BPG binding – 2,3-bisphosphoglycerate (2,3-BPG) is a potent modulator of the affinity of hemoglobin for oxygen. Increasing levels of 2,3-BPG decrease oxygen affinity, shift the dissociation curve to the right, and increase the delivery of oxygen to tissues (figure 1).

Lysine 82 is a conserved residue in beta globin. Three beta globin variants affecting lysine 82 have been described, all of which have drastically reduced binding to 2,3-BPG due to the elimination of one of the normal binding sites:

•Hb Rahere (HBB lys82thr) [18]

•Hb Helsinki (HBB lys82met) [19]

•Hb Providence (HBB lys82asn—>asp) [20]

All of these hemoglobins have moderately high oxygen affinity and moderate erythrocytosis.

●Unexplained – Some high affinity hemoglobins, such as Hb Heathrow, lack an explanation for their functional behavior. Hb Heathrow (HBB phe103leu) is an electrophoretically silent variant with moderately high oxygen affinity and erythrocytosis [21,22]. This variant affects a conserved residue that guards the entrance to the heme pocket. It is possible that a smaller side chain at the bottom of the heme pocket might alter ligand access or the electronic environment of the prosthetic group.

Selected low oxygen affinity Hbs — As noted above, there are several potential alterations of the hemoglobin molecule that can decrease hemoglobin affinity for oxygen. (See 'Hemoglobin variants that can affect oxygen affinity' above.)

Alterations in critical molecular regions involved in the R—>T state transition can produce low oxygen affinity by favoring T state stabilization or R state destabilization. Asparagine 102 is a highly conserved residue that participates in the only hydrogen bond between asn102 and asp94 across the alpha1beta2 interface in oxyhemoglobin; this bond is broken when the hemoglobin molecule assumes the T state. Three low oxygen affinity beta globin variants have been described at this position, all of which cause cyanosis:

●Hb Beth Israel – Hb Beth Israel (HBB asn102ser) has the greatest decrease in oxygen affinity and the most intense cyanosis of the asparagine 102 variants [23].

Hb Beth Israel was found in a patient with cyanosis of the lips, fingers, and nail beds [23]. He had been severely disciplined early in life for constantly having "dirty hands," although the abnormal skin color was not noticeable to the patient or his parents. Cyanosis was detected by an observant surgeon about to perform a hernia repair. The whole blood p50 was markedly elevated at 88 mmHg, and arterial blood was only 63 percent saturated at a normal pO₂ of 97 mmHg. The hemolysate also had a low oxygen affinity, a normal Bohr effect, and elevated erythrocyte 2,3-BPG.

●Hb Kansas – Hb Kansas (HBB asn102thr) has an intermediate oxygen affinity and intermediate cyanosis [24,25]. It was the first low affinity hemoglobin described and is the best studied; it causes a whole blood p50 of approximately 70 mmHg (normal 27 mmHg), decreased cooperativity, and a normal Bohr effect.

Examination of deoxygenated Hb Kansas by difference Fourier X-ray diffraction analysis at 5.5 Å resolution has shown that the new threonine residue is incapable of forming this bond. Thus, low oxygen affinity results from destabilization of the R conformer. In addition, other structural changes, which occur at the alpha1beta2 interface, account for the increased tendency of Hb Kansas to dissociate into dimers. This variant is the mirror image, pathogenetically, of Hb Chesapeake. (See 'Selected high oxygen affinity Hbs' above.)

●Hb Saint Mande – Hb Saint Mande (HBB asn102tyr) has the smallest decrease in oxygen affinity and the least intense cyanosis of the asparagine 102 variants [26].

Hb Saint Mande was first described in an individual with cyanosis of the lips and anemia (hemoglobin 9.6 g/dL) [26]. The hemoglobin accounted for 38 percent of the hemolysate and migrates similar to HbF on electrophoresis. The p50 was 52 mmHg and the oxygen saturation of arterial blood was 81 percent.

Nuclear magnetic resonance (NMR) spectroscopy demonstrated that the quaternary structure of the liganded conformer was different from the R state but also different from the T state. This special quaternary structure also differed from Hb Kansas, which had an NMR spectrum similar to the T state quaternary structure in the liganded molecule. The tertiary conformation around the heme of both globin chains was altered, suggesting a long range interaction involving leucine 141.

●Hb Bruxelles – Hb Bruxelles (HBB phe42del) was first described in an individual with cyanosis and severe hemolytic anemia from the age of four years, requiring transfusion one time. Later in life, her hemoglobin concentration stabilized at 10 g/dL; the reasons for the change in phenotype were not well understood.

The variant involves a deletion of the most conserved amino acid residue of hemoglobin. Phenylalanine residues at beta41 and beta42 are conserved in all mammalian non-alpha-globin chains and are indispensable for the structural integrity and oxygen-binding functions of the molecule.

●Hb Sarajevo – Hemoglobin Sarajevo is a variant that affects one of the gamma globin chains (HBG2 asn102thr). It was reported in a child with neonatal cyanosis that resolved by the age of six months, associated with the switch from fetal to adult hemoglobin. Other gamma-globin variants have a similar phenotype [27].

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: Sickle cell disease and thalassemias".)

SUMMARY AND RECOMMENDATIONS

●Pathophysiology – Adult hemoglobin (HbA) is a tetramer of two alpha globin chains and two beta globin chains, each of which can bind one molecule of oxygen. Cooperative binding enables the hemoglobin molecule to bind oxygen tightly in the high-oxygen environment of the lungs and release it in the tissues, where the oxygen content is lower (figure 1). Pathogenic variants in certain genes shift the curve to the left (increasing oxygen affinity) or to the right (decreasing oxygen affinity). Variants affecting beta globin (HBB gene) are more common than variants affecting alpha globin (HBA gene). (See 'Pathophysiology' above and 'Alpha versus beta globin variants' above.)

●High oxygen affinity variants

•Presentation – These variants cause erythrocytosis, often asymptomatic; some individuals can have headaches or other symptoms. Some may come to attention when newborn screening identifies an abnormal hemoglobin. Oxygen saturation by pulse oximetry is normal. (See 'Clinical presentation (high oxygen affinity Hbs)' above.)

•Evaluation – Start by excluding other causes of erythrocytosis (cardiopulmonary disease, carbon monoxide, a myeloproliferative neoplasm [MPN], or an erythropoietin-producing tumor (table 1)). If the complete blood count (CBC) is otherwise unremarkable and the erythropoietin level is normal or high, obtain a p50 measurement, hemoglobin analysis (electrophoresis or high-performance liquid chromatography [HPLC]), and/or genetic testing. (See 'Evaluation (high oxygen affinity Hbs)' above and 'Differential diagnosis' above.)

•Diagnosis – Diagnostic confirmation requires a low p50 and a known high oxygen affinity variant or a novel (previously unknown) pathogenic variant in a hemoglobin gene. (See 'Diagnostic confirmation (high oxygen affinity Hb)' above.)

•Management – Most individuals do not require interventions. Phlebotomy may be helpful in selected cases with symptomatic erythrocytosis. Low-dose aspirin may be considered if other cardiovascular or thrombotic risks are present. Education can help the patient and clinicians avoid inappropriate diagnostic evaluations and therapies. Testing and counseling first-degree relatives can be helpful. (See 'Management (high oxygen affinity Hbs)' above.)

•Specific variants – High oxygen affinity variants include Hbs Chesapeake, Montefiore, Crete, Malmö, and others. (See 'Selected high oxygen affinity Hbs' above.)

●Low oxygen affinity variants

•Presentation – These variants cause cyanosis, often asymptomatic, with slate gray skin and mucous membranes. Anemia may be present or intermittent and may be more marked if the hemoglobin is unstable. (See 'Clinical presentation (low oxygen affinity Hbs)' above.)

•Evaluation – Start by excluding cardiopulmonary disorders and other causes of cyanosis (methemoglobinemia or sulfhemoglobinemia). An arterial blood gas (ABG) on room air will show a normal PaO₂ despite a low oxygen saturation by pulse oximetry. The oxygen saturation can be raised to 100 percent by having the patient breath 100 percent oxygen, which generally does not occur with other causes of cyanosis. If these findings are present, it is appropriate to obtain a p50 and genetic testing. (See 'Evaluation (low oxygen affinity Hbs)' above and 'Differential diagnosis' above.)

•Diagnosis – Diagnosis is confirmed in the appropriate clinical setting by a finding of a high p50 and a known low affinity hemoglobin or a pathogenic variant in a hemoglobin gene. (See 'Diagnostic confirmation (low oxygen affinity Hb)' above.)

•Management – Medical intervention for hypoxia is not required. If the hemoglobin is also unstable, provide counseling about avoiding medications that may precipitate hemolysis and anemia. Education can help the patient and clinicians avoid inappropriate diagnostic evaluations and therapies. Testing and counseling first-degree relatives can be helpful. (See 'Management (low oxygen affinity Hbs)' above.)

•Specific variants – Low oxygen affinity variants include Hbs Beth Israel, Kansas, Saint Mande, Bruxelles, and Sarajevo. (See 'Selected low oxygen affinity Hbs' above.)

●Resources – Laboratories for p50 and genetic testing are listed above. (See 'Resources (p50, genetic testing)' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges extensive contributions of Donald H Mahoney, Jr, MD, to earlier versions of this topic review.