INTRODUCTION — Examination of the peripheral blood smear is an inexpensive but powerful diagnostic tool in both children and adults. In some ways it is becoming a "lost art" but it often provides rapid, reliable access to information about a variety of hematologic disorders.
The smear offers a window into the functional status of the bone marrow, the factory producing all blood elements. It is particularly important when assessing cytopenic states (eg, anemia, leukopenia, thrombocytopenia). Review of the smear is an important adjunct to other clinical data; in some cases, the peripheral smear alone is sufficient to establish a diagnosis .
Automated machines that deliver increasingly sophisticated data about blood counts and morphology tend to generalize and include a wide array of morphologic abnormalities. However, only an experienced reviewer can weigh the relative significance of observed findings and assess their importance within the context of other clinical data. A trained eye will also appreciate other subtleties of morphology that may be undetected by automated review. (See "Automated hematology instrumentation".)
Use of "digital blood smears" is also increasing. (See 'Digital blood smears' below.)
Review of the peripheral smear is not required in all patients with a hematological disorder. Certain straightforward conditions such as iron deficiency anemia can be easily diagnosed on the basis of clinical information and basic laboratory data (eg, mean corpuscular volume [MCV], serum ferritin) alone. However, there are a number of settings in which interpretation of the peripheral smear is especially important. Three examples include:
●Hemolytic anemia – Review of red cell morphology may identify the cause of erythrocyte destruction (eg, the presence of bite cells points to a Heinz body hemolytic anemia) and the ultimate diagnosis (eg, oxidant damage to the red cell secondary to drugs).
●Thrombocytopenia – Review of platelet size and morphology can sometimes suggest whether thrombocytopenia is due to increased platelet consumption (generally associated with larger platelets) and reduced platelet production (often associated with smaller platelets or abnormal platelet morphology).
●White blood cell disorders – The precise disease classification may rely upon evaluation of abnormal circulating cells (eg, the presence of Auer rods in a blast form in patients with acute myeloid leukemia).
Evaluation of the peripheral blood smear will be discussed here. Evaluation of bone marrow aspirate smears is discussed separately. (See "Evaluation of bone marrow aspirate smears".)
INITIAL APPROACH — The reviewer must develop a systematic approach to the assessment of the peripheral smear. Below is one proposed system, which can be tailored to meet individual preferences and efficiency.
Slide preparation — In most cases, peripheral blood smears are prepared using a wedge technique, either manually or via an automated technique. Smears should be made from one small drop of blood which has not been allowed to clot, and which has been completely mixed, if made from an anticoagulated blood sample that may have been allowed to settle for some time.
The importance of a clean slide, free from dust, dirt, grease, and fingerprints, cannot be overstated. The presence of such contaminants can often be surmised when a large number of target cells or stomatocytes is present only in localized areas on the smear . Repeating the procedure with alcohol-cleaned slides corrects this problem.
Optimal area for review — Review of the peripheral smear starts with choosing the best prepared and stained slide for examination. Scanning the entire slide under low power enables selection of an optimal area. All slides have areas that can interfere with an accurate assessment of morphology (figure 1):
●One end of smear is too thick; stacks or clusters of red cells in this area cause erythrocytes to appear small and dark (picture 1). This area of the slide may be useful when searching for the presence of malarial parasites.
●The other end of the slide (the feather edge) will be spread too thin; red cells assume a "brick-like" or "cobblestone" type of pattern in this area (picture 2). Red cells in this area are not biconcave discs. However, this area may be useful when searching for small cytoplasmic inclusions, cellular fragments, cells containing Auer rods, and large circulating tumor cells (picture 3).
●In the optimal area, red cells will be more or less evenly spaced, and central pallor will be appreciated. In normal patients it should be rare to see two or more cells abutting (picture 4).
Abnormal cell distribution — If an otherwise optimal area of the peripheral smear shows abnormal distribution of red cells, this may be due to a number of medical conditions or artifacts, such as:
●Rouleaux formation due to elevated levels of plasma proteins – In this setting the red cells may take on the appearance of a stack of coins, a phenomenon called rouleaux formation. This is most commonly seen in multiple myeloma (picture 5), but is also present in patients with increased levels of fibrinogen or total proteins from whatever cause (eg, polyclonal or monoclonal gammopathies) .
●Irregular collections of red cells – This may signify the presence of cold agglutinins (picture 6), as seen following certain infections and in cold agglutinin disease. (See "Cold agglutinin disease".)
●Collections of amorphous or crystalline material – This may signify the presence of precipitates of cryoglobulins, as might occur in patients with hepatitis C infection . This can result in WBC counts as high as 50,000/microL and a doubling of the platelet count, both of which are attributed to various sizes of precipitated cryoglobulin particles, which may be counted as white blood cells and/or platelets in automated cell counters. (See "Approach to the patient with thrombocytosis", section on 'Blood smear'.)
●Presence of large, clear areas – The presence of large, clear areas on an otherwise well-prepared smear, along with circular gaps between red cells, rouleaux formation, and red cell aggregates has been attributed to the presence of a circulating surfactant or oil-based material. One example would be the presence of polyoxyethylated castor oil (Cremophor), a nonionic surfactant used for solubilization of such hydrophobic agents as anesthetic agents, sedatives, immunosuppressives, sensitizers, antifungals, and antineoplastics (eg, paclitaxel) . Another would be presence of chemoembolization material containing a viscous oil together with a suspension of microparticles .
●Presence of lipid droplets surrounding red cells – Small lipid droplets overlying and surrounding the periphery of red cells, along with lipemic (milky) plasma (picture 7), may be seen in patients with hypertriglyceridemia .
Digital blood smears — Some hematology analyzers are connected to equipment that can use preset criteria to generate a blood smear if there are certain abnormalities on the complete blood count (CBC). The analyzers can also be connected to automated image analysis software capable of capturing high-magnification images of the blood smear and storing them digitally.
Image analysis software that relies on deep learning methods and pattern recognition can then analyze the findings and identify specific abnormal findings, which in turn can be provided to a hematologist for further review.
RED BLOOD CELLS — Erythrocytes are the most numerous cells encountered in the peripheral smear. Morphologic examination should include assessment of size, shape, and color (pallor), and the presence of inclusions.
Size — Normal red cells approximate the size of the lymphocyte nucleus, with a diameter of 7 to 8 microns and a mean corpuscular volume (MCV) of approximately 90 femtoliters (picture 4). Automated Coulter Counters confirm the actual size, providing a numerical value in the form of the MCV. However, the MCV can be misleading in the presence of a dimorphic population of microcytic (small) and macrocytic (large) cells, since the average may be normal. A high red cell distribution width (RDW) is suggestive of such a divergent population of red cells of different sizes (ie, the presence of anisocytosis), but does not permit direct appreciation of the components explaining the variation in size. (See "Microcytosis/Microcytic anemia", section on 'RDW (size variability)' and "Automated hematology instrumentation", section on 'The red blood cell volume histogram' and "Automated hematology instrumentation", section on 'Red cell distribution width'.)
Shape — Red blood cells normally appear more or less round and have a smooth contour. Permutations in red blood cell shape (poikilocytosis) have different implications depending on the specific shapes observed. Examples of shape abnormalities that suggest an important pathologic process include the following:
●Large oval-shaped cells (macroovalocytes, (picture 8)) suggest a megaloblastic process (eg, deficiency of vitamin B12 or folic acid). A high percentage of oval or elliptical cells (ovalocytes, elliptocytes) is characteristic of a number of inherited red cell abnormalities (picture 9). (See "Hereditary elliptocytosis and related disorders", section on 'Clinical syndromes'.)
●Fragmented erythrocytes (schistocytes, helmet shaped cells, pieces of red cells) point to destruction within the vascular spaces as might occur in thrombotic thrombocytopenic purpura, disseminated intravascular coagulation (picture 10 and picture 11), or a defective prosthetic heart valve (picture 12). (See "Non-immune (Coombs-negative) hemolytic anemias in adults", section on 'Fragmentation'.)
●Tear drop-shaped red cells are commonly found in patients with extramedullary hematopoiesis (eg, primary myelofibrosis) (picture 13) as well as in the thalassemic disorders.
Color — Approximately one-third of the red cell should be clear, in the form of central pallor (picture 4). A decrease in this proportion indicates hyperchromia (increase in hemoglobin concentration). Complete loss of central pallor is characteristic of spherocytes; these are dense, dark cells which are seen in hereditary spherocytosis and autoimmune hemolytic anemia (picture 14). Hypochromic red cells, which are often also microcytic, are seen in conditions such as thalassemia, iron deficiency, and the sideroblastic anemias, and have just a thin rim of pink hemoglobin (picture 15). A bluish tinge suggests excessive amounts of RNA, as would be seen in the reticulocyte (picture 16).
WHITE BLOOD CELLS — A normal peripheral smear should contain a spectrum of mature leukocytes including lymphocytes, neutrophils, and monocytes.
Lymphocytes — Small lymphocytes comprise about 30 to 40 percent of the circulating white cells. They are identified by clumped nuclear chromatin and a scant rim of deep blue cytoplasm (picture 17).
The large granular lymphocyte (LGL) is a morphologically distinct lymphoid subset comprising 10 to 15 percent of normal peripheral blood mononuclear cells. These cells are approximately twice the size of red cells, with abundant cytoplasm, a round to oval nucleus, and a small number of azurophilic cytoplasmic granules (picture 18). (See "Clinical manifestations, pathologic features, and diagnosis of T cell large granular lymphocyte leukemia", section on 'Morphology'.)
●Atypical lymphocytes with a more generous and malleable cytoplasm, often indented by surrounding red cells, can be seen following viral infections such as infectious mononucleosis (picture 19). (See "Approach to the child with lymphocytosis or lymphocytopenia", section on 'Infectious mononucleosis'.)
●Lymphocytosis, accompanied by mature-appearing lymphocytes with scant cytoplasm, condensed chromatin, and clefted nuclei are commonly seen following Bordetella pertussis infection [8,9]. (See "Approach to the child with lymphocytosis or lymphocytopenia", section on 'Pertussis'.)
Neutrophil series — The neutrophil series matures in an orderly fashion (figure 2), from myeloblast (picture 20) to promyelocyte (picture 21) to myelocyte (picture 22) to metamyelocyte (picture 23) to band form (picture 24) to mature neutrophil (picture 25). Only the last two of these stages, the band form and the mature neutrophil, are normally present in the peripheral smear.
An increased absolute number of neutrophilic band forms is called a "left shift" or "bandemia", and is most often associated with infection. This subject is discussed separately. (See "Approach to the patient with neutrophilia", section on 'Neutrophil abnormalities'.)
Metamyelocytes, and rarely myelocytes, may be seen during infections, pregnancy, leukemoid reactions, and recovery from myelosuppression. Forms less mature than the myelocyte (eg, promyelocytes, myeloblasts) are almost exclusively present in the peripheral blood in hematologic malignancies. The combined presence of early neutrophil forms, nucleated red blood cells, and tear drop-shaped red blood cells is called a "leuko-erythroblastic" blood picture, and suggests the presence of bone marrow invasion and/or fibrosis. (See 'Leukoerythroblastic smear' below.)
The presence of a greater percent of myelocytes than metamyelocytes on the WBC differential ("leukemic hiatus") is strongly suspicious of the diagnosis of chronic myeloid leukemia. (See "Clinical manifestations and diagnosis of chronic myeloid leukemia".)
Lobulation — Neutrophils should have a three- to four-lobed nucleus and a pink-sandy, granular cytoplasm.
●Increased lobulation – More than five lobes defines hypersegmentation and suggests either a megaloblastic process (picture 26) or, rarely, iron deficiency anemia. The finding of hypersegmented neutrophils in iron deficiency has been described in a study of 50 individuals with iron deficiency in whom vitamin B12 and folate deficiency had been excluded, in which 31 (62 percent) had hypersegmented neutrophils; and in a study of 94 children with iron deficiency, in whom 81 percent had hypersegmented neutrophils, compared with only 9 percent of controls [10,11]. Grape-like (botryoid) multiple lobulations can also be seen in patients suffering from heat stroke (picture 27) [12,13].
●Decreased lobulation – In the Pelger-Huet anomaly, which can occur as an inherited disorder or can be acquired in patients with myelodysplastic syndromes (referred to as pseudo-Pelger-Huet), there is reduced lobulation of mature neutrophils. Such cells typically have a bilobed nucleus connected by a thin strand, giving a "pince-nez" appearance, often accompanied by reduced or absent granulation (picture 28). (See "Clinical manifestations and diagnosis of myelodysplastic syndromes (MDS)", section on 'Blood smear'.)
Granulation — Dark blue, coarse granules ("toxic granulations") are non-specific findings characteristic of toxic systemic illnesses (picture 29). They represent azurophilic granules with abnormal staining properties. (See "Approach to the patient with neutrophilia", section on 'Neutrophil abnormalities'.)
Giant cytoplasmic granules within neutrophils, especially in a child with recurrent pyogenic infections, may indicate presence of the Chediak-Higashi syndrome (picture 30).
Giant green neutrophil inclusions are discussed below.
Dohle bodies — Döhle bodies are light blue in color, peripheral in location, and are most commonly seen in the neutrophils of patients with infection (picture 29 and picture 31). When accompanied by other changes in neutrophils (eg, left shift, toxic granulation, cytoplasmic vacuoles), this finding is very sensitive for the presence of infectious or inflammatory disease. (See "Approach to the patient with neutrophilia", section on 'Peripheral blood smear'.)
However, Döhle bodies have also been described in patients with burns, myelodysplasia, and in pregnancy. They represent areas of rough endoplasmic reticulum with bound ribosomes, giving them their blue color. Similar-looking inclusions, along with giant platelets, are seen in patients with the May-Hegglin anomaly.
Eosinophils — Eosinophils, normally present in small numbers (less than 5 percent of white cells), are recognized by their vibrant orange granules and a characteristic bilobed nucleus (picture 32). Increased numbers of eosinophils can be a clue to an underlying allergic state, parasitic infection, or other conditions. (See "Approach to the patient with unexplained eosinophilia".)
Basophils — Basophils, the least common of the circulating white blood cells, comprise less than 1 percent of the total white blood cell count, and are recognized by their prominent dark blue-black granules (picture 33).
Basophilic leukocytosis is a distinctly unusual condition, and is most often associated with basophilic or mast cell variants of acute or chronic leukemia. The most common causes of basophilia include myeloproliferative disorders, hypersensitivity or inflammatory reactions, hypothyroidism (myxedema), and certain infections.
Monocytes — Monocytes are the largest normal cells encountered in the peripheral blood. They have a grayish blue cytoplasm, often replete with vacuoles, and a distinctive folded nucleus (picture 17).
Presence of abnormal or giant granules — Although many of the circulating white blood cells (eg, neutrophils, eosinophils, basophils, monocytes, large granular lymphocytes) normally contain granules of varying sizes, giant or abnormal granules may be present in peripheral blood cells in various disease states :
●Azurophilic cytoplasmic inclusions (Alder-Reilly granules) have been described in neutrophils, lymphocytes, and monocytes in the mucopolysaccharidoses (picture 34), but also have been seen in myeloperoxidase mutations, and myelodysplasia [15-18].
●Giant cytoplasmic granules within neutrophils may indicate Chediak-Higashi syndrome. (See 'Granulation' above.)
PLATELETS — Platelets are small purplish anuclear cells. There is normally at least one platelet visualized per oil-immersion field, and seven platelets per 100-power field; less than this number should alert the observer to possible thrombocytopenia. As an example, in an area of the peripheral blood smear where RBCs barely touch, the number of platelets per 100-power field, when multiplied by 20,000/microL, gives an estimate of the platelet count. (See "Automated hematology instrumentation", section on 'Platelet count and size'.)
Review of the smear is particularly important when the platelet count is depressed; pseudothrombocytopenia can be diagnosed by finding large clumps of platelets in smears taken from blood samples anticoagulated with EDTA (picture 35), but not in samples anticoagulated with heparin or citrate.
Large platelets suggest a heightened marrow response secondary to a destructive process such as immune thrombocytopenia. When associated with fragmentation of red cells, microangiopathic processes such as disseminated intravascular coagulation (DIC), thrombotic thrombocytopenic purpura (TTP), hemolytic uremic syndrome (HUS), or drug-induced thrombotic microangiopathy (DITMA) should be suspected (picture 10). (See "Diagnostic approach to suspected TTP, HUS, or other thrombotic microangiopathy (TMA)".)
For a pregnant patient with thrombocytopenia and microangiopathic changes, these diagnoses as well as other pregnancy-associated conditions should be possible causes. (See "Thrombocytopenia in pregnancy".)
The presence of a very high platelet count, extremely large platelets, and/or megakaryocyte fragments is abnormal and suggests an underlying myeloproliferative neoplasm, such as essential thrombocythemia (picture 36) or primary myelofibrosis. (See "Clinical manifestations and diagnosis of primary myelofibrosis", section on 'Platelet and white blood cell abnormalities'.)
While platelets the size of red blood cells ("giant platelets") may be seen in patients with increased platelet turnover or a myeloproliferative neoplasm, they may also be seen in a variety of congenital bleeding disorders (picture 37). (See "Congenital and acquired disorders of platelet function".)
RED CELL ABNORMALITIES — Several red cell abnormalities are common and/or important to recognize:
Permutation in size — Microcytosis in the adult has a limited differential diagnosis which includes iron deficiency, thalassemia, the anemia of chronic disease, and the sideroblastic anemias (see "Diagnostic approach to anemia in adults"). In addition, in the child microcytosis can be caused by chronic lead exposure. (See "Approach to the child with anemia".)
It can be difficult to distinguish between iron deficiency and thalassemia trait on the peripheral smear alone. The former is suggested by variation in cell size and shape (reflected by an increased red cell distribution width) and characteristic "pencil cells", whereas uniformly sized cells with increased numbers of target cells and teardrop cells are characteristic of thalassemia trait. (See "Causes and diagnosis of iron deficiency and iron deficiency anemia in adults" and "Microcytosis/Microcytic anemia".)
Macrocytosis has a broader differential. Reticulocytes are larger than the normal red cells and have a bluish tinge (polychromatophilia) (picture 16). Macroovalocytes are large, oval shaped red cells, and are typical of megaloblastic processes such as folate and cobalamin deficiency (picture 8). Macrocytosis can also be seen in liver disease and primary bone marrow failure states such as aplastic anemia and myelodysplastic syndromes. (See "Macrocytosis/Macrocytic anemia".)
Permutation in shape — Several red cells disorders are associated with distinctive changes in red cell shape and contour.
●Mushroom-shaped cells – COVID-19 can be associated with anisocytosis, spherocytes, stomatocytes, and polychromasia. In a series of 50 people hospitalized with COVID-19, two-thirds had mushroom-shaped cells (also called pincer cells) on the blood smear (picture 38) . This could suggest that the virus may have a significant impact on red cell physiology with a role for oxidative stress.
●Fragmented cells – Fragmentation of the red cell (ie, schistocytes, helmet cells) is probably the most important to recognize since it can indicate a life-threatening condition such as DIC, HUS, or TTP (picture 10 and picture 11). Morphologic changes can be subtle and easily overlooked in mild cases or early in the course of these diseases. Schistocytes generally comprise less than 0.5 percent of the red cells in a normal blood smear. (See "Non-immune (Coombs-negative) hemolytic anemias in adults", section on 'Fragmentation'.)
●Bite cells – Bite cells are caused by phagocytes which have extracted rigid precipitates of denatured hemoglobin (Heinz bodies) (picture 39); these cells may be the earliest clue to the pathogenesis of a hemolytic anemia due to oxidant sensitivity, such as a deficiency of glucose-6-phosphate dehydrogenase (G6PD).
●Sickle cells – Sickle cells are unique in their spiculated shape (picture 40). In patients with SC disease, the cells are only partially sickled, tempering the morphologic appearance and rendering them more "canoe-like" or "pita bread-like" (picture 41).
●Target cells – Target cells have a "bull's eye" extra drop of hemoglobin in their center. They are characteristic of liver disease (particularly obstructive liver disease), postsplenectomy states, and hemoglobinopathies such as thalassemia and Hb C, Hb D, and Hb E (picture 42).
●Projections – Spiculated red cells have an irregular outline. Those with similarly sized, regularly spaced projections are called echinocytes, burr cells, or crenated cells, and are seen most commonly in uremia, or as an artifact of preparation (picture 43). Those with irregularly sized and spaced projections are called acanthocytes or spur cells, and are seen most commonly in liver disease (picture 44). The mechanisms by which these changes occur and the associated underlying disorders are presented separately. (See "Burr cells, acanthocytes, and target cells: Disorders of red blood cell membrane", section on 'Burr cells and acanthocytes'.)
●Tear drops – Tear drop-shaped red cells are commonly found in patients with extramedullary hematopoiesis (eg, primary myelofibrosis, (picture 13)) as well as in the thalassemic disorders.
Red cell inclusions and other changes — The mature circulating red cell contains no inclusions which can be seen on routine (Wright or Giemsa) staining.
Reticulocytes — Reticulocytes, the youngest of the circulating red cells contain polyribosomes which cannot be seen on routine staining, but which give the cytoplasm a blue tint (picture 16). A reticular network can be seen in reticulocytes only in the presence of certain supravital dyes, such as new methylene blue (picture 45). Reticulocytes normally comprise about 1 percent of red cells. An increased percentage of reticulocytes is seen in response to bleeding or hemolysis (picture 46) or following use of hematinics such as iron, vitamin B12, or folic acid for the appropriate deficiency state. Reticulocytes are reduced in the presence of a suppressed or otherwise abnormal bone marrow, such as in aplastic anemia, pure red cell aplasia, or following systemic chemotherapy.
Nucleated red blood cells — Nucleated red blood cells (normoblasts) are not normally seen in the peripheral blood. When present, they usually indicate the presence of severe degrees of hemolysis (picture 40 and picture 47), profound stress or hypoxemia , or a myelophthisic condition, such as myelofibrosis (picture 48).
The presence of binucleated normoblasts on the smear in a patient with congenital hemolytic anemia suggests the diagnosis of congenital dyserythropoietic anemia. (See "Overview of causes of anemia in children due to decreased red blood cell production", section on 'Congenital dyserythropoietic anemia'.)
Howell-Jolly bodies — Howell-Jolly bodies are nuclear remnants within red cells that are ordinarily removed by the spleen. They are usually single, round, dark purple-red in color and peripheral in location. The presence of Howell-Jolly bodies most frequently indicates either absence of the spleen (eg, surgical removal) (picture 49)  or splenic hypofunction (eg, sickle cell disease) (picture 40). (See "Evaluation of splenomegaly and other splenic disorders in adults", section on 'Asplenia or hyposplenia'.)
Heinz bodies — Heinz bodies, which are aggregates of denatured hemoglobin, are not normally present in red cells. If present, they cannot be seen on routine staining, but become obvious following use of a supravital dye such as crystal violet (picture 50). They are most commonly found in people with glucose-6-phosphate dehydrogenase (G6PD) deficiency, especially following exposure to oxidant compounds. The can also be seen in people with thalassemia and certain unstable hemoglobin variants. (See "Drug-induced hemolytic anemia", section on 'Oxidant injury'.)
Basophilic stippling — Basophilic stippling refers to the presence of blue granules of various sizes dispersed throughout the cytoplasm of the red cell, which represent ribosomal precipitates. They are most often seen in the thalassemias, alcohol abuse, lead and heavy metal poisoning (picture 51), and the rare condition hereditary pyrimidine 5'-nucleotidase deficiency (picture 52) .
Pappenheimer bodies — Pappenheimer bodies are iron-containing dark blue granules found in red cells in patients with sideroblastic anemia. The red cells are usually hypochromic, with basophilic stippling that stains positive for iron (picture 53). Red cells containing Pappenheimer bodies are called siderocytes in contrast to iron-containing nucleated red cells, which are called sideroblasts. (See "Sideroblastic anemias: Diagnosis and management".)
Red blood cell parasites — Red blood cell parasites such as babesiosis (picture 54 and picture 55) and malaria (picture 56 and figure 3) are often detected only by well-trained hematology technicians or clinicians who are specifically looking for RBC inclusions in a patient with unexplained hemolytic anemia. (See "Non-immune (Coombs-negative) hemolytic anemias in adults".)
Hemoglobin crystals — Hemoglobin crystals are occasionally seen in hemoglobin C disease or hemoglobin SC disease, especially if the blood sample has become slightly dehydrated before the peripheral smear is made. The crystals are often hexagonal or rhomboid in shape (picture 57). (See "Hemoglobin variants including Hb C, Hb D, and Hb E", section on 'Hb C'.)
Red cell ghosts — Red cell ghosts, which are red cell membranes devoid of hemoglobin, are never normally seen in the peripheral blood. They are red cells which have undergone intravascular lysis, with leakage of their hemoglobin content into the plasma (picture 58). Red cell ghosts may be seen in certain fulminant bacterial infections; the most common organism associated with this finding is Clostridium perfringens. (See "Non-immune (Coombs-negative) hemolytic anemias in adults", section on 'Infections (RBC parasites and intracellular bacteria)'.)
Cabot rings — Cabot rings are red cell inclusions appearing as fine, purple filamentous loops or "figure of eight" forms . Their precise origin is not well understood, although they might be remnants from the mitotic spindle [24,25]. They have been described in a number of settings such as megaloblastic anemia, severe anemia, lead poisoning, and leukemia.
WORRISOME FINDINGS — Certain abnormalities should never be found on the normal peripheral smear and always signify a pathologic process:
Blasts or tumor cells — It is normal to identify a range of early white cells in pregnancy or during a leukemoid reaction. However, it is never normal to see blast forms (eg, lymphoblasts, myeloblasts) on the peripheral smear. Further evaluation of such patients (eg, review of the peripheral smear, hematologic consultation, bone marrow examination) is warranted.
The presence of myeloblasts, which are immature cells with large nuclei, nucleoli, and a scant rim of dark blue cytoplasm, suggests an underlying malignant hematologic disorder (picture 20). Other circulating cells which suggest the presence of lymphoma or leukemia include the following:
●Cells with Auer rods (a rod-like conglomeration of granules in the cytoplasm) within a blast cell are pathognomonic of acute myeloid leukemia (picture 59). (See "Clinical manifestations, pathologic features, and diagnosis of acute myeloid leukemia", section on 'Diagnosis'.)
●Small lymphoid cells with cleaved nuclei (small cleaved B-cells, centrocytes) may be seen in the circulation in patients with follicular lymphoma (picture 60). (See "Clinical manifestations, pathologic features, diagnosis, and prognosis of follicular lymphoma".)
●Lymphoid cells with bipolar villous projections (picture 61) may be seen in patients with splenic marginal zone lymphoma. (See "Splenic marginal zone lymphoma".)
●Lymphoid cells with ragged or "hairy" cytoplasm (picture 62) may be seen in hairy cell leukemia. (See "Clinical features and diagnosis of hairy cell leukemia".)
●Lymphoid cells with hyperlobulated nuclei (clover leaf or flower cells) (picture 3) may be seen in patients with adult T-cell leukemia/lymphoma. (See "Clinical manifestations, pathologic features, and diagnosis of adult T cell leukemia-lymphoma".)
●Atypical lymphoid cells with "cerebriform" nuclei (Sézary cells) may be seen in the circulation of patients with cutaneous T-cell lymphoma (picture 63). (See "Clinical manifestations, pathologic features, and diagnosis of mycosis fungoides", section on 'Pathology'.)
Leukoerythroblastic smear — The combined presence of tear drop-shaped red cells (picture 13), circulating nucleated red cells, and early white cells (picture 48) suggest a myelophthisic process in the bone marrow. This outpouring of immature forms (leukoerythroblastic reaction) usually results from marrow fibrosis and/or invasion, which is either idiopathic (eg, primary myelofibrosis) or reactive to conditions such as cancer metastatic to the bone marrow.
Plasma cells — Plasma cells are normally not found on the peripheral blood smear. They appear as lymphocytes and are counted as such, appearing with a distinct clear perinuclear region that contains large numbers of Golgi bodies.
Circulating plasma cells in the peripheral blood can be seen in multiple myeloma and rarely in primary systemic amyloidosis [26,27]. Plasma cell leukemia, a rare phenomenon, is defined by an absolute plasma cell count in the peripheral blood of >2000/microL (>2 x 109 /L) or >20 percent of WBCs. (See "Plasma cell leukemia".)
Circulating plasma cells may also be seen in other hematologic malignancies (multiple myeloma and non-Hodgkin lymphoma). (See "Multiple myeloma: Clinical features, laboratory manifestations, and diagnosis" and "Clinical presentation and initial evaluation of non-Hodgkin lymphoma".)
Reactive plasma cells may mimic plasma cell leukemia and can be seen in a variety of infections and inflammatory diseases .
When plasma cell are seen in the peripheral blood, immunologic studies and flow cytometry will help with the differential. In plasma cell leukemia and multiple myeloma, plasma cells will be monoclonal, whereas reactive plasma cells will be polyclonal. (See "Approach to the adult with lymphocytosis or lymphocytopenia", section on 'Assessment of clonality'.)
Inclusions within neutrophils and monocytes
Organisms — On occasion, and especially in patients with overwhelming sepsis, circulating neutrophils or monocytes may be found to contain the invading organism.
The following types of organisms may be observed:
●Bacteria, including ehrlichia, anaplasma (picture 64), or others [29-34]. (See "Human ehrlichiosis and anaplasmosis", section on 'Blood smears'.)
●Fungi such as histoplasma (picture 65) [35,36].
●Parasites, both extracellular (trypanosomes, microfilaria) and intracellular (eg, malaria, babesia) [32,37]. (See "Human African trypanosomiasis: Epidemiology, clinical manifestations, and diagnosis", section on 'Blood examination' and "Lymphatic filariasis: Epidemiology, clinical manifestations, and diagnosis", section on 'Blood smears'.)
When certain types of bacteremia or parasitemia is massive, organisms may also be found on the peripheral smear outside of cells (picture 66 and picture 67).
The sensitivity of the peripheral smear for detecting such organisms is low. In one study, detection of candidemia by peripheral blood smear examination required a yeast concentration of 1 to 5 x 105 CFU/mL or greater; this degree of fungemia is unusual . Sensitivity of smear review for yeast detection was greatly increased when the microscopist was specifically directed to look for the presence of yeast.
Pigmented inclusions — Pigmented inclusions, composed of hemoglobin incompletely digested by plasmodial organisms (hemozoin, malarial pigment), may be found in circulating neutrophils and monocytes in patients with severe degrees of malaria (eg, severe anemia, cerebral malaria) (picture 68) . (See "Laboratory tools for diagnosis of malaria", section on 'Blood smear interpretation'.)
Bright green inclusions — Bright green cytoplasmic inclusions in neutrophils or monocytes have occasionally been documented in patients with severe illness (sepsis, liver failure) [40,41].
●These inclusions have been reported in individuals infected with SARS-CoV-2 during the coronavirus 2019 (COVID-19) pandemic. One series described six individuals with blue-green inclusions on the blood smear, all of whom died within days of the finding .
●A case report described an individual who died of sepsis and had bright green cytoplasmic inclusions (picture 69) noted two days before he died . The inclusions tested negative on special stains for iron, bilirubin, and myeloperoxidase.
●In a series of 20 patients in whom bright green inclusions were documented in neutrophils or monocytes, 13 (65 percent) died within a few days of the finding . All but one had elevated hepatic transaminases. The term "critical green inclusions" was proposed as a name for this finding.
Nuclear material — Neutrophils which have ingested nuclear material can be sometimes be found in the peripheral blood of patients with systemic lupus erythematosus (SLE; the "LE cell") (picture 70). These cells can also be seen on occasion in the bone marrow or in body fluids of patients with SLE . (See "Clinical manifestations and diagnosis of systemic lupus erythematosus in adults", section on 'Classification criteria'.)
Howell-Jolly-like inclusions can be seen in neutrophils in a number of settings, such as viral infection and the use of immunosuppressive agents or chemotherapy [45,46]. (See 'Howell-Jolly bodies' above.)
Schistocytes — Schistocytes (see 'Permutation in shape' above) are suggestive of microangiopathic hemolysis, which may be caused by a life-threatening condition such as thrombotic thrombocytopenia purpura (TTP) or other thrombotic microangiopathy or serious systemic condition such as sepsis with disseminated intravascular coagulation (DIC). (See "Diagnostic approach to suspected TTP, HUS, or other thrombotic microangiopathy (TMA)".)
Smudge cells — Upon examination of the peripheral blood smear in patients with chronic lymphocytic leukemia (CLL), mature-appearing small lymphocytes may account for 50 to 100 percent of the leukocytes. One may also see lymphocytes which appear flattened or smudged in the process of being spread on the glass slide. Such "smudge" cells, reflecting fragility or vulnerability to distortion of B-CLL cells upon mechanical manipulation, are considered characteristic of CLL (picture 71). (See "Clinical features and diagnosis of chronic lymphocytic leukemia/small lymphocytic lymphoma", section on 'Peripheral smear'.)
SUMMARY AND RECOMMENDATIONS
●Uses – Examination of the peripheral blood smear provides a window into the functional status of the bone marrow. The peripheral smear is indispensable for evaluating the number, size, and shape of red cells, white cells, and platelets. It is particularly important when assessing cytopenic states (eg, anemia, leukopenia, thrombocytopenia). In some cases, the peripheral smear alone is sufficient to establish a reliable diagnosis. (See 'Introduction' above.)
●Preparation – The peripheral smear should be made using a clean, oil-free slide and the optimal portion of the slide should be viewed (figure 1). Irregular distribution of red cells should be evaluated, if present. (See 'Initial approach' above.)
●Red blood cells – Review the color, shape, irregular borders, and presence of nuclei, inclusions, or parasites. (See 'Red blood cells' above and 'Red cell abnormalities' above and 'Schistocytes' above.)
●White blood cells – Review the presence of early (immature) cells, abnormal lobulation or nuclear contour, abnormal or absent granulation, presence of parasites or other inclusions. (See 'White blood cells' above and 'Worrisome findings' above.)
●Platelets – Review any increase or decrease in platelet number and size, absence of granules, presence of platelet aggregation, megakaryocyte fragments. (See 'Platelets' above.)
ACKNOWLEDGMENT — 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.
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