Evaluation of the peripheral blood smear

INTRODUCTION — 

Examination of the peripheral blood smear is an inexpensive but powerful diagnostic tool for identifying a variety of medical conditions, especially hematologic and infectious disorders. It is becoming a "lost art," but it often provides rapid, reliable access to key information.

This topic reviews preparation and evaluation of the peripheral blood smear, with information about common findings and findings of concern.

Evaluation of bone marrow aspirate smears is discussed separately. (See "Evaluation of bone marrow aspirate smears".)

WHEN IS A BLOOD SMEAR NEEDED? — 

The blood smear offers a window into the functional status of the bone marrow where blood elements are produced. It is particularly important when assessing the following findings [1]:

●Cytopenias (anemia, leukopenia, thrombocytopenia)

●Leukemias

●Certain infections (mononucleosis, babesiosis, malaria)

●Thrombotic microangiopathies (thrombotic thrombocytopenic purpura [TTP] and others)

●Hemolytic anemias

●Inherited platelet disorders

A blood smear should generally be accompanied by a complete blood count (CBC) with differential, as quantitative information like severity of anemia is more effectively captured by automated cell counters than manual review. In contrast, many CBCs do not require a corresponding blood smear.

There are a number of settings in which interpretation of the peripheral smear is especially important; in some cases, the peripheral smear alone is sufficient to establish a diagnosis [2].

Review of the peripheral smear is not required in all patients with a hematologic disorder. Certain straightforward conditions such as iron deficiency anemia can be diagnosed from clinical information and basic laboratory data such as serum ferritin alone.

Automated machines can provide sophisticated data about blood counts and morphology. However, unlike an expert in blood smear review, they tend to include a wide array of morphologic abnormalities. Only an experienced reviewer who is familiar with the patient's clinical status can weigh the relative significance of blood smear 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 complete blood count (CBC)".)

Use of digital blood smears is also increasing. (See 'Digital blood smears' below.)

TECHNICAL DETAILS

Slide preparation

●Technique – In most cases, peripheral blood smears are prepared using a wedge technique, either manually or via an automated procedure. Smears should be made from a small drop of blood that has not been allowed to clot and, if made from an anticoagulated blood sample that may have been allowed to settle, that has been completely mixed.

The slide should be clean and free from dust, dirt, grease, and fingerprints. These contaminants may be suspected when a large number of target cells or stomatocytes is present only in localized areas on the smear [3]. Repeating the procedure with alcohol-cleaned slides corrects this problem.

Dirt or precipitates in the stain may also make interpretation more challenging. (See 'Extracellular microorganisms and precipitates' below.)

●Staining – An unstained blood smear provides limited utility in identifying subtle cellular features. The most commonly used stains are Romanowsky-type dyes including combinations of those developed by Wright and Giemsa in the early 1900s [4].

These stains produce the characteristic appearance of peripheral blood smears where shades of pink, red, purple, and blue are seen because of the Romanowsky effect, whereby a blue cationic dye and red anionic dye combine to produce more than two colors. Ideal staining time and drying time vary by dye manufacturer [5].

Romanowsky-type dyes have proven to be the most versatile, but alternative dyes are sometimes used to highlight a specific feature, such as use of crystal violet dye to identify denatured hemoglobin (Heinz bodies), Prussian blue or Perls stain to identify iron-containing granules, or methylene blue dye to identify reticulocytes. (See 'RBC inclusions' below and 'Reticulocytes and nucleated RBCs' below.)

Optimal area for review — All slides have regions that are too thick at one end and too thin at the other end, both of which can interfere with an accurate assessment of morphology; the intermediate area near the center is best for optimal review (figure 1):

●Too thick – One end of smear is too thick; stacks or clusters of red blood cells (RBCs) in this area cause the RBCs to appear small and dark (picture 1). This area of the slide may be useful when searching for the presence of malarial parasites.

●Too thin – The other end of the slide (the feathered edge) will be spread too thin; RBCs assume a "brick-like" or "cobblestone" type of pattern in this area (picture 2). RBCs 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).

●Just right – In the optimal area, RBCs will be evenly spaced, and central pallor will be appreciated. It should be rare to see two or more RBCs abutting (picture 4).

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). (See "Automated 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 [6].

Image analysis software that relies on deep learning methods and pattern recognition can then analyze the images and identify specific abnormal findings, which in turn can be provided to a hematologist for further review. Commercially available products require significant human reclassification [7,8]; however, the field is rapidly developing [9].

INITIAL LOW POWER 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. (See 'Optimal area for review' above.)

There are several blood smear patterns that are best initially appreciated at low power because they affect more than one cell line and/or have alterations in the percentages of different types of cells. Some typical patterns are discussed below.

While relative increases or decreases in numbers of white blood cells (WBCs) and platelets can be appreciated from a low power review, the hemoglobin and hematocrit cannot be determined from the blood smear.

Leukoerythroblastic smear — A leukoerythroblastic blood smear (picture 5) refers to a pattern with teardrop-shaped red blood cells (RBCs) (picture 6), circulating nucleated RBCs, and immature WBCs.

A leukoerythroblastic picture suggests a myelophthisic (marrow replacing) process in the bone marrow. It can be due to a primary process such as primary myelofibrosis or can occur secondary to conditions such as cancer metastatic to the bone marrow.

Teardrop-shaped RBCs and nucleated RBCs can also be seen in hemolytic anemias, megaloblastic anemias, and severe thalassemias, although other characteristic abnormalities of those conditions would typically be seen as well [10-13].

A small number of teardrop-shared RBCs may be present in healthy individuals, but they typically represent <0.5 percent of RBCs [14].

Megaloblastic smear — A megaloblastic smear refers to the finding of macrocytic RBCs, with some taking an oval shape (macro-ovalocytes) (picture 7). These changes may be accompanied by other changes such as hypersegmented neutrophils (neutrophils with six or more segments) (picture 8).

These findings can be seen in vitamin B12 and folate deficiencies as well as with medications that inhibit DNA replication. (See "Macrocytosis/Macrocytic anemia", section on 'Megaloblastic anemia'.)

Left shift — Left shift refers to an increased absolute number of immature neutrophils with a high percentage of neutrophilic band forms; "bandemia" is another name for this finding.

A left shift is most often associated with infection. (See "Approach to the patient with neutrophilia", section on 'Neutrophil abnormalities'.)

Less mature neutrophils such as metamyelocytes, and rarely myelocytes, may be seen during infections, pregnancy, leukemoid reactions, and recovery from myelosuppression. (See 'Normal neutrophils, eosinophils, basophils, and monocytes' below.)

In contrast, the most immature forms (promyelocytes and myeloblasts) in the peripheral blood are almost always due to a hematologic malignancy. (See 'Blasts or tumor cells' below.)

A greater percent of myelocytes than metamyelocytes on the WBC differential (referred to as a "leukemic hiatus" or "myelocyte bulge") is strongly suspicious for chronic myeloid leukemia (CML) [15]. Simultaneous occurrence of leukocytosis with left shift plus eosinophilia (increase in absolute eosinophil count) is also strongly suspicious for CML [15]. (See "Chronic myeloid leukemia: Pathogenesis, clinical manifestations, and diagnosis", section on 'Peripheral blood'.)

RBC agglutination and rouleaux formation — Circulating immunoglobulins can bind to RBCs and cause them to agglutinate or form stacks known as rouleaux.

●Agglutination – Agglutination (picture 9) can occur when an IgM autoantibody is present. IgM causes agglutination because it is pentameric and can bind up to five RBCs simultaneously. This may signify the presence of cold agglutinins, as seen following certain infections and in cold agglutinin disease. (See "Cold agglutinin disease".)

●Rouleaux formation – Rouleaux are collections of RBCs that are stacked together in series (picture 10). The most common cause is multiple myeloma, in which high levels of immunoglobulins are responsible; other causes may include increased fibrinogen, other monoclonal gammopathies, or polyclonal gammopathies [16]. (See "Multiple myeloma: Clinical features, laboratory manifestations, and diagnosis", section on 'Peripheral smear'.)

An otherwise well-prepared smear that has agglutinated RBCs, rouleaux formation, and large, clear areas with circular gaps between RBCs may occur when a surfactant or oil-based material is present in the circulation. One example is polyoxyethylated castor oil (Cremophor), a nonionic surfactant used for solubilization of hydrophobic agents such as anesthetic agents, sedatives, immunosuppressive medications, sensitizers, antifungal agents, and chemotherapeutic agents such as paclitaxel [17]. Chemoembolization material containing a viscous oil together with a suspension of microparticles can also create this appearance [18].

Extracellular microorganisms and precipitates — It may be challenging to distinguish microorganisms from other material.

●Dirt or precipitates from the stain – This is the most common abnormality. These slide preparation artifacts are often distributed haphazardly inside and outside of cells with a dark blue appearance. They may be out of focus from the cells. A freshly prepared smear may be helpful if there is uncertainty about whether these are microorganisms or artifacts.

●Parasites – Extracellular parasites that can be seen on a blood smear include trypanosomes and microfilaria [19]. (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'.)

●Massive infection – When certain types of bacteremia or parasitemia are massive, organisms may also be found on the peripheral smear outside of cells (picture 11 and picture 12) [20].

●Amorphous or crystalline material – Crystalline-appearing material may be due to precipitated cryoglobulins, as might occur in patients with chronic hepatitis C virus (HCV) infection [21]. This can result in white blood cell (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 WBCs and/or platelets in automated cell counters. (See "Approach to the patient with thrombocytosis", section on 'Blood smear'.)

●Lipid droplets surrounding RBCs – Small lipid droplets overlying and surrounding the periphery of RBCs, along with lipemic (milky) plasma (picture 13), may be seen with hypertriglyceridemia [22].

Reticulocytes and nucleated RBCs — Reticulocytes are young RBCs that have just extruded their nuclei and contain some remaining polyribosomes. Reticulocytes normally comprise approximately 1 percent of RBCs. Reticulocytes are slightly larger than mature RBCs, and the polyribosomes give them a bluish tint on standard Wright Giemsa staining (picture 14).

Supravital dyes such as new methylene blue are used to better visualize the reticular network of polyribosomes (picture 15) and to produce a reticulocyte count.

An increased reticulocyte percentage (or increased absolute reticulocyte count) occurs when RBC production is increased, as in response to bleeding or hemolysis (picture 16) or after treatment of a deficiency such as iron, vitamin B12, or folate. (See "Diagnostic approach to anemia in adults", section on 'Reticulocyte production'.)

A decreased reticulocyte percentage (or decreased absolute reticulocyte count) occurs when the bone marrow is suppressed, such as in aplastic anemia, pure red cell aplasia (PRCA), or following systemic chemotherapy. (See "Acquired aplastic anemia: Pathogenesis, clinical manifestations, and diagnosis" and "Acquired pure red cell aplasia in adults".)

Nucleated RBCs (normoblasts) are not normally seen in the peripheral blood. When present, they usually indicate massively stimulated erythropoiesis, such as with severe hemolysis (picture 17 and picture 18), profound stress or hypoxemia [23], or a myelophthisic condition such as myelofibrosis (picture 5). (See 'Leukoerythroblastic smear' above.)

Binucleated normoblasts on the smear in a patient with congenital hemolytic anemia suggest congenital dyserythropoietic anemia (CDA). (See "Overview of causes of anemia in children due to decreased red blood cell production", section on 'Congenital dyserythropoietic anemia'.)

RED BLOOD CELLS — 

Red blood cells (RBCs, erythrocytes) are the most numerous cells on the peripheral smear. Morphologic examination should include assessment of size, shape, color (pallor), and any inclusions.

●Some conditions cause the same uniform abnormality in most or all RBCs, such as in hereditary spherocytosis or hereditary stomatocytosis.

●In other conditions, only a small subset of RBCs show the abnormality, and careful review of multiple RBCs is required. Examples include Howell-Jolly bodies in functional or anatomic splenectomy, intracellular parasites in malaria or babesiosis, or schistocytes in thrombotic thrombocytopenic purpura (TTP).

Normal RBC size, shape, and color

●Size – Normal RBCs have a diameter of 7 to 8 microns (approximately the size of a lymphocyte nucleus) (picture 4).

Automated hematology instruments confirm the actual size, providing a numerical value in the form of the mean corpuscular volume (MCV), which is normally approximately 90 femtoliters (fL).

Causes of microcytosis (low MCV) and macrocytosis (high MCV) are discussed separately.

•Microcytosis – (See "Microcytosis/Microcytic anemia".)

•Macrocytosis – (See "Macrocytosis/Macrocytic anemia".)

The MCV is an average, and it can be misleading if there is a dimorphic population of microcytic (small) and macrocytic (large) cells, since the average may be normal. The red cell distribution width (RDW) reflects the degree of variation in RBC size.

●Shape – RBCs normally have a smooth contour and appear round in two dimensions, with an area of central pallor (picture 4). The area of central pallor generally takes up approximately one-third of the cell. This is due to the biconcave shape in the third dimension.

Abnormal RBC shapes can occur in numerous conditions; review of the blood smear may be the first or best indication of one of these conditions. (See 'RBC size and shape abnormalities' below and 'Schistocytes' below.)

RBC size and shape abnormalities — Abnormal RBC size and/or shape, with or without anemia, may be an important clue to an underlying condition.

●Microcytosis – Microcytosis has a limited differential diagnosis that includes iron deficiency, thalassemia, anemia of chronic disease/anemia of inflammation (ACD/AI), and some sideroblastic anemias (algorithm 1).

Microcytosis is often accompanied by hypochromia (decreased hemoglobin content causing a larger area of central pallor) (picture 19). (see "Microcytosis/Microcytic anemia").

Microcytosis in a child can also 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. Iron deficiency is suggested by variation in cell size and shape (reflected by an increased RDW) and characteristic "pencil cells"; in contrast, cells of a more uniform size with increased numbers of target cells and teardrop cells is characteristic of thalassemia trait. (See "Diagnosis of iron deficiency and iron deficiency anemia in adults" and "Microcytosis/Microcytic anemia".)

●Macrocytosis – Macrocytosis has a broad differential diagnosis that includes several conditions (see "Macrocytosis/Macrocytic anemia"):

•Reticulocytosis – Reticulocytes are larger than the normal RBCs and have a bluish tinge (polychromatophilia) (picture 14). Reticulocytosis can occur in any condition with increased erythropoiesis. (See 'Reticulocytes and nucleated RBCs' above.)

•Megaloblastic changes – Megaloblastic changes are seen with folate or vitamin B12 deficiency (picture 7) as well as drugs that inhibit DNA synthesis. Abnormal RBC morphologies include large, oval shaped RBCs known as macroovalocytes. (See 'Megaloblastic smear' above.)

•Liver disease – Liver disease can cause macrocytosis due to increased RBC membrane. Other morphologic changes such as target cells may also be seen. (See "Burr cells, acanthocytes, and target cells: Disorders of red blood cell membrane", section on 'RBC changes in liver disease'.)

•MDS – Myelodysplastic syndromes (MDS) can cause macrocytic anemia due to dysplastic RBC production in the bone marrow. Anemia and other cytopenias may be present. (See "Clinical manifestations, diagnosis, and classification of myelodysplastic syndromes (MDS)", section on 'Blood smear'.)

●Loss of central pallor – Loss of central pallor is seen when RBCs become spherocytic, such as in hereditary spherocytosis or autoimmune hemolytic anemia (picture 20). Apparent loss of central pallor can also be seen at the feather edge of the blood smear; this is an artifact.

●High RDW – A high RDW suggests larger than normal variation in RBC size or two populations of RBCs with different sizes. (See "Microcytosis/Microcytic anemia", section on 'RDW (size variability)' and "Automated complete blood count (CBC)", section on 'MCV and RDW' and "Automated complete blood count (CBC)", section on 'RBC indices'.)

●Poikilocytosis – Poikilocytosis (variability of RBC shapes) has different implications depending on the specific shapes observed. Examples include:

•Elliptocytes – A high percentage of oval or elliptical cells (ovalocytes, elliptocytes) is characteristic of a number of inherited abnormalities such as hereditary elliptocytosis (picture 21). (See "Hereditary elliptocytosis and related disorders", section on 'Clinical syndromes'.)

Acquired elliptocytosis (picture 22) is occasionally seen in myelodysplastic syndromes (MDS), most often with a deletion of 20q [24].

•Mushroom-shaped cells (also called fish-shaped cells or pincer 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 RBCs on the blood smear (picture 23) [25]. This could suggest that the virus may have a significant impact on RBC physiology, with a role for oxidative stress.

Similar abnormalities have been reported in a wide range of other conditions including iron deficiency, vitamin B12 or folate deficiency, liver disease, hereditary elliptocytosis, MDS, myeloproliferative neoplasms, and acute leukemia [26,27].

•Bite cells – Bite cells (picture 24) are caused by phagocytosis of precipitates of denatured hemoglobin (Heinz bodies); these cells may be the earliest clue to the pathogenesis of a hemolytic anemia due to oxidant sensitivity, such as glucose-6-phosphate dehydrogenase (G6PD) deficiency. (See "Glucose-6-phosphate dehydrogenase (G6PD) deficiency", section on 'Typical presentation and blood smear findings'.)

•Sickle cells – Sickle cells are unique in their curved shape (picture 17). In patients with hemoglobin SC disease, the cells are only partially sickled, giving them the shape of a canoe or pita bread (picture 25). (See "Diagnosis of sickle cell disorders", section on 'Findings in sickle cell anemia'.)

•Target cells – Target cells have a bull's eye appearance due to an increase in the surface-to-volume ratio (picture 26). Often this is caused by increased membrane due to liver disease (particularly obstructive liver disease). Target cells can also be seen in hemoglobinopathies such as thalassemias, Hb C, Hb D, and Hb E disease, and post-splenectomy. (See "Burr cells, acanthocytes, and target cells: Disorders of red blood cell membrane", section on 'Target cells'.)

•Projections – Spiculated RBCs have an irregular outline. 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'.)

-RBCs with similarly sized, regularly spaced projections are called echinocytes, burr cells, or crenated cells. These are seen most commonly in uremia or as an artifact of preparation (picture 27).

-RBCs with irregularly sized and spaced projections are called acanthocytes (extreme form: spur cells). These are seen most commonly in liver disease (picture 28). (See "Burr cells, acanthocytes, and target cells: Disorders of red blood cell membrane", section on 'Burr cells and acanthocytes'.)

•Tear drop cells – Teardrop-shaped RBCs are commonly found in patients with extramedullary hematopoiesis (eg, primary myelofibrosis, (picture 6)) as well as in the thalassemias.

•RBC ghosts – These are RBC membranes devoid of hemoglobin (picture 29). This occurs when intravascular lysis causes leakage of their hemoglobin into the plasma. RBC ghosts may be seen in fulminant bacterial infections, most commonly with Clostridium perfringens. (See "Non-immune (Coombs-negative) hemolytic anemias in adults", section on 'Infections (RBC parasites and intracellular bacteria)'.)

Schistocytes — Schistocytes refer to fragmented RBCs (picture 30) or in some cases helmet cells (picture 31). The finding may be subtle. (See "Non-immune (Coombs-negative) hemolytic anemias in adults", section on 'Fragmentation'.)

●Fragmentation can be mechanical, such as in march hemolysis or with a mechanical heart valve (picture 32).

●More concerning is microangiopathic hemolysis or microangiopathic hemolytic anemia (MAHA), in which microthrombi in small capillaries and venules causes shearing of RBCs (picture 33). This can occur in life-threatening conditions such as thrombotic thrombocytopenic purpura (TTP), complement-mediated thrombotic microangiopathy (CM-TMA), other thrombotic microangiopathies (TMAs), or other serious systemic disorders such as sepsis with disseminated intravascular coagulation (DIC). (See "Diagnostic approach to suspected TTP, HUS, or other thrombotic microangiopathy (TMA)", section on 'Key distinguishing features among different TMA syndromes'.)

Thrombocytopenia plus schistocytes due to microangiopathic hemolysis necessitates immediate hospitalization and hematology evaluation. (See "Diagnostic approach to suspected TTP, HUS, or other thrombotic microangiopathy (TMA)", section on 'Immediate management decisions'.)

Schistocytes usually represent <0.5 percent to <1 percent of RBCs in healthy individuals [28].

The highest levels of schistocytes are seen in TMAs, but schistocytes may also be present at lower levels (usually <3 percent of RBCs) in conditions not classically associated with TMA including:

●Post-splenectomy [29]

●Vitamin B12 deficiency [12]

●Severe burns [30,31]

●Advanced cancer [32]

●Advanced chronic kidney disease [33]

●Sepsis [33]

RBC inclusions

Howell-Jolly, Heinz, and Pappenheimer bodies — These three findings are similar in that they appear as small round intracellular inclusions of a similar size. They differ in composition and clinical implications; the table summarizes their different features (figure 2).

●Howell-Jolly bodies – These are nuclear remnants that are ordinarily removed by the spleen. They are usually single, round, dark purple-red in color and peripheral in location. Howell-Jolly bodies most frequently indicated absence of the spleen (eg, post-splenectomy) (picture 34) or functional asplenia, such as seen in sickle cell disease (picture 17) [34]. (See "Splenomegaly and other splenic disorders in adults", section on 'Asplenia or hyposplenia'.)

●Heinz bodies – These are aggregates of denatured hemoglobin. They are not normally present in RBCs. If present, they cannot be seen on routine staining but become obvious when cells are stained with a supravital dye such as crystal violet (picture 35). They are most commonly found in G6PD deficiency, especially following exposure to oxidant compounds. They can also be seen in thalassemia and certain unstable hemoglobin variants. (See "Drug-induced hemolytic anemia", section on 'Oxidant injury'.)

●Pappenheimer bodies – These inclusions (picture 36) are iron-containing dark blue granules found in RBCs in patients with sideroblastic anemia, excess alcohol, or lead poisoning. The RBCs are usually hypochromic, with basophilic stippling that stains positive for iron using Prussian blue or Perls stain (picture 37). RBCs containing Pappenheimer bodies are called siderocytes, in contrast to iron-containing nucleated RBCs, which are called sideroblasts. (See "Sideroblastic anemias: Diagnosis and management".)

Hemoglobin crystals — Hemoglobin crystals are occasionally seen in Hb C disease or Hb 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 38). (See "Hemoglobin variants including Hb C, Hb D, and Hb E", section on 'Hb C'.)

Basophilic stippling — Basophilic stippling (picture 39) refers to blue granules of various sizes dispersed throughout the RBC cytoplasm; they are precipitated ribosomes. They are most often seen in thalassemia, excess alcohol use, lead and heavy metal poisoning, and the rare condition hereditary pyrimidine 5'-nucleotidase deficiency [35]. (See "Rare RBC enzyme disorders", section on 'Pyrimidine 5' nucleotidase (P5'N) deficiency'.)

Intracellular parasites — Red blood cell parasites 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 or fever with an appropriate travel history. (See "Non-immune (Coombs-negative) hemolytic anemias in adults".)

Intracellular RBC parasites include babesiosis and malaria:

●Babesiosis – Babesia is an intracellular parasite (picture 40 and picture 41) that can cause fever, hemolytic anemia and other cytopenias. It is endemic in regions of the United States in the Northeast and Upper Midwest, as well as regions of Europe, China, and other regions. (See "Babesiosis: Clinical manifestations and diagnosis" and "Babesiosis: Microbiology, epidemiology, and pathogenesis", section on 'Epidemiology'.)

●Malaria – Malaria is an intracellular parasite (picture 42 and figure 3) that can cause fever, hemolytic anemia, and other cytopenias. It is endemic in Africa, where most infections occur; infections also occur in Southeast Asia and some Mediterranean locations. (See "Malaria: Clinical manifestations and diagnosis in nonpregnant adults and children" and "Malaria: Epidemiology, prevention, and elimination".)

●Parasite mimics

•Hemoglobin Southampton (Casper) is a rare hemoglobinopathy caused by a point mutation in the HBB gene (c.320T>C, p.L107P). The RBC inclusions (picture 43) caused by the unstable hemoglobin look similar to intracellular parasites [36].

•Platelets overlying RBCs can give the false impression of intracellular parasites. Comparing the appearance to surrounding platelets can be helpful.

Cabot rings — Cabot rings (picture 44) are RBC inclusions appearing as fine, purple filamentous loops or "figure of eight" forms [37]. Their precise origin is not well understood, although they might be remnants from the mitotic spindle [38,39]. They have been described in a number of settings such as megaloblastic anemia, severe anemia, lead poisoning, and leukemia.

WHITE BLOOD CELLS — 

A normal peripheral smear contains a spectrum of mature leukocytes (white blood cells [WBCs]) including lymphocytes, neutrophils, and monocytes.

Normal lymphocytes — Small lymphocytes comprise about 30 to 40 percent of the circulating WBCs. They are identified by clumped nuclear chromatin and a scant rim of deep blue cytoplasm (picture 45).

Large granular lymphocytes (LGLs) are 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 blood cells (RBCs), with abundant cytoplasm, a round to oval nucleus, and a small number of azurophilic cytoplasmic granules (picture 46). (See "Clinical manifestations, pathologic features, and diagnosis of T cell large granular lymphocyte leukemia", section on 'Morphology'.)

Some lymphocyte forms increase after infections.

●Atypical lymphocytes with a more generous and malleable cytoplasm, often indented by surrounding RBCs, can be seen following viral infections such as infectious mononucleosis (picture 47). (See "Approach to the child with lymphocytosis or lymphocytopenia", section on 'Infectious mononucleosis'.)

●Lymphocytosis is commonly seen following Bordetella pertussis infection, with mature-appearing lymphocytes with scant cytoplasm, condensed chromatin, and clefted nuclei [40,41]. (See "Approach to the child with lymphocytosis or lymphocytopenia", section on 'Pertussis'.)

Lymphocyte abnormalities — Numerical lymphocyte abnormalities and evaluation for clonality are discussed separately. (See "Approach to the child with lymphocytosis or lymphocytopenia" and "Approach to the adult with lymphocytosis or lymphocytopenia".)

Smudge cells — Smudge cells are mature-appearing small lymphocytes that appear to have been flattened or smudged in the process of being spread on the glass slide (picture 48). These cells form due to fragility or vulnerability to distortion upon mechanical manipulation.

Smudge cells usually represent <2 percent of cells in a smear [42]. However, a slightly larger percentage of cells may smudge if a smear is prepared from blood that has been sitting for 12 hours or more [43]. Preparing a smear from a fresh blood sample is preferred where possible.

The likely cause differs depending on patient age (algorithm 2).

●Older adults – Smudge cells are considered characteristic of chronic lymphocytic leukemia (CLL). The blood smear in CLL generally has an abundance of mature-appearing small lymphocytes and smudge cells. Virtually all individuals with CLL have some degree of smudge cells; the percentage may range from 1 to 80 percent (median 25 to 30 percent) [44-46]. (See "Clinical features and diagnosis of chronic lymphocytic leukemia/small lymphocytic lymphoma", section on 'Peripheral smear'.)

In an older adult with persistent lymphocytosis and smudge cells, the diagnosis of CLL is likely. Flow cytometry should be performed to confirm the diagnosis, since other lymphoproliferative disorders including mantle cell lymphoma may also have smudge cells, although typically to a lower degree [47]. (See "Clinical features and diagnosis of chronic lymphocytic leukemia/small lymphocytic lymphoma", section on 'Immunophenotype'.)

●In contrast, in an individual under age 30 years, in whom CLL is exceedingly rare, the most common cause of smudge cells is infectious mononucleosis. Other upper respiratory tract infections may transiently have a small percentage of smudge cells (usually <10 percent), but the percentage can be greater than 50 percent in mononucleosis [48]. (See "Infectious mononucleosis".)

Smudge cells are also present transiently after recovery from cardiac arrest [49,50].

Plasma cells — Plasma cells appear as lymphocytes and are counted as such. However, unlike normal small lymphocytes, they have a distinct clear perinuclear region that contains large numbers of Golgi bodies (picture 49).

Plasma cells are normally not found on the peripheral blood smear. Circulating plasma cells in the peripheral blood can be seen in multiple myeloma, plasma cell leukemia, non-Hodgkin lymphoma, and rarely in primary systemic amyloidosis [51,52]. Flow cytometry would indicate clonality. Plasma cell leukemia is defined by clinical criteria for multiple myeloma plus plasma cells accounting for ≥5 percent of WBCs on the blood smear. (See "Multiple myeloma: Clinical features, laboratory manifestations, and diagnosis" and "Plasma cell leukemia" 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 including HIV, and inflammatory diseases [53-55]. Reactive plasma cells are polyclonal. (See "Approach to the adult with lymphocytosis or lymphocytopenia", section on 'Assessment of clonality'.)

Normal neutrophils, eosinophils, basophils, and monocytes — Granulocytes include:

●Neutrophils

●Eosinophils

●Basophils

Monocytes share a common progenitor cell with granulocytes. Morphologically, they are the largest normal cells seen in the peripheral blood. They have a grayish blue cytoplasm, often replete with vacuoles, and a distinctive folded nucleus (picture 45).

Granulocytes mature in an orderly fashion through a series of morphologic changes as the nucleus condenses and the cytoplasm is reduced (figure 4). From least mature to most mature, the stages are as follows:

●Myeloblast (picture 50)

●Promyelocyte (picture 51)

●Myelocyte (picture 52)

●Metamyelocyte (picture 53)

●Band form (picture 54)

●Mature neutrophil (picture 55)

Only the last two of these stages, the band form and the mature neutrophil, are normally present in the peripheral smear. Causes of increased numbers of bands and neutrophils include infection, inflammation, redistribution (eg, after a seizure or in an asplenic individual), smoking, some medications, myeloproliferative neoplasms, and others. These causes and the evaluation to distinguish among them are discussed separately. (See "Approach to the patient with neutrophilia", section on 'Causes of neutrophilia'.)

Neutrophil granulation and granule function are discussed separately. (See "An overview of the innate immune system", section on 'Neutrophils'.)

Eosinophils typically represent <5 percent of WBCs. They have vibrant reddish-orange granules and a characteristic bilobed nucleus (picture 56). Increased numbers of eosinophils can be due to an allergic state, parasitic infection, or other conditions. (See "Approach to the patient with unexplained eosinophilia".)

Basophils are the least abundant type of the circulating granulocytes and comprise <1 percent of WBCs. They have prominent dark blue-black granules (picture 57). Basophilic leukocytosis is unusual 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.

Neutrophil abnormalities — Neutrophils normally have a three- to four-lobed nucleus and a pink-sandy, granular cytoplasm.

●Increased lobulation – Hypersegmented neutrophils have increased lobulation (>5 lobes) (picture 58).

Typically this is seen in megaloblastic anemia due to vitamin B12 or folate deficiency. (See 'Megaloblastic smear' above.)

Rarely, iron deficiency anemia can cause hypersegmented neutrophils. The finding was 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 [56,57].

Grape-like (botryoid) multiple lobulations can also be seen in patients with heat stroke (picture 59) [58,59].

●Decreased lobulation – Decreased lobulation of mature neutrophils (eg, two lobes with fully condensed chromatin) is seen in the Pelger-Huet anomaly, which can occur as an inherited disorder or can be acquired in patients with MDS (referred to as pseudo-Pelger-Huet). Typically the bilobed nucleus is connected by a thin strand, giving a "pince-nez" appearance, often accompanied by reduced or absent granulation (picture 60). (See "Clinical manifestations, diagnosis, and classification of myelodysplastic syndromes (MDS)", section on 'Blood smear'.)

Abnormal granulation — There are several conditions characterized by abnormal neutrophil granulation.

●Toxic granulations – These are dark blue, coarse granules that represent non-specific findings characteristic of toxic systemic illnesses (picture 61). They represent azurophilic granules with abnormal staining properties. (See "Approach to the patient with neutrophilia", section on 'Neutrophil abnormalities'.)

●Giant cytoplasmic granules - These may indicate Chediak-Higashi syndrome (picture 62), especially in a child with recurrent pyogenic infections.

●Azurophilic cytoplasmic inclusions (Alder-Reilly granules) – These may be seen in neutrophils, lymphocytes, and monocytes in the mucopolysaccharidoses (picture 63), with myeloperoxidase mutations, and in myelodysplastic syndromes (MDS) [60-63].

●Bright green cytoplasmic inclusions – These have occasionally been documented in neutrophils or monocytes in patients with severe illness (sepsis, liver failure) [64,65]. Examples include:

•Coronavirus 2019 (COVID-19) – One series described six individuals with blue-green inclusions on the blood smear, all of whom died within days of the finding [66].

•Sepsis – A case report described an individual who died of sepsis and had bright green cytoplasmic inclusions (picture 64) noted two days before he died [64]. The inclusions tested negative on special stains for iron, bilirubin, and myeloperoxidase.

•Increased transaminases – 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 [67]. All but one had elevated hepatic transaminases. The term "critical green inclusions" was proposed as a name for this finding.

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 61 and picture 65). 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.

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 [68,69]. (See 'Howell-Jolly, Heinz, and Pappenheimer bodies' above.)

Apoptotic neutrophils — Apoptotic neutrophils (picture 66) may be seen in infections and occasionally in myelodysplastic syndrome [70-72].

Vacuolated neutrophils — Vacuoles may be seen in neutrophil cytoplasm in conditions of high phagocytic activity such as infectious and inflammatory conditions as well as in VEXAS syndrome [73]. (See "Autoinflammatory diseases mediated by NFkB and/or aberrant TNF activity", section on 'Vacuoles, E1 enzyme, X-linked, autoinflammatory, somatic (VEXAS) syndrome'.)

Microorganisms within neutrophils

●Bacteria – Bacteria seen in neutrophils include ehrlichia, anaplasma (picture 67), and others [20,74-78]. (See "Human ehrlichiosis and anaplasmosis", section on 'Blood smears'.)

●Fungi – Fungi such as histoplasma (picture 68) or candida may be seen [79,80]. In one study, detection of candidemia by peripheral blood smear examination required a yeast concentration of ≥1 to 5 x 10⁵ colonies/mL; this degree of fungemia is unusual [81]. Sensitivity of smear review for yeast detection was greatly increased when the microscopist was directed to look for yeast.

●Hemozoin – Hemozoin (also called malarial pigment) is not a microorganism, but it is seen in individuals with malaria (picture 69). It is a pigmented inclusion composed of hemoglobin that has been incompletely digested by plasmodial organisms. It may be seen in circulating neutrophils and monocytes in individuals with severe malarial anemia or cerebral malaria [82]. (See "Laboratory tools for diagnosis of malaria", section on 'Blood smear interpretation' and "Anemia in malaria", section on 'Pathogenesis'.)

LE cells — LE cells are neutrophils that have ingested nuclear material (picture 70); they can be sometimes be found in the peripheral blood of patients with systemic lupus erythematosus (SLE). They can also be seen on occasion in the bone marrow or in body fluids of patients with SLE [83]. (See "Systemic lupus erythematosus in adults: Clinical manifestations and diagnosis", section on 'Classification criteria'.)

Blasts or tumor cells — A range of immature WBCs can be seen in pregnancy or during a leukemoid reaction. However, it is never normal to see blast forms (lymphoblasts or myeloblasts) on the peripheral smear.

Patients with unexplained blasts on the blood smear require hematologist evaluation with review of the peripheral smear and bone marrow examination. (See "Bone marrow aspiration and biopsy: Indications and technique", section on 'Indications'.)

The following types of blasts and malignant cells may be seen:

●Myeloblasts – These are immature cells with large nuclei, nucleoli, and a scant rim of dark blue cytoplasm, suggesting an underlying malignant hematologic disorder (picture 50).

Auer rods are rod-like conglomerations of granules in the cytoplasm; this finding within a blast cell is pathognomonic of acute myeloid leukemia (picture 71). (See "Acute myeloid leukemia: Clinical manifestations, pathologic features, and diagnosis".)

●Small cleaved B-cells or centrocytes – These are small lymphoid cells with cleaved nuclei (may be seen in the circulation in patients with follicular lymphoma) (picture 72). (See "Clinical manifestations, pathologic features, diagnosis, and prognosis of follicular lymphoma".)

●Hairy cells – These are lymphoid cells with ragged or "hairy" cytoplasm (picture 73); they may be seen in hairy cell leukemia. (See "Clinical features and diagnosis of hairy cell leukemia".)

Lymphoid cells with bipolar villous projections (picture 74) may be seen in patients with splenic marginal zone lymphoma. (See "Splenic marginal zone lymphoma".)

●Clover leaf or flower cells – These are lymphoid cells with hyperlobulated nuclei (picture 3); they may be seen in patients with adult T-cell leukemia/lymphoma. (See "Clinical manifestations, pathologic features, and diagnosis of adult T cell leukemia-lymphoma".)

●Sézary cells – These are atypical lymphoid cells with "cerebriform" nuclei (picture 75); they may be seen in the circulation of patients with cutaneous T-cell lymphoma. (See "Clinical manifestations, pathologic features, and diagnosis of mycosis fungoides", section on 'Pathology'.)

PLATELETS

Normal platelets — Platelets are small purplish anuclear cells formed from megakaryocytes. (See "Megakaryocyte biology and platelet production".)

There is normally at least one platelet visualized per oil-immersion (1000-power) 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 red blood cells (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 complete blood count (CBC)", section on 'Platelet parameters'.)

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 76), but not in samples anticoagulated with heparin or citrate.

Platelet abnormalities — Large platelets suggest a heightened marrow response secondary to a destructive process such as immune thrombocytopenia (ITP); however, the assessment of large platelets is very subjective. Automated measurement of the mean platelet volume has poor sensitivity and specificity for distinguishing platelet destructive processes from marrow production issues [84]. (See "Automated complete blood count (CBC)", section on 'Reticulocyte indices'.)

Extremely large platelets and/or megakaryocyte fragments with thrombocytosis are abnormal and suggest an underlying myeloproliferative neoplasm, such as essential thrombocythemia (picture 77) or primary myelofibrosis. (See "Clinical manifestations and diagnosis of primary myelofibrosis", section on 'Platelet and white blood cell abnormalities'.)

Giant platelets (picture 78) and/or abnormalities in platelet granulation may be seen in certain inherited platelet disorders. (See "Inherited platelet function disorders (IPFDs)", section on 'Clinical spectrum (thrombocytopenia, platelet size, syndromic features)'.)

SUMMARY

●Uses – The blood smear offers a window into the bone marrow. It is particularly important when assessing cytopenias, leukemias, certain infections (babesiosis, malaria), thrombotic microangiopathies, hemolytic anemias, and inherited platelet disorders. (See 'When is a blood smear needed?' above.)

●Slide preparation and viewing – The smear should be made using a clean, oil-free slide. Wright-Giemsa is the most commonly used stain. The optimal area for viewing is near the middle of the smear where red blood cells (RBCs) are evenly spaced, and central pallor will be appreciated (figure 1). (See 'Technical details' above.)

●Low power review – Scanning the slide at low power helps locate the optimal area for review and may identify leukoerythroblastic, megaloblastic, or left-shifted patterns, RBC agglutination or rouleaux formation, extracellular microorganisms, and reticulocytosis or nucleated RBCs. (See 'Initial low power review' above.)

●RBCs

•Normal – Normal RBCs are 7 to 8 microns in diameter. They appear round in two dimensions, with an area of central pallor that takes up approximately one-third of the cell (picture 4). (See 'Normal RBC size, shape, and color' above.)

•Abnormal – Abnormal RBC size and/or shape may be an important clue to an underlying condition; possible causes are listed above. Schistocytes (picture 30) are an especially concerning finding; schistocytes accompanied by thrombocytopenia indicate a potentially life-threatening condition, and immediate hospitalization and hematology evaluation are required. Other RBC shape abnormalities and inclusions (Howell-Jolly bodies, Heinz bodies, Pappenheimer bodies, basophilic stippling, RBC parasites) may provide useful diagnostic information (figure 2). (See 'RBC size and shape abnormalities' above and 'Schistocytes' above and 'RBC inclusions' above.)

●White blood cells (WBCs)

•Normal – Normal WBCs include small lymphocytes with clumped chromatin and a rim of blue cytoplasm (picture 45), accounting for 30 to 40 percent of WBCs, neutrophils (picture 55) with three to four lobes, accounting for 60-70 percent of WBCs, and small numbers of eosinophils, basophils, and monocytes. (See 'Normal lymphocytes' above and 'Normal neutrophils, eosinophils, basophils, and monocytes' above.)

•Abnormal – Smudge cells (picture 48) are mature-appearing small lymphocytes that appear to have been flattened or smudged; they are typical of chronic lymphocytic leukemia (CLL) but can also be seen in infectious mononucleosis; the figure illustrates an approach to evaluation (algorithm 2). Neutrophils with <3 lobes may signify myelodysplasia or an inherited disorder; neutrophils with >4 lobes may signify a megaloblastic process. Immature neutrophils, abnormal granulation, Döhle bodies, or microorganisms require careful review for the cause. Blasts require hematologist evaluation with review of the peripheral smear and bone marrow examination. (See 'Smudge cells' above and 'Neutrophil abnormalities' above and 'Blasts or tumor cells' above and "Bone marrow aspiration and biopsy: Indications and technique".)

●Platelets

•Normal – Platelets are small purplish anuclear cells formed from megakaryocytes. There is normally at least one platelet visualized per oil-immersion field, and seven platelets per 100-power field. (See 'Normal platelets' above.)

•Abnormal – Giant platelets (picture 78) and/or abnormalities in platelet granulation may be seen in certain inherited platelet disorders. Extremely large platelets and/or megakaryocyte fragments suggest a myeloproliferative neoplasm (picture 77). (See 'Platelet abnormalities' above.)

ACKNOWLEDGMENT — 

UpToDate gratefully acknowledges Stanley L Schrier, MD, who contributed as Section Editor on earlier versions of this topic and was a founding Editor-in-Chief for UpToDate in Hematology.