Chapter 61: Examination of Blood Smears and Bone Marrow

ERYTHROCYTE (RBC) MORPHOLOGY

• Normal: 7.5 μm diameter. Roughly the size of the nucleus of a small lymphocyte.

• Reticulocytes (Wright’s stain): large, grayish-blue, admixed with pink (polychromasia).

• Anisocytosis: variation in RBC size; large cells imply delay in erythroid precursor DNA synthesis caused by folate or B₁₂ deficiency or drug effect; small cells imply a defect in hemoglobin synthesis caused by iron deficiency or abnormal hemoglobin genes. The automated red cell distribution width (RDW) is a measure of anisocytosis.

• Poikilocytosis: abnormal RBC shapes; the following are examples:

  1. Acanthocytes (spur cells): irregularly spiculated; abetalipoproteinemia, severe liver disease, rarely anorexia nervosa.

  2. Echinocytes (burr cells): regularly shaped, uniformly distributed spiny projections; uremia, RBC volume loss.

  3. Elliptocytes: elliptical; hereditary elliptocytosis.

  4. Schistocytes (schizocytes): fragmented cells of varying sizes and shapes; microangiopathic or macroangiopathic hemolytic anemia.

  5. Sickled cells: elongated, crescentic; sickle cell anemias.

  6. Spherocytes: small hyperchromic cells lacking normal central pallor; hereditary spherocytosis, extravascular hemolysis as in autoimmune hemolytic anemia, G6PD deficiency.

  7. Target cells: central and outer rim staining with intervening ring of pallor; liver disease, thalassemia, hemoglobin C, and sickle C diseases.

  8. Teardrop cells: myelofibrosis, other infiltrative processes of marrow (e.g., carcinoma).

  9. Rouleaux formation: alignment of RBCs in stacks; may be artifactual or due to paraproteinemia (e.g., multiple myeloma, macroglobulinemia).

RBC INCLUSIONS

• Howell-Jolly bodies: 1-μm-diameter basophilic cytoplasmic inclusion that represents a residual nuclear fragment, usually single; asplenic pts.

• Basophilic stippling: multiple, punctate basophilic cytoplasmic inclusions composed of precipitated mitochondria and ribosomes; lead poisoning, thalassemia, myelofibrosis.

• Pappenheimer (iron) bodies: iron-containing granules usually composed of mitochondria and ribosomes resemble basophilic stippling but also stain with Prussian blue; lead poisoning, other sideroblastic anemias.

• Heinz bodies: spherical inclusions of precipitated hemoglobin seen only with supravital stains, such as crystal violet; G6PD deficiency (after oxidant stress such as infection, certain drugs), unstable hemoglobin variants.

• Parasites: characteristic intracytoplasmic inclusions; malaria, babesiosis.

LEUKOCYTE INCLUSIONS AND NUCLEAR CONTOUR ABNORMALITIES

• Toxic granulations: dark cytoplasmic granules; bacterial infection.

• Döhle bodies: 1- to 2-μm blue, oval cytoplasmic inclusions; bacterial infection, Chédiak-Higashi anomaly.

• Auer rods: eosinophilic, rodlike cytoplasmic inclusions; acute myeloid leukemia (some cases).

• Hypersegmentation: neutrophil nuclei contain more than the usual 2–4 lobes; usually >5% have ≥5 lobes or a single cell with 7 lobes is adequate to make the diagnosis; folate or B₁₂ deficiency, drug effects.

• Hyposegmentation: neutrophil nuclei contain fewer lobes than normal, either one or two: Pelger-Hüet anomaly, pseudo–Pelger-Hüet or acquired Pelger-Hüet anomaly in acute leukemia.

PLATELET ABNORMALITIES

• Platelet clumping: an in vitro artifact—is often readily detectable on smear; can lead to falsely low platelet count by automated cell counters.

• Giant platelets: can be a sign of a very young platelet or increased platelet production or abnormal karyocyte maturation; if the platelets are >5–6 μm in diameter, they may not be counted as platelets by electronic counters.

Aspiration assesses cell morphology. Biopsy assesses overall marrow architecture, including degree of cellularity. Biopsy should precede aspiration to avoid aspiration artifact (mainly hemorrhage) in the specimen.

INDICATIONS

Aspiration

Hypoproliferative or unexplained anemia, leukopenia, or thrombocytopenia, suspected leukemia or myeloma or marrow defect, evaluation of iron stores, workup of some cases of fever of unknown origin.

Special Tests

Histochemical staining (leukemias), cytogenetic studies (leukemias, lymphomas), microbiology (bacterial, mycobacterial, fungal cultures), Prussian blue (iron) stain (assessment of iron stores, diagnosis of sideroblastic anemias).

Biopsy

Performed in addition to aspiration for pancytopenia (aplastic anemia), metastatic tumor, granulomatous infection (e.g., mycobacteria, brucellosis, histoplasmosis), myelofibrosis, lipid storage disease (e.g., Gaucher’s, Niemann-Pick), any case with “dry tap” on aspiration; evaluation of marrow cellularity. When biopsy and aspirate are both planned, the biopsy should be performed first because of the risk of bleeding artifact from biopsy of an aspiration site.

Special Tests

Histochemical staining (e.g., acid phosphatase for metastatic prostate carcinoma), immunoperoxidase staining (e.g., immunoglobulin or cell surface marker detection in multiple myeloma, leukemia, or lymphoma; lysozyme detection in monocytic leukemia), reticulin staining (increased in myelofibrosis), microbiologic staining (e.g., acid-fast staining for mycobacteria).

INTERPRETATION

Cellularity

Defined as percentage of space occupied by hematopoietic cells. The space that is not hematopoietic tissue is usually fat. Cellularity decreases with age after age 65 years from about 50% to 25–30% with a corresponding increase in fat.

Erythroid:Granulocytic (E:G) Ratio

Normally about 1:2, the E:G ratio is decreased in acute and chronic infection, leukemoid reactions (e.g., chronic inflammation, metastatic tumor), acute and chronic myeloid leukemia, myelodysplastic disorders (“preleukemia”), and pure red cell aplasia; increased in agranulocytosis, anemias with erythroid hyperplasia (megaloblastic, iron-deficiency, thalassemia, hemorrhage, hemolysis, sideroblastic), and erythrocytosis (excessive RBC production); normal in aplastic anemia (though marrow hypocellular), myelofibrosis (marrow hypocellular), multiple myeloma, lymphoma, anemia of chronic disease. Some centers use the term M:E (myeloid to erythroid) ratio; normal value is 2:1 and increases with diseases that promote myeloid activity or inhibit erythroid activity and decreases with diseases that inhibit myeloid activity or promote erythroid activity.