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).