Red Blood Cells (RBCs): Count and Morphology


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Subject: Red Blood Cells (RBCs): Count and Morphology

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Definition and Use

  • The RBC count is part of the CBC as obtained by automated counters. It is less useful than the Hb or Hct.

  • Normal range: 4.2–5.4 cells/μL in women and 4.4–6.0 cells/μL in men (reported by automated counters in a random adult population)

    • Different values are reported for newborns, infants, and children until they reach adulthood.

    • Automated counters adjust normal values for age groups.


  • The RBC count is interpreted in conjunction with red cell indices, hemoglobin, and hematocrit.

Increased In

  • Certain myeloproliferative neoplasms (e.g., polycythemia vera).

  • Severe dehydration. RBC counts may be appropriately decreased or increased in certain physiologic states.

Decreased In

  • Various types of anemia

Abnormal RBC Morphology

  • It is flagged by automated counters, triggering microscopic examination of stained peripheral blood smears (see above).

  • Abnormalities (see Tables 16.72 and 16.73) may be specific for certain conditions (e.g., spherocytes for hemolytic anemias, sickle cells for sickle cell anemias) or may be informative but not specific. Anisocytosis refers to variation in RBC size, poikilocytosis refers to variation in shape, and polychromasia refers to bluish discoloration of RBC reflecting high reticulocytes.

TABLE 16–72
Abnormal Shapes of Red Blood Cells
TABLE 16–73
Red Blood Cell Inclusions


  • Patient's circumstances (e.g., vomiting or diarrhea)

  • Other preanalytic factors

    • Marked leukocytosis marginally increases the RBC count.

    • Inappropriate blood collection is a major source of preanalytic errors. For instance, inappropriate filling of test tube results in excess anticoagulant, thereby diluting the blood and decreasing the red cell parameters.

    • Very low temperatures may lyse the red cells. Anticoagulated blood may be stored at 4°C for 24 hours, but beyond this interval, the results become increasingly altered.