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Hematology

Erythrocytes

In the clinical laboratory, the evaluation of erythrocytes - the circulating red blood cells (RBCs) and their precursors in the bone marrow - is a fundamental component of hematologic diagnosis. Disease states involving erythrocytes typically disrupt the homeostatic balance between RBC production (erythropoiesis) and destruction (hemolysis) or loss. These disorders generally manifest in two opposing directions: Anemia, representing a deficit in red cell mass or function, and Erythrocytosis, representing an excess. For the laboratory scientist, the ability to correlate numerical data from the Complete Blood Count (CBC) with morphological findings on the peripheral blood smear is critical for directing the differential diagnosis

Classification of Anemias

Anemia is functionally defined as a reduction in the oxygen-carrying capacity of the blood, clinically observed as a decrease in Hemoglobin (HGB), Hematocrit (HCT), or RBC count. To systematically diagnose anemia, the laboratory utilizes two primary classification systems:

  • Morphological Classification: This approach utilizes RBC indices, specifically the Mean Corpuscular Volume (MCV), to categorize anemia based on cell size
    • Microcytic (MCV < 80 fL): Often associated with defects in hemoglobin synthesis, such as Iron Deficiency Anemia (IDA), Thalassemia, and Sideroblastic Anemia
    • Normocytic (MCV 80–100 fL): Commonly seen in acute blood loss, hemolytic events, bone marrow failure, or renal disease
    • Macrocytic (MCV > 100 fL): divided into Megaloblastic causes (B12/Folate deficiency interfering with DNA synthesis) and Non-Megaloblastic causes (liver disease, alcoholism)
  • Pathophysiological Classification: This approach categorizes anemia based on the underlying mechanism of the deficit
    • Hypoproliferative: The bone marrow fails to produce adequate RBCs (e.g., Aplastic Anemia, nutritional deficiencies)
    • Maturation Defects: The bone marrow produces cells, but they do not mature properly (e.g., Megaloblastic anemia, Thalassemia)
    • Survival Defects (Hemolysis): The bone marrow produces RBCs effectively, but they are destroyed prematurely due to intrinsic defects (membrane, enzyme, hemoglobin) or extrinsic factors (antibodies, trauma)

Erythrocytosis

Erythrocytosis (or Polycythemia) refers to an abnormal increase in red cell mass. Differentiating between the types of erythrocytosis is essential for proper clinical management, as causes range from benign physiological responses to malignant neoplasms

  • Relative Erythrocytosis: An apparent increase in RBCs caused by a decrease in plasma volume (dehydration/hemoconcentration). The total RBC mass in the body is normal
  • Absolute Erythrocytosis: A true increase in total RBC mass
    • Primary: An intrinsic bone marrow defect, such as Polycythemia Vera (PV), a myeloproliferative neoplasm where RBC production occurs independent of Erythropoietin (EPO)
    • Secondary: A physiological response to tissue hypoxia (e.g., high altitude, COPD) or inappropriate EPO secretion (e.g., renal tumors), leading to elevated EPO levels

The Diagnostic Role of the Laboratory

The diagnosis of these states relies heavily on a tiered laboratory approach. The initial CBC provides the baseline indices (MCV, MCH, MCHC, RDW). The Reticulocyte Count is the single best indicator of bone marrow function, helping distinguish between production failure and increased destruction. The Peripheral Blood Smear allows for the visual assessment of poikilocytosis (shape), anisocytosis (size), and inclusions (e.g., Howell-Jolly bodies, basophilic stippling). Finally, definitive diagnosis often requires confirmatory testing such as iron studies, hemoglobin electrophoresis, flow cytometry, or bone marrow examination