Clinically significant red cell disorders can be classified into:
— Disorders in which the red cell mass is decreased (anemias). The principal effect is decreased oxygen-carrying capacity of the blood. They are best expressed in terms of hemoglobin concentration.
— Disorders in which the red cell mass is increased (polycythemias). The principal effect is related to an increased viscosity of the blood (see Fig. 2–1). In addition to their specific effects, they are best expressed in terms of packed red cell volume (hematocrit).
The red cell mass is the volume of red cells in the circulation.
— The normal red cell mass of women is 23–29 mL/kg.
— The normal red cell mass of men is 26–32 mL/kg.
— More accurate formulas based on body surface have been recommended.
Because the red cells are measured either as a concentration in the blood as the red cell count, the hemoglobin content of the blood, or the hematocrit (packed red cell volume per 100 mL of blood), rather than the volume of red cells in the total circulation, the anemias and polycythemias can each be subclassified as:
— Relative, where the red cell mass is normal but the amount of plasma is decreased (relative polycythemia) or increased (relative anemia).
— Absolute, where the red cell mass is increased (in true polycythemia) or decreased (in true anemia).
It is essential that the specific cause of anemia be determined. The initial laboratory approach to the diagnosis of anemia follows. (Table 2–1).
— Hematocrit, hemoglobin, or red cell count to determine degree of anemia. In most cases, these three variables are closely correlated. Hemoglobin concentration is the most direct measure of oxygen-carrying capacity.
— Red cell indices (MCV, MCH, MCHC) to determine whether normocytic, macrocytic, or microcytic and normochromic or hypochromic red cells are present on average.
— Exaggerated red cell distribution width (RDW) is a measure of anisocytosis.
— Reticulocyte count or index to estimate whether marrow response suggests inadequacy of red cell production or an appropriate erythropoietic response to hemolysis (or acute bleeding). The latter is usually readily apparent clinically.
— Examination of the blood film to determine red cell shape, hemoglobin content, presence of red cell inclusions, and accompanying abnormalities of white cells and platelets.
— These five studies should be the prelude to guide further specific testing.
— Red cell size and hemoglobin content is best determined from their indices because the blood film will usually make evident only gross deviations (e.g., the need to estimate red cell volume from a two-dimensional area). Moreover, the blood in macrocytic anemia usually contains many microcytic cells and in microcytic anemias, many normocytic cells, which make determination of the average red cell volume from a blood film difficult.
— In general, the abnormalities in size, hemoglobin content, and shape are approximately correlated with severity of anemia. If the anemia is slight, the other changes are often subtle.
— Anemia classically categorized as macrocytic or microcytic may be present in the face of red cell volumes that are in the normal range. This may be the case because the anemia is so mild that red cell volumes have not yet deviated beyond the normal range, or may be the case with more severe anemias because of confounding effects of two causal factors (e.g., iron deficiency and folate deficiency), or for unexplained reasons (e.g., well-established megaloblastic anemia may have normocytic index).
A classification of the major causes of anemia is shown in Table 2–1.
A classification of the major causes of polycythemia is shown in Table 2–2.
Polycythemias are discussed in Chap. 29. It is important to search for the specific cause of polycythemia.