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. Their severity is best expressed in terms of hemoglobin concentration.
— Disorders in which the red cell mass is increased (polycythemias also known as erythrocytoses). The principal effect is related to an increased viscosity of the blood (see Figure 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 the mass of red cells in the circulation.
— The normal red cell mass among women is 23 to 29 mL/kg.
— The normal red cell mass among men is 26 to 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 the red cell mass 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 increased (relative anemia) or decreased (polycythemia)
— Absolute, where the red cell mass is decreased (true anemia) or increased (true polycythemia)
The various types of anemia are classified in Table 2–1.
It is essential that the specific cause of anemia be determined. The initial laboratory approach to the diagnosis of anemia follows, and these four studies should be the prelude to guide further specific testing.
— 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, such as mean cell volume (MCV), mean cell hemoglobin (MCH), and mean cell hemoglobin concentration (MCHC) to determine whether normocytic, macrocytic, or microcytic and normochromic or hypochromic red cells are present on average
— Measurement of red cell distribution width (RDW) to obtain 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 size and shape, hemoglobin content, presence of red cell inclusions, presence of agglutination or rouleaux formation, nonhematopoietic particles such as parasites (ie, Babesia and Plasmodium species) and helminths (ie, Wuchereria bancrofti, nematodes), and accompanying abnormalities of white cells and platelets
— Red cell size and hemoglobin content are best determined from their indices because the blood film will usually make evident only gross deviations (eg, the need to estimate red cell volume from a two-dimensional area). Moreover, the blood in macrocytic anemia ...