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 impact is best expressed in terms of hemoglobin concentration.
— Disorders in which the red cell mass is increased (erythrocytoses); often referred to as polycythemia. Here we have chosen to refer to polycythemia vera as polycythemia and other increases in red cell mass as erythrocytosis. 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 erythrocytoses can each be subclassified as:
— Relative, where the red cell mass is normal but the amount of plasma is increased (relative anemia) or decreased (relative erythrocytosis).
— Absolute, where the red cell mass is decreased (true anemia) or increased (true erythrocytosis).
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 parameters 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, which include 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; this parameter is an average value.
— 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 event 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, presence of 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 ...