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Anemia is defined as a reduced red blood cell (RBC) mass resulting in decreased oxygen-carrying capacity of the blood. Oxygen levels depend on the RBC mass, lung function, available erythropoietin (EPO), and the bone marrow. Some would define it as an abnormally low hematocrit/hemoglobin (H/H) level, but there are exceptions to this definition. Long-distance runners and pregnant women may have an increased plasma volume, diluting the H/H to abnormal or borderline low levels and suggesting a diagnosis of anemia when, in fact, the RBC mass and oxygen-carrying capacity of the blood are normal. Thus, the physiologic definition is most accurate. Anemia is the most frequent hematologic disorder seen by family physicians. Iron deficiency is the most common anemia seen in family medicine and the most common nutritional deficiency in the world. Children age 1–2 years (7%) and females age 12–48 years (8–16%) are the most commonly affected due to inadequate supply or increased requirement via blood loss, respectively. Anemia in men is most likely due to blood loss. Although in some instances elderly patients tend to have borderline low H/H levels, anemia in the elderly deserves evaluation instead of writing it off as an “anemia of aging.”
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A normal RBC lasts 120 days, whereas white blood cells (WBCs) last several days but only spend 6 hours in the circulation. The normal myeloid-to-erythroid ratio of the bone marrow is 3:1, but in severe blood loss, this will be reversed so that erythroid precursors become the dominant form. The bone marrow can change and expand its production of any cell line under appropriate circumstances. In responding to anemia, the bone marrow can increase production of RBCs by 10%, and this response can be measured by the percentage of new RBCs called reticulocytes. These are new RBCs in which the RNA has not yet disintegrated. They can be counted using a supravital stain and reported as a percentage per 100 RBCs. In the face of acute blood loss or treatment of an anemia with iron, this percentage may or may not accurately reflect the expected bone marrow response. To correct for this, one must multiply the reticulocyte percentage on blood smear:
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A reticulocytosis of 10% after an acute bleed would seem to be an appropriate response, but if the hemoglobin dropped to 7.5 gms% (grams of Hgb/100 dl of blood) from a normal 15 gms%, the corrected response is:
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This is still an effective response, but more accurately reflects bone marrow capacity in a particular patient.
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Many laboratories report reticulocytes as an actual number such as 35–70 × 103. In this case, one can see if an appropriate response occurs relative to the degree of anemia if the actual count dramatically rises 3- to 10-fold. Reticulocyte measurement and correction can ...