Anemia, a decrease in the number of red blood cells (RBCs), hemoglobin (Hb) content, or hematocrit (Hct), can result from decreased RBC production (erythropoiesis), increased RBC destruction, blood loss, or a combination of these factors. (See also Approach to the Patient with Anemia.)
Anemias due to decreased erythropoiesis (termed hypoproliferative anemias) are recognized by a reticulocyte count that is inappropriately low for the degree of the anemia.
The RBC indices, mainly the mean corpuscular volume (MCV), can narrow the differential diagnosis of deficient erythropoiesis and help determine what further testing is necessary.
Microcytic anemias result from deficient or defective heme or globin synthesis. Microcytic anemias include
Patients with a microcytic anemia typically require evaluation of iron stores.
Normocytic anemias are characterized by a normal RBC distribution width (RDW) and normochromic indices. The two most common causes are
The anemia of chronic disease or inflammation
Macrocytic anemias can be caused by impaired DNA synthesis leading to megaloblastosis, as occurs with .
Deficiencies of vitamin B12 or folate (see Megaloblastic Macrocytic Anemias)
Other causes of macrocytic anemia include
Chronic alcohol intake (independent of vitamin deficiency)
Hemolysis with reticulocytosis
Drugs (eg, zidovudine, azathioprine, methotrexate, hydroxyurea, imatinib)
Some patients with hypothyroidism have macrocytic RBC indices, including some without anemia.
Anemias can have variable findings on the peripheral smear. The anemia of chronic disease may be microcytic or normocytic. Anemias due to myelodysplastic syndromes may be normocytic, macrocytic, or even microcytic. Anemias due to endocrine disorders (such as hypothyroidism) or elemental deficiencies (eg, copper deficiency, zinc deficiency) can have variable manifestations, including a normocytic or macrocytic anemia.
Treatment of deficient RBC production depends on the cause.