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These result from either defective hemoglobin synthesis, leading to cytoplasmic maturation defects and small relatively empty red cells, or abnormally slow DNA replication, leading to nuclear maturation defects and large full red cells. Defects in hemoglobin synthesis usually result from insufficient iron supply (iron deficiency) or decreased globin production (thalassemia) or are idiopathic (sideroblastic anemia). Defects in DNA synthesis are usually due to nutritional problems (vitamin B12 and folate deficiency), toxic (methotrexate or other cancer chemotherapeutic agent) exposure, or intrinsic marrow maturation defects (refractory anemia, myelodysplasia).
Laboratory tests useful in the differential diagnosis of the microcytic anemias are shown in Table 62-2. Mean corpuscular volume (MCV) is generally 60–80 fL. Increased lactate dehydrogenase (LDH) and indirect bilirubin levels suggest an increase in RBC destruction and favor a cause other than iron deficiency. Iron status is best assessed by measuring SI, TIBC, and ferritin levels. Macrocytic MCVs are >94 fL. Folate status is best assessed by measuring red blood cell folate levels. Vitamin B12 status is best assessed by measuring serum B12, homocysteine, and methylmalonic acid levels. Homocysteine and methylmalonic acid levels are elevated in the setting of B12 deficiency.
|Tests||Iron Deficiency||Inflammation||Thalassemia||Sideroblastic Anemia|
|Smear||Micro/hypo||Normal micro/hypo||Micro/hypo with targeting||Variable|
|SI||<30||<50||Normal to high||Normal to high|
|Hemoglobin pattern on electrophoresis||Normal||Normal||Abnormal with β thalassemia; can be normal with α thalassemia||Normal|