Iron deficiency anemia refers to an anemia that is associated with additional laboratory evidence of iron depletion as a result of one or more of the following tests results: low serum ferritin concentration, low transferrin saturation, or an elevation in the erythrocyte protoporphyrin level.

Iron deficiency without anemia represents a relatively mild iron deficiency that is diagnosed on the basis of a combination of biochemical indicators of iron status but in which the hemoglobin concentration remains within the normal range. Although no single indicator of iron status is diagnostic of functional iron deficiency, a low serum ferritin concentration indicates that iron reserves are depleted.

Measurement of hemoglobin and hematocrit levels is used to screen for anemia and putative iron deficiency because they are easy and inexpensive to measure and reflect the largest iron compartment in the body. However, individuals with mild degrees of iron deficiency are missed by such screenings because of the overlap in values between normal and iron deficient individuals. Hemoglobin and hematocrit values vary by age, sex, and stage of pregnancy. Hemoglobin values normally are lower in children than in nonpregnant adults. During puberty, the average hemoglobin concentration of males rises above that of females—a gender difference sustained throughout the reproductive years. During pregnancy, hemoglobin values gradually fall to a low point in the second trimester, largely because of a normal expansion in blood volume. From the end of the second trimester to term, the concentration of hemoglobin rises again. The average hemoglobin concentration of healthy blacks is lower than that of other races, by about 0.3 g/dl in young children and 0.8 g/dl in adults, as recorded in the NHANES II database.