Anemia is defined as an abnormally low circulating red blood cell (RBC) mass, reflected by low serum hemoglobin (Hb). However, the normal range of Hb varies among different populations. For menstruating women, anemia is present if the Hb level is at or below 11.6-12.3 g/dL. In men and postmenopausal women, anemia is present if the Hb level is at or below 13.0-14.0 g/dL. Other factors, such as age, race, altitude, and exposure to tobacco smoke, can also alter Hb levels.
Anemia is usually classified by cell size (Table 31-1). Microcytic anemias, or those with mean corpuscular volume (MCV) below 80 fL, are usually due to iron deficiency, chronic inflammation, or thalassemia. Macrocytic anemias, those with MCV above 100 fL, are classified as megaloblastic or nonmegaloblastic. Megaloblasts, which are large, immature, nucleated precursors to RBCs, are seen with vitamin B12 deficiency and folic acid deficiency. Nonmegaloblastic causes of macrocytosis include alcoholism, hypothyroidism, and chronic liver disease. Normocytic anemia (MCV between 80 and 100 fL) can be due to hemolytic or nonhemolytic causes. Hemolysis can result from hereditary abnormalities of the cell contents or cell membrane. Hemolysis can also result from acquired insults caused by autoantibodies, alloantibodies (in, eg, transfusion reactions), or a nonimmune process such as malaria or hypersplenism. Important nonhemolytic causes of normocytic anemia include poor production of RBCs due to aplastic anemia, renal insufficiency, and bone marrow infiltration.
Table 31-1. Anemia Classification by Cell Size. |Favorite Table|Download (.pdf)
Table 31-1. Anemia Classification by Cell Size.
Anemia of chronic disease
Vitamin B12 deficiency
Folic acid deficiency
Membrane defects (spherocytosis)
Enzyme deficiencies (G6PD deficiency)
Hemoglobinopathies (sickle cell disease)
Warm antibody mediated (chronic lymphocytic leukemia, systemic lupus erythematosus, idiopathic)
Cold antibody mediated (Mycoplasma, idiopathic)
Physical trauma (thrombotic thrombocytopenic purpura, disseminated intravascular coagulation, burns)
Acute blood loss
Anemia of chronic disease
Chronic renal insufficiency
Normal Hb levels vary with age. At birth, mean Hb is about 16.5 g/dL. This level increases to 18.5 g/dL during the first week of life, followed by a drop to 11.5 g/dL by 1-2 months of age. This physiologic anemia of infancy is mediated by changes in erythropoietin levels. By 1-2 years of age, the Hb level begins to rise, to 14 g/dL in adolescent girls and 15 g/dL in adolescent boys. Other relevant laboratory values also vary in children. The median MCV, for example, can be as high as 120 fL in premature infants and as low as 78 fL in 1-year-old infants. Thus, laboratory values in children should always be compared with age-appropriate norms.