Anemia is the most common blood disorder in children. Anemia can be defined as a reduction in the total number of erythrocytes in the circulating blood or a reduction in the quality or quantity of hemoglobin. There are two classifications for anemia based on either morphology or etiology. Morphological classification is based on size and hemoglobin content. Etiological classification is based on the premise that anemia develops because of a decrease in the production of erythrocytes or an increase in the destruction of erythrocytes. Examples of anemia with a decreased production of red blood cells are iron-deficiency anemia and aplastic anemia. Examples of anemia with an increased destruction of red blood cells are sickle cell anemia and thalassemia.

Hemoglobinopathy is the umbrella term for a number of inherited disorders of the red blood cell (erythrocytes) due to the abnormalities in the protein hemoglobin. Sickle cell disease (SCD), and the older term sickle cell anemia (Hb SS), is the most common hemoglobinopathy variant seen worldwide, is associated with severity of illness, and primarily affects people of African and southwest Asian decent. Sickle cell anemia is a homozygous condition resulting from inheriting a sickle cell gene (Hb S) from both parents. There are several hundred hemoglobin variants. Other common heterozygous hemoglobinopathy disorders include the Hb S gene interacting with Hb C, Hb D, and Hb E; α- and β-thalassemia (primarily affecting people of central and southeast Asian and Mediterranean decent), and other variant combinations. SCD is a genetic mutation of the hemoglobin β gene thought to have come about as a protection against malaria in people living in equatorial climates. The severity of the disease varies in individuals.

Sickle cell trait (Hb AS) is the heterozygous carrier state where one receives a normal hemoglobin gene (Hb A) from one parent and an abnormal variant gene from the other parent. Sickle cell trait may be very common in many of these ethnic groups, and although patients with sickle cell trait are relatively healthy, they may show signs of disease as they age or may experience symptoms under extraordinary conditions such as exhaustion from vigorous exercise, dehydration, exposed to high altitudes, or deep-sea diving pressures.