Sickle cell anemia is a qualitative hemoglobin synthesis disorder known as a hemoglobinopathy. Sickle hemoglobin (HbS) is a structural disorder caused by valine replacing glutamic acid in the sixth position on the beta chain. The heterozygous state, HbSA, is known as sickle cell trait, while the homozygous state, HbSS, is sickle cell anemia or sickle cell disease.
A double heterozygous condition known as Hemoglobin SC disease also exists where one beta chain carries the mutation for HbS and the other beta chain carries the mutation for HbC.
In addition, HbS can be present with thalassemia.
Sickle cell anemia can also demonstrate hereditary persistance of fetal hemoglobin (HbS/HPFH).
Other HbS combinations are very rare and include HbS/HbE, HbS/HbD LosAngeles, HbS/HbG-Philadelphia, and HbS/HbO Arab.
