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Screening for hemoglobin S (sickle cell trait)
Homozygous hemoglobin S (sickle cell disease) is a serious chronic hemolytic anemia most commonly found in those of African or Middle Eastern descent.
Hemoglobin S is freely soluble when fully oxygenated; when oxygen is removed, polymerization of the abnormal hemoglobin occurs, forming tactoids that are rigid and deformed cells. This leads to sickling of the cells, hemolysis, and many other complications.
Heterozygous hemoglobin S (sickle cell trait) is the most common hemoglobinopathy in the United States. This condition is present in about 8% of African Americans. Usually, hemoglobin S trait exhibits no clinical or hematological effects. A small fraction of people with sickle cell trait have recurrent hematuria.
Precautions: The procedure does not distinguish hemoglobin S trait from homozygous sickle cell disease nor any of the following combinations: S/C, S/D, S/G, S/E, S/thalassemia, S/O-Arab, S/New York and C-Georgetown trait (Hb C-Harlem).
A positive result should be followed by hemoglobin electrophoresis to confirm the presence and concentration of hemoglobin S.
A positive test is presumptive evidence for hemoglobin (Hb) S. However, rare sickling hemoglobins such as Hb C-Harlem (C-Georgetown) and Hb I will also produce a positive result.
This test only detects the presence of Hb S. It cannot differentiate sickle cell trait (heterozygous Hb S) from sickle cell disease (homozygous Hb S), or Hb S in combination with other abnormalities (eg, S/C, S/D, S/G, S/E, S/beta-thalassemia, S/O-Arab, S/New York, and C-Georgetown trait).
The use of packed RBCs instead of whole blood significantly reduces false negatives due to anemia and false positives due to hypergammaglobulinemia (eg, multiple myeloma).
False positives can occur due to large numbers of nucleated RBCs.
False negatives can occur due to an insufficient quantity of Hb S due to age (neonates) or transfusion. Hb S concentrations 15% to 20% or less may give a negative result.
Fairbanks VF, Klee GG: Biochemical aspects of hematology. In Tietz Textbook of Clinical Chemistry. Third edition. Edited by CA Burtis, ER Ashwood, Philadelphia, WB Saunders Company, 1999, pp 1670-1673