Sickle Cell Screen

Hemoglobin S is an abnormal hemoglobin created by a point mutation at the sixth position on the beta globin chain that results in replacement of glutamic acid by valine.  It causes sickling of red cells under conditions of reduced oxygen concentration.

The hemoglobin solubility test (Sickledex®, Streck Inc) is a rapid method to detect hemoglobin S in a patient’s blood sample. Blood is mixed with a reagent containing saponin, 2.3M potassium phosphate buffer and sodium hydrosulfite. Saponin is a detergent that lyses red blood cell membranes releasing hemoglobin. Sodium hydrosulfite is a reducing agent that deoxygenates hemoglobin. Deoxygenated Hgb S is insoluble in concentrated phosphate buffer and precipitates, while deoxygenated Hgb A remains soluble. A technologist tries to observe black lines on a card placed behind the sample. In a negative screen the black lines are visible because the specimen is clear, but In a positive screen the lines are not visible due to turbidity.

A positive screen indicates the presence of Hgb S or another sickling hemoglobin, usually at an amount of at least 20%. A positive test may be associated with either sickle cell anemia or sickle cell trait, in which Hgb S is usually 30-45%. Positive results can also be seen in cases of hemoglobin C Harlem and hemoglobin C Georgetown.

False positive reactions can occur in patients with high plasma protein levels, such as occurs in multiple myeloma. Other causes of false positive reactions include erythrcytosis, leukocytosis and hyperlipidemia.

 False negative results may occur if the percentage of Hgb S is < 20%, if the concentration of fetal hemoglobin in high, or if the total hemoglobin is less than 6 g/dL. Recent transfusion can give a false negative result. This test is not recommended for infants less than 6 months of age because they have a high concentration of hemoglobin F. Some variants, such a s Hgb D-Punjab and Hgb O-Arab, that can result in clinically significant sickle cell disorders when paired with Hgb S in offspring, are not detected by sickle solubility testing.

 Results are reported as positive or negative.  Reference value is negative.

Specimen requirement is one lavender top (EDTA) tube.

Hematology: Clinical Principles and Applications, Rodak et al., 4th ed. 2012, p. 376

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