Sickledex is a test for the abnormal hemoglobin which causes sickle cell anemia. The test is performed to detect both sickle cell disease and sickle cell trait. Sickle cell anemia results from the hereditary presence of abnormal Hgb S in place of Hgb A. When Hgb S becomes deoxygenated (loses oxygen), it tends to form polymers that deform the red blood cells (RBCs) into a sickle shape. Sickled RBCs cannot pass freely through capillaries and cause blockage of small vessels. This can compromise the blood supply to various organs. Hgb S trait is found in 8 to 10% of the black population. The routine peripheral blood smear does not contain sickled red blood cells unless hypoxemia (inadequate levels of oxygen in the blood) or a precipitating factor (such as a red blood cell poison or drug) is present. In the Sickledex test a deoxygenating agent is added; if 25% or more of the Hgb is Hgb S, the cells will sickle.  Another test that can be performed is hemoglobin electrophoresis.  This test separates different types of hemoglobin based on electrical charge.  Hgb S can be differentiated from Hgb A and other types of Hgb.

In performing the sickledex test among adults or children, blood is drawn from a vein usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding. In performing the sickledex test among infants or young children, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.