Sickle cell test

Definition

This test indicates the presence or absence of the abnormal hemoglobin that causes sickle cell anemia.

How the test is performed

Blood is drawn from a vein on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an airtight vial or a syringe. During the procedure, the band 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.

For an infant or young child:

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.

How the test will feel

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed

Both sickle cell disease (where two faulty hemoglobin S genes are present and disease is present) and sickle cell trait (where only one of the faulty Hgb S genes is present, and the person has no symptoms, or only mild ones) can be detected by this test.

Sickle cell anemia results when a child inherits two Hgb S genes -- one from each parent -- instead of normal Hgb A genes. When it loses oxygen, this faulty hemoglobin tends to cause red blood cells (RBCs) to take on a sickle shape. Sickled RBCs cannot pass freely through capillaries and cause blockage of small vessels. This disrupts the blood supply to various organs. The faulty Hgb S trait is found in 8-10% 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.

Normal Values

A negative test result is normal.

In hemoglobin electrophoresis, no Hgb S should be present. Normal hemoglobins in an adult are mostly Hgb A with small amounts of Hgb A2 and Hgb F.

What abnormal results mean

• Sickle cell trait
• Sickle cell anemia

What the risks are

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Special considerations

Blood transfusions within the past 3 months or iron deficiency can cause a false negative result.

Sickledex does not distinguish between sickle cell trait and sickle cell disease. Hemoglobin electrophoresis is required to distinguish between these two disorders.

Illustrations

Red blood cells, sickle cell

Red blood cells, multiple sickle cells

Red blood cells, sickle cells

Red blood cells, sickle and pappenheimer

Page Content:

Sickledex; Hgb S test