Hgb S test

Alternative names

Sickledex; 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 (venipuncture), 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 airtight 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.

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 to prepare for the test

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:

• Infant test or procedure preparation (birth to 1 year)
  
• Toddler test or procedure preparation (1 to 3 years)
  
• Preschooler test or procedure preparation (3 to 6 years)
  
• Schoolage test or procedure preparation (6 to 12 years)
  
• Adolescent test or procedure preparation (12 to 18 years)
  

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 (homozygous for hemoglobin S) and sickle cell trait (heterozygous for Hgb S) can be detected by this test. 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.