Hgb S test

Alternative names
Sickledex; Sickle cell test

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.

The routine peripheralblood 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 for the Sickledex, though it will be abnormal also in patients with sickle trait.

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.

Johns Hopkins patient information

Last revised: December 5, 2012
by David A. Scott, M.D.

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