Hemoglobin S

Hemoglobin is made up of heme (the iron-containing portion of hemoglobin) and globin (amino acid chains that form a protein). Hemoglobin (Hgb) molecules are found in all red blood cells. They bind oxygen in the lungs, carry the oxygen throughout the body, and release it to the body's cells and tissues. Hemoglobin S is the primary hemoglobin in people with sickle cell disease. Approximately 8% of Americans of African descent carry the sickle Hb mutation in one of their two beta genes (0.15% of African Americans have sickle cell disease). The mutation exists in the beta (β) chain gene, so those with Hgb S disease have two beta (βS) chains and two normal alpha (α) chains. The presence of hemoglobin S causes the red blood cell to deform and assume a sickle shape when exposed to decreased amounts of oxygen (such as might happen when someone exercises or in the peripheral circulation). Sickled red blood cells can block small blood vessels, causing pain and impaired circulation, decrease the oxygen-carrying capacity of the red blood cell, and decrease the cell's lifespan. A single beta (β) copy does not cause symptoms unless it is combined with another hemoglobin mutation, such as that causing Hgb C (βC).

Normal hemoglobin types include:

  • Hemoglobin A (about 95% - 98%): Hgb A contains two alpha (α) chains and two beta (β) chains

  • Hgb A2 (2% - 3%): has two alpha (á) and two delta (ä) chains

  • Hgb F (up to 2%): the primary hemoglobin produced by the fetus during gestation; its production usually falls to a low level shortly after birth; Hgb F has two alpha (α) and two gamma (γ) chains

Hemoglobin variants are abnormal forms of hemoglobin that occur when changes (point mutations, deletions) in the globin genes cause changes in the amino acids that make up the globin protein. These changes may affect the structure of the hemoglobin, its behavior, its production rate, and/or its stability. Several hundred hemoglobin variants have been documented; however, only a few are common and clinically significant. The majority of these are beta chain variants.

These variants are inherited in an autosomal recessive fashion. A person inherits one copy of each beta globin gene from each parent. If one normal beta gene and one abnormal beta gene are inherited, the person is said to be a carrier or to be heterozygous for the abnormal hemoglobin. The abnormal gene can be passed on to any offspring but does not cause symptoms or health concerns in the carrier.

If two abnormal beta genes of the same type are inherited, the person is considered to have the disease and is homozygous for the abnormal hemoglobin. A copy of the abnormal beta gene will be passed on to any offspring.

If two abnormal beta genes of different types are inherited, the person is doubly or compound heterozygous. One of the abnormal beta genes will be passed on to each offspring.

MEANING OF TEST

Your caregiver will go over the test results with you and discuss the importance and meaning of your results, as well as treatment options and the need for additional tests if necessary.

OBTAINING THE TEST RESULTS

It is your responsibility to obtain your test results. Ask the lab or department performing the test when and how you will get your results.