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Sickle Cell Tests

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Also known as: Hemoglobin S; Hb S; Hgb S
Formal name: Hemoglobin S Evaluation
Related tests: Hemoglobin Electrophoresis; Hemoglobin Isoelectric Focusing; Hemoglobin S Solubility; Hemoglobin Fractionation by HPLC; Hemoglobinopathy Evaluation; Complete Blood Count; Blood Smear; Iron Tests

The Test Sample

What is being tested?

Sickle cell tests are used to help diagnose sickle cell anemia (also called sickle cell disease) and to identify those who may have sickle cell trait. Sickle cell anemia is an inherited disorder that leads to the production of an abnormal hemoglobin called hemoglobin S (Hb S or Hgb S). Sickle cell tests determine the presence and relative amount of hemoglobin S in a blood sample.

Hemoglobin is a protein found in red blood cells (RBCs) that binds to oxygen in the lungs and carries it to tissues throughout the body. Typically, hemoglobin A (Hb A, adult hemoglobin) makes up most of the hemoglobin found in normal RBCs in adults, with small amounts of hemoglobin A2 and hemoglobin F. Before babies are born, they normally produce large amounts of hemoglobin F (Hb F, fetal hemoglobin), which is then replaced by Hb A as the predominant hemoglobin shortly after birth.

Mutations in the genes that code for the production of hemoglobin can lead to abnormal types of hemoglobin (variants). Common mutations include the mutations causing beta thalassemia, a blood disorder that results in reduced production of hemoglobin, and mutations associated with hemoglobin variants such as Hb S and hemoglobin C (Hb C). With a normal hemoglobin gene copy from one parent and a Hb S gene copy from the other parent (heterozygous), a person is said to have sickle cell trait and to be a sickle cell carrier. When someone has two Hb S gene copies (one from each parent; homozygous), then the person has sickle cell anemia (disease). If the person has one Hb S gene and one other abnormal gene, such as a Hb C gene, then the person will experience some of the same symptoms associated with sickle cell disease.

Blood smear showing sickled red blood cells

Hb S can form crystals that change the shape of the RBC from a round disc to a characteristic sickle shape. This altered shape limits the RBC's ability to flow smoothly throughout the blood vessels in the body, limits the hemoglobin's ability to transport oxygen to tissues, and decreases RBC lifespan from 120 days to about 10-20 days. A person with sickle cell disease (homozygous for Hb S) can become severely anemic because the body cannot produce RBCs as fast as they are destroyed. The affected person can suffer painful episodes and a variety of complications when sickled cells become lodged in and obstruct small blood vessels (vaso-occlusion).

Sickle cell tests are done to determine whether someone is producing hemoglobin S and thus carrying a sickle gene. They are ordered routinely as part of newborn screening programs and are mandated by every state in the U.S. and the District of Columbia. If results of a newborn screen are abnormal, then one or more sickle cell tests may be ordered to confirm abnormal findings. Sickle cell tests may also be ordered along with or following an abnormal complete blood count (CBC) and blood smear to help evaluate someone who has an unexplained hemolytic anemia or who demonstrates symptoms that suggest the presence of sickle cell anemia.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm or, for infants, by pricking a heel or finger.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed; however, testing for diagnosis should not be performed after a recent blood transfusion.