Sickle Cell Test

Test Overview

A sickle cell test is a blood test done to screen for sickle cell trait or sickle cell disease. Sickle cell disease is an inherited blood disease that causes red blood cells to be deformed (sickle-shaped). The red blood cells deform because they contain an abnormal type of hemoglobin, called hemoglobin S, instead of the normal hemoglobin, called hemoglobin A.

Sickled blood cells are destroyed by the body faster than normal blood cells. This causes anemia. Also, sickled cells can get trapped in blood vessels and reduce or block blood flow. This can damage organs, muscles, and bones and may lead to life-threatening conditions.

The best way to screen for sickle cell trait or sickle cell disease is to examine the blood using a method called high-performance liquid chromatography (HPLC). This test identifies which type of hemoglobin is present. To confirm the results of HPLC, a genetic test may be done.

Sickle cell disease is an autosomal recessive disease. This means that to have the disease, a person must inherit a gene for the disease from both parents. Each person inherits two genes (one from each parent). As a result, a person may have:

• Two genes that make normal hemoglobin (hemoglobin A). These people have normal red blood cells, unless they have some other disease.
• One gene that makes hemoglobin A and one that makes hemoglobin S. These people carry the sickle cell trait (and are called "a carrier"), but they do not have sickle cell disease. Sickle cell trait is usually a harmless condition.
• Two genes that make hemoglobin S. These people have sickle cell disease. Both parents either carry the sickle cell trait or have the disease. Sickled red blood cells often cause recurring health problems called sickle cell crises.
• One gene that makes hemoglobin S and one that makes some other abnormal type of hemoglobin. Depending on the other type of abnormal hemoglobin, these people may have mild or severe sickle cell disorder.

The United States Preventive Services Task Force recommends that all newborns be tested for sickle cell disease.¹

Why It Is Done

A sickle cell test screens for sickle cell trait or sickle cell disease in people at high risk. Detecting sickle cell trait is important for couples who want to have children and who may be carriers of sickle cell trait.

How To Prepare

Be sure to tell your health professional if you have had a blood transfusion in the past 4 months because it can interfere with the test results.

How It Is Done

Blood sample from a vein

The health professional drawing blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure to the site and then a bandage.

Blood sample from a heel stick

During newborn testing, the blood sample is usually taken from your baby's heel (called a heel stick).

• Your baby's heel is cleaned with alcohol and then the heel is poked with a small needle.
• Several drops of blood are collected inside circles on a special piece of paper.
• When enough blood has been collected, a small bandage is put on the site.

How It Feels

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

Your baby may feel a sting or a pinch with a heel stick.

Risks

Blood sample from a vein

There is very little chance of a problem from having blood sample taken from a vein.

• You may get a small bruise at the site. You can lower the chance of bruising by keeping pressure on the site for several minutes.
• In rare cases, the vein may become swollen after the blood sample is taken. This problem is called phlebitis. A warm compress can be used several times a day to treat this.
• Ongoing bleeding can be a problem for people with bleeding disorders. Aspirin, warfarin (Coumadin), and other blood-thinning medicines can make bleeding more likely. If you have bleeding or clotting problems, or if you take blood-thinning medicine, tell your doctor before your blood sample is taken.

Blood sample from a heel stick

Usually, there are no problems from a heel stick. A small bruise may develop. Babies with bleeding problems may bleed more with usual. Sometimes bleeding problems are found when blood is being collected for the sickle cell test.

Results

A sickle cell test is a blood test done to screen for sickle cell trait or sickle cell disease.

In babies, a sickle cell blood test may be repeated at 6 months old, or a genetic information (DNA) test may be done.

What Affects the Test

Having a blood transfusion in the past 4 months can cause a false-negative test result because of the normal hemoglobin from the blood donor.

What To Think About

• Most states routinely do a sickle cell blood test on all newborns.
• If you have a family history of sickle cell disease, you may be advised to have a blood test to determine whether you carry the sickle cell trait. If you have sickle cell trait or sickle cell disease, you may choose genetic counseling before deciding to have children.
• Testing is available to check for sickle cell disease in an unborn baby (fetus). This can be done through amniocentesis or chorionic villus sampling (CVS). For more information, see the medical tests Amniocentesis and Chorionic Villus Sampling (CVS).
• Babies under 6 months of age may have false-negative results because they have more hemoglobin F (fetal hemoglobin) in their blood.
• In the United States, sickle cell disease mainly affects African Americans and Latin Americans.

References

Citations

1. U.S. Preventive Services Task Force (2007). Screening for sickle cell disease in newborns. Available online: http://www.ahrq.gov/clinic/uspstf/uspshemo.htm.

Other Works Consulted

• Chernecky CC, Berger BJ (2008). Laboratory Tests and Diagnostic Procedures, 5th ed. St. Louis: Saunders.
• Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.

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