The test for Tay-Sachs disease measures the amount of an enzyme called hexosaminidase A in the blood. Hexosaminidase A breaks down fatty substances in the brain and nerves. Tay-Sachs is an inherited disease in which the body does not break down fatty substances as it should, so the fatty substances collect in the body and damage brain and nerve cells.

Normally a child inherits one chromosome from each parent that tells the body to make hexosaminidase A. If you inherit:

• A pair of chromosomes that does not tell the body to make hexosaminidase A, you have Tay-Sachs disease.
• One chromosome that tells the body to make hexosaminidase A and one that does not, you have the Tay-Sachs trait. This means you are a carrier of Tay-Sachs and can pass the trait on to your children. You do not have the disease because your body still makes enough hexosaminidase A. If both parents are carriers, there is a 1-in-4 chance (25%) that any child they have will have Tay-Sachs disease.

A Tay-Sachs test may also measure the amount of another enzyme, called hexosaminidase B. People who cannot make either hexosaminidase A or B have a rare, more serious form of Tay-Sachs called Sandhoff's disease.

The Tay-Sachs test is usually done on blood taken from a vein or from the umbilical cord right after birth. It can also be done on a sample of amniotic fluid (collected during amniocentesis) or on special cells in the placenta (collected during chorionic villus sampling).

Why It Is Done

A test to measure hexosaminidase A is done to:

• See whether a newborn has Tay-Sachs disease.
• Find carriers of the Tay-Sachs trait. People of Ashkenazi Jewish or French-Canadian descent who have a family history of Tay-Sachs disease or who live in a community or population with a high amount of Tay-Sachs disease need to be tested for Tay-Sachs trait.
• See whether an unborn baby (fetus) has Tay-Sachs disease. This is done early in pregnancy by amniocentesis or chorionic villus sampling.

How To Prepare

You do not need to do anything before having this test. If you are having this test to see whether you carry the Tay-Sachs trait, you should tell your doctor if you have had a blood transfusion in the past 3 months.

Discuss with your doctor any concerns you have about the need for the test, its risks, how it will be done, or what the results may mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

How It Is Done

The health professional taking a sample of your 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 on the site and then put on a bandage.