Health & Baby

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

Tay-Sachs can occur when parents pass on a changed gene to their child.

• If the baby gets the gene from both parents, he or she will get the disease.
• If the baby gets the gene from only one parent, he or she will be a carrier. This means that the child will have one gene that produces hex A and one that doesn't. The child's body makes enough hex A so that he or she doesn't get the disease. But the child can pass the gene on to his or her children.

A Tay-Sachs test may also measure the amount of another enzyme, called hexosaminidase B. People who cannot make either hex A or B have a rare, more serious condition 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 cells from the placenta (collected during chorionic villus sampling).

Why It Is Done

A test to measure hexosaminidase A is done to:

• See whether a baby has Tay-Sachs disease.
• Find Tay-Sachs carriers. People of Ashkenazi Jewish, French-Canadian, or Cajun 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 may want to be tested.
• 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 are a Tay-Sachs carrier, you should tell your doctor if you have had a blood transfusion in the past 3 months.

Talk to your doctor about 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.