Test Overview

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 nerve cells of the brain and damage them.

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

• If the baby gets the changed gene from both parents, he or she will get the disease.
• If the baby gets the changed 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 changed 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 condition called Sandhoff's disease.

The Tay-Sachs enzyme test is usually done on blood taken from a vein or from the umbilical cord right after birth.

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 descent, who have family from the Quebec Bas-St-Laurent or Gaspésie regions or New Brunswick territories, or who have a family history of the disease, or who live in a community or population with a high amount of Tay-Sachs disease may want to be tested. ^{footnote 1}
• See whether an unborn baby (fetus) has Tay-Sachs disease. This is done early in pregnancy using 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.

How It Is Done

A health professional uses a needle to take a blood sample, usually from the arm.

Watch

How It Feels

When a blood sample is taken, you may feel nothing at all from the needle. Or you might feel a quick sting or pinch.

Risks

There is very little chance of having a problem from this test. When a blood sample is taken, a small bruise may form at the site.

Results

Each lab has a different range for what's normal. Your lab report should show the range that your lab uses for each test. The normal range is just a guide. Your doctor will also look at your results based on your age, health, and other factors. A value that isn't in the normal range may still be normal for you.

• A person who has about half the normal amount of hex A is a Tay-Sachs carrier.
• A person who does not have any hex A has Tay-Sachs disease.
• In rare cases, a person may not have either the hex A or hex B enzyme. This causes a condition called Sandhoff's disease.