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 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 gene from each parent that tells the body to make hex A. If you inherit:
- A pair of genes that does not tell the body to make hex A, you have Tay-Sachs disease.
- One gene that tells the body to make hex A and one that does not, you are a Tay-Sachs carrier. Because your body still makes enough hex A, you do not have the disease. But you can pass the gene to your children. 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 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 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 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.
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.
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.
There is very little chance of a problem from having a 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.
Amount in blood
Total hexosaminidase (A+B):
9.9–15.9 units per liter (U/L)
Amount in blood
7.5–9.8 units per liter (U/L)
- Normal values vary depending on the laboratory and test method used. Check with your doctor for normal values.
- 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 hex A or hex B enzyme. This causes a more severe condition called Sandhoff's disease.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
- Having a recent blood transfusion. If you have a blood transfusion from a blood donor who has normal levels of hexosaminidase A, your level may temporarily be higher than usual.
What To Think About
- A positive Tay-Sachs test may need to be confirmed with other genetic tests. For more information, see the medical test Genetic Test.
- If you are pregnant or use birth control pills, your doctor would test the hexosaminidase A (hex A) amount in your white blood cells.
- People who have a high chance of being a carrier of Tay-Sachs may want to have a blood test to see whether they are carriers before they have children. 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 may want to be tested. Genetic counseling is available for people who have the disease or are carriers.
- If the parents of an unborn baby
(fetus) are Tay-Sachs carriers, the following tests
can be done to see if the baby has the disease.
- An amniocentesis checks a sample of amniotic fluid and is usually done between the 14th and 18th weeks of pregnancy. For more information, see the medical test Amniocentesis.
- Chorionic villus sampling checks special cells in the placenta and can be done between the 9th and 12th weeks of pregnancy. For more information, see the medical test Chorionic Villus Sampling.
Other Works Consulted
- Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.
- Pagana KD, Pagana TJ (2006). Mosby’s Manual of Diagnostic and Laboratory Tests, 3rd ed. St. Louis: Mosby.
|Author||Debby Golonka, MPH|
|Editor||Susan Van Houten, RN, BSN, MBA|
|Associate Editor||Tracy Landauer|
|Primary Medical Reviewer||Michael J. Sexton, MD - Pediatrics|
|Specialist Medical Reviewer||Siobhan M. Dolan, MD, MPH - Reproductive Genetics|
|Last Updated||March 31, 2008|