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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, 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 using amniocentesis, chorionic villus sampling, or mutation analysis.
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 .
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.
The normal values listed here—called a reference range—are just a guide. These ranges vary from lab to lab, and your lab may have a different range for what's normal. Your lab report should contain the range your lab uses. Also, your doctor will evaluate your results based on your health and other factors. This means that a value that falls outside the normal values listed here may still be normal for you or your lab.
Normal value footnote 1
Amount in blood:
Total hexosaminidase (A+B)
9.8–15.9 units per litre (U/L)
Amount in blood:
7.2–9.8 units per litre (U/L)
- 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 disease.
What Affects the Test
If you had a recent blood transfusion, you may not be able to have the test, or the test results may not be helpful. 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. To learn more, see the topic Genetic Test.
- People who have a high chance of being a carrier of Tay-Sachs may want to have a blood test before they have children, to see whether they are carriers. People of Ashkenazi Jewish, French-Canadian, or Cajun descent, or those 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 counselling is available for people who have the disease or are carriers.
- Before you have testing for a genetic disease such as Tay-Sachs disease, you may want to learn about possible ethical, legal, and religious issues of genetic testing.
- Fischbach F, Dunning MB III (2015). A Manual of Laboratory and Diagnostic Tests, 9th ed. Philadelphia: Wolters Kluwer Health.
Other Works Consulted
- Pagana KD, Pagana TJ (2010). Mosby's Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby Elsevier.
Current as of:
July 17, 2020
Author: Healthwise Staff
Sarah Marshall MD - Family Medicine
Brian D. O'Brien MD - Internal Medicine
Martin J. Gabica MD - Family Medicine
E. Gregory Thompson MD - Internal Medicine
Adam Husney MD - Family Medicine
Kathleen Romito MD - Family Medicine
Siobhan M. Dolan MD, MPH - Reproductive Genetics
Current as of: July 17, 2020
Author: Healthwise Staff
Medical Review:Sarah Marshall MD - Family Medicine & Brian D. O'Brien MD - Internal Medicine & Martin J. Gabica MD - Family Medicine & E. Gregory Thompson MD - Internal Medicine & Adam Husney MD - Family Medicine & Kathleen Romito MD - Family Medicine & Siobhan M. Dolan MD, MPH - Reproductive Genetics
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