Tay-Sachs Test
Test OverviewThe 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 DoneA 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 or chorionic villus sampling.
How To PrepareYou 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 DoneThe 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 FeelsThe 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. RisksThere 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.
ResultsThe test for
Tay-Sachs disease measures the amount of an
enzyme called hexosaminidase A (hex A) in the
blood. 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. - 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.
What Affects the TestIf 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 counseling 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.
ReferencesCitations- 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.
CreditsByHealthwise Staff
Primary Medical ReviewerSarah Marshall, MD - Family Medicine
Martin J. Gabica, MD - Family Medicine
E. Gregory Thompson, MD - Internal Medicine
Specialist Medical ReviewerSiobhan M. Dolan, MD, MPH - Reproductive Genetics Current as ofOctober 13, 2016 Current as of:
October 13, 2016 Fischbach F, Dunning MB III (2015). A Manual of Laboratory and Diagnostic Tests, 9th ed. Philadelphia: Wolters Kluwer Health. Last modified on: 8 September 2017
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