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
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.
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
- See whether a baby has Tay-Sachs
- 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
- 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
- Put the needle into the vein. More than one needle stick
may be needed.
- Attach a tube to the needle to fill it with
- Remove the band from your arm when enough blood is
- 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