Tay-Sachs Test

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 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. Your body makes enough hex A so that 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).