Galactose-1-phosphate uridyltransferase is a blood test that measures the level of a substance called GALT, which helps break down milk sugars (lactose/galactose).

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

For help preparing your baby for the test, see infant test or procedure preparation (birth to 1 year).

When the needle is inserted to draw blood, some infants feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

This is a screening test for galactosemia.

In normal diets, most galactose comes from the breakdown (metabolism) of lactose, which is found in milk and dairy products. About 1 out of 65,000 newborns lack a substance (enzyme) called GALT. Without this substance, the body can't break down galactose, and the substance builds up in the blood. Continued use of milk products can lead to failure to thrive, as well as cataracts, jaundice, liver enlargement, cirrhosis, and mental retardation.

The normal range is 18.5 to 28.5 U/g Hb (units per gram of hemoglobin).

It is important to note that normal values vary from laboratory to laboratory.

An abnormal result suggests galactosemia. Further tests must be done to confirm the diagnosis.

If your child has galactosemia, a genetics specialist should be consulted promptly. The child should immediately be placed on a no-milk diet. This means no breast milk and no animal milk. Soy milk and infant soy formulas are generally used as substitutes. For more detailed information, see the article on galactosemia.

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

Early diagnosis is important, since the disease can be treated by removing milk from the diet.

Some states require that all newborns be screened for this disorder. These screening tests are set to be very sensitive so they do not miss many infants with galactosemia. Because of this, false-positives can occur. If your child has an abnormal screening result, follow-up tests must be done to confirm the result.

See: Newborn screening tests