Alternative names 
Galactosemia screen; GALT; Galactose-1-phosphate uridyltransferase

This is a blood test that measures the level of GALT, an enzyme involved in breakdown of milk sugars (lactose/galactose).

How the test is performed

Blood is drawn from a vein, usually on the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

For an infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip of paper, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

How to prepare for the test
No special preparation is necessary.

For infants and children:
The preparation you can provide for this test depends on your child’s age, previous experience, and level of trust. For specific information regarding how you can prepare your child, see the following topics:

  • infant test or procedure preparation (birth to 1 year)  
  • toddler test or procedure preparation (1 to 3 years)  
  • preschooler test or procedure preparation (3 to 6 years)  
  • schoolage test or procedure preparation (6 to 12 years)  
  • adolescent test or procedure preparation (12 to 18 years)

How the test will feel
When the skin is pricked with the needle, the amount of discomfort varies. Some people experience moderate pain while others feel only a prick or stinging sensation. Afterward, the site may be bruised for a few days.

Why the test is performed

This is a screening test for galactosemia.

In normal diets, the major source of galactose is in the metabolism of lactose. The major source of dietary lactose is milk and dairy products. In most people, galactose is converted to glucose by a series of chemical reactions. Each chemical reaction is catalyzed by an enzyme.

About 1 out of 65,000 newborns has a deficiency of the GALT enzyme (deficiencies of the other enzymes are much less common). As a consequence, galactose accumulates in the blood (galactosemia) and tissues because it cannot be metabolized.

Because milk is an important part of the diet of most newborns, these infants develop fluid imbalance and failure to thrive. Continued use of galactose can also result in cataracts, jaundice, liver enlargement, cirrhosis, and mental retardation.

Normal Values

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.

What abnormal results mean

  • galactosemia is indicated by a significantly reduced level of activity.  
  • further tests should be done to confirm the diagnosis of galactosemia  
  • a specialist in biochemical genetics should be consulted promptly  
  • the child should immediately be placed on a no-milk diet (no breast milk, no animal milk)

What the risks are
Risks are associated with venipuncture:

  • multiple needle sticks may be performed if the first one ‘misses’ a vein  
  • excessive bleeding  
  • hematoma (blood collection under the skin)  
  • infection (a risk any time the skin is broken)

Special considerations
Although the disease can be treated by removing milk from the diet, early recognition is essential. Some states, in fact, require that all newborns be screened for this disorder. These screening tests are set to be very sensitive so as not to miss many infants with galactosemia. Therefore, screening tests will come back abnormal but follow-up testing to confirm the abnormal result show the baby to be normal in many cases.

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.

Johns Hopkins patient information

Last revised: December 4, 2012
by Amalia K. Gagarina, M.S., R.D.

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