Gamma-glutamyl transpeptidase is a test to measure the amount of the enzyme GGT in the blood.

Blood is drawn from a vein 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 band is removed to restore circulation (blood flow). 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, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

The health care provider may advise you to stop taking any drugs that can affect the test (see Special Considerations).

For infants and children:

The preparation you can provide for this test depends on your child's age and experience. For specific information regarding how you can prepare your child, see the following:

• 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)
• School age test or procedure preparation (6 to 12 years)
• Adolescent test or procedure preparation (12 to 18 years)

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

This test is used to detect diseases of the liver, bile ducts, and kidney. It is also used to differentiate liver or bile duct disorders from bone disease.

GGT participates in the transfer of amino acids across the cell membrane, and in glutathione (an anti-oxidant) metabolism. High concentrations of GGT are found in the liver, bile ducts, and the kidney.

GGT is measured in combination with other tests. In particular, the enzyme ALP is increased in liver and bile duct disease as well as in bone disease. GGT is elevated in liver and bile duct disease, but not in bone disease. So, a patient with an elevated ALP and a normal GGT probably has bone disease, not liver or bile ducts disease.

The normal range is 0 to 51 IU/L.

Note: IU/L = international units per liter

Greater-than-normal levels of GGT may indicate:

• Congestive heart failure
• Cholestasis (congestion of the bile ducts)
• Cirrhosis
• Hepatitis
• Liver ischemia (blood flow deficiency)
• Liver necrosis
• Liver tumor
• Use of hepatotoxic drugs (drugs toxic to liver)

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (bleeding under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Drugs that can increase GGT levels include alcohol, phenytoin, and phenobarbital.

Drugs that can decrease GGT levels include clofibrate and oral contraceptives (birth control pills).

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.