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

Blood is typically 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.

The health care provider may advise you to stop taking any drugs that can affect the test.

• Drugs that can increase GGT levels include alcohol, phenytoin, and phenobarbital.
• Drugs that can decrease GGT levels include clofibrate and birth control pills.

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 or bile ducts. It is also done along with other tests, such as the ALP test, to tell the difference between liver or bile duct disorders and bone disease.

Alkaline phosphatase (ALP) is increased in liver and bile duct disease as well as in bone disease. GGT is only increased 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 international units per liter (IU/L).

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

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)

There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood 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)

Berk PD, Korenblat KM. Approach to the patient with jaundice or abnormal liver test results. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 150.