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The Test Sample
What is being tested?
Gamma-glutamyl transferase (GGT) is an enzyme that is found in many organs, such as the kidney, liver, gallbladder, spleen, and pancreas. Among these, the liver is the main source of GGT in the blood. This test measures the level of GGT in a blood sample.
GGT is increased in most diseases that cause damage to the liver or bile ducts. Normally, GGT is present in low levels, but when the liver is injured, the GGT level can rise. GGT is usually the first liver enzyme to rise in the blood when any of the bile ducts that carry bile from the liver to the intestines become obstructed, for example, by tumors or stones. This makes it the most sensitive liver enzyme test for detecting bile duct problems.
However, the GGT test is not very specific and is not useful in differentiating between various causes of liver damage because it can be elevated with many types of liver diseases, such as cancer and viral hepatitis as well as other non-hepatic conditions such as acute coronary syndrome.
For this reason, use of GGT is controversial, and guidelines published by the National Academy of Clinical Biochemistry and the American Association for the Study of Liver Diseases do not recommend routine use of GGT. These guidelines do suggest, however, that it can be useful in determining the cause of a high alkaline phosphatase (ALP) level, another enzyme found in the liver.
Both GGT and ALP are increased in liver diseases, but only ALP will be increased with diseases affecting bone tissue. Therefore, GGT can be used as a follow up to an elevated ALP to help determine if the high ALP result is due to liver or bone disease.
GGT levels are sometimes increased with consumption of even small amounts of alcohol. Higher levels are found more commonly in chronic heavy drinkers than in people who consume less than 2 to 3 drinks per day or who only drink heavily on occasion (binge drinkers). The GGT test may be used in evaluating someone for acute or chronic alcohol abuse.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
GGT levels fall after meals. You may be instructed to fast (have nothing to eat or drink except water) for at least 8 hours prior to the test. Alcohol and certain prescription medications can affect GGT levels, so you may be asked to abstain from them prior to the test as well.