Gamma-glutamyl transpeptidase (GGT) is an enzyme found in many body tissues, the most notable being the liver. A GGT test may be used to determine the cause of elevated alkaline phosphatase (ALP). GGT and ALP are both elevated in disease of the bile ducts and in some liver diseases, but only ALP will be elevated in bone disease. Comparing the two test results can help determine if liver or bone disease is the cause of abnormal enzyme levels. For this reason, a GGT test is often run alongside other tests when a liver issue is suspected, such as: ALP, ALT, AST, and bilirubin tests, to differentiate between liver disease, bile duct disorders, and bone disease. Some signs and symptoms of liver damage include:
- Weakness, fatigue
- Loss of appetite
- Nausea and vomiting
- Abdominal swelling / pain
- Jaundice (yellowing of the skin / eyes)
- Dark urine, light-colored stool
Alcohol abuse is also known to affect GGT levels; therefore, GGT tests are sometimes ordered to monitor alcohol-dependency treatments and compliance with treatment programs in those with a history of alcohol abuse.
Decreased GGT levels are associated with vitamin B6 and magnesium deficiency.
A low or normal GGT test indicates that liver damage in unlikely or the person hasn’t consumed any alcohol recently.
It is worth noting that use of clofibrate and oral contraceptives can decease GGT levels.
An elevated GGT level indicates that something is damaging the liver but does not indicate specifically what. Generally, the higher the GGT level the greater the damage to the liver. Therefore, a high GGT level typically warrants further testing. Elevated levels may be due to liver diseases (such as hepatitis or cirrhosis), or they may be due to other conditions (such as congestive heart failure, diabetes, or pancreatitis). As mentioned above, alcohol abuse and smoking can also cause elevated GGT, as well as drugs that are toxic to the liver, including: phenytoin, carbamazepine, and barbiturates. In addition, use of many other prescription and non-prescription drugs, including NSAIDs, lipid-lowering drugs, antibiotics, stomach acid reducers, antifungal agents, antidepressants, and testosterone can increase GGT levels.
It is worth noting that GGT levels increase with age in women, but not in men, and are always somewhat higher in men than in women.
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