Gamma-glutamyltransferase, or GGT, is an enzyme present in your body with the highest concentrations in the liver, pancreas, and kidneys. The GGT test measures the activity of GGT in your body and helps physicians to assess the condition of your liver, to check for bile duct obstructions and to differentiate between bone and liver disease. Sometimes, this test is used as a screening test to detect or monitor alcohol abuse. 

GGT is the first enzyme whose numbers increase when any damage or disease is present in the liver. It can be viral hepatitis, cirrhosis, or tumors. In general, higher GGT levels mean greater damage to the liver. Levels also rise very quickly when any bile ducts that carry bile in and out of the liver are obstructed by either stones or tumors. Elevated levels of GGT can also indicate conditions not originating from the liver, such as congestive heart failure, diabetes, metabolic syndrome, or inflammation of the pancreas. Around 75% of heavy or chronic alcohol drinkers have elevated levels of GGT. The GGT test is so sensitive that it can detect the consumption of small amounts of alcohol, smoking, or certain medications – all of these cause GGT levels to rise.

Since many types of liver issues can cause GGT levels to rise, the test is not considered very specific. It is therefore done together with other liver function tests. The test most often done together with a GGT test is for alkaline phosphatase (ALP). ALP can be elevated due to liver or bone disease, while GGT is elevated only when there are issues with the liver. Therefore the doctor will perform both tests to determine the origin of the damage. When ALP levels are elevated, and GGT levels are in the normal range, it's most likely that the problems are caused because of bone and not liver disease. On the other hand, when both ALP and GGT levels are elevated, it's most likely that the problems originate from the liver. 

While high GGT levels are clinically important and are associated with many medical conditions, low GGT levels only mean that your liver is not suffering any damage and you haven't been consuming any alcohol.