Gamma-Glutamyl Transferase (GGT) a.k.a. Gamma-Glutamyl Transpeptidase (GGTP) is an enzyme which plays a rule in the process of transferring amino acids and peptides among the cellular membrane. GGT may also play a rule in glutathione metabolism. GTT is exits in the liver and biliary tract at very high concentrations. GTT is is a sensitive indicator of Hepatobiliary disease. GGT test is used to diagnose several liver diseases and as an indicator of heavy alcohol consumption.
GGT is also found in the brain, heart, kidneys, spleen, intestine, and prostate. Very small amounts of GGT may also exist in the endothelial cells of capillaries. Gamma-Glutamyl Transpeptidase concentrations in the liver are normally higher than they are in the other organs. Since GGT is found in the prostate gland, males tend to have higher GGT levels than females. GGT levels in females also decrease after age of 45 years.
GGT test is used to detect liver cell dysfunction, and it is highly accurate in indicating even the slightest degree of cholestasis. Compared to other liver enzymes, GGT is acutally the most sensitive when it comes to detecting liver diseases such like biliary obstruction, cholangitis, or cholecystitis. As with leucine aminopeptidase and 5’-nucleotidase, the elevation of GGT generally parallels that of alkaline phosphatase; however, GGT is more sensitive. Also, as with 5’-nucleotidase and leucine aminopeptidase, GGT is not increased in bone diseases as is alkaline phosphatase. A normal GGT level with an elevated alkaline phosphatase level would imply skeletal disease. An elevated GGT and elevated alkaline phosphatase level would imply hepatobiliary disease. GGT is also not elevated in childhood or pregnancy as alkaline phosphatase (ALP) usually is.
GGT test can also used to indicate occurrence of chronic alcohol ingestion. It is, therefore, very useful in the screening and evaluation of alcoholic patients. GGT is elevated in approximately 75% of patients who chronically drink alcohol.
Why this enzyme is elevated after an acute myocardial infarction (AMI) is not clear. It may represent the associated hepatic insult (if elevation occurs in the first 7 days) or the proliferation of capillary endothelial cells in the granulation tissue that replaces the infarcted myocardium. The elevation usually occurs 1 to 2 weeks after infarction.
Causes of False GGT Test Indications
- Values may be decreased in late pregnancy.
- Alcohol Consumption may increase GGT levels.
- Drugs that may cause increased GGT levels include Phenobarbital and Phenytoin (Dilantin).
- Drugs that may cause decreased GGT levels include Clofibrate and Oral contraceptives.
Normal GGT Levels
Newborns normally have the highest Gamma-Glutamyl Transferase levels. Normal GGT levels for newborns are five times the normal GGT levels of children and adults. Normal GGT levels tend to be decreased in females above 45 years of age and increase in the elderly. The following are the normal GGT levels in adults and females over 45 years:
Children and Adults: 8 to 38 units/L.
Females < 45 years: 5-27 units/L.
Causes of High GGT Levels
Liver and biliary cells contain GGT in very high concentrations. When injured or diseased, these cells lyse and the GGT leaks into the bloodstream. High GGT levels are associated with Liver Diseases including:
- Hepatic Necrosis.
- Hepatic Tumor Or Metastasis.
- The use of Hepatotoxic Drugs.
Pancreatic cells also contain GGT. When injured or diseased, these cells lyse and the GGT leaks into the bloodstream causing high GGT levels. High GGT levels are associated with Pancreatic Diseases including Pancreatitis and Cancer of the Pancreas.
Myocardial Infarction (MI): The pathophysiology behind the relationship between high GGT and Myocardial Infection is not understood. It is thought that the hepatic insult or the proliferation of capillary endothelial cells in the granulation tissue (which contains low amounts of GGT) may increase GGT levels as they replace the infarcted myocardium cells.
Alcohol ingestion: The pathophysiology is not clear. It may be associated with hepatic insult.
Pancreatic Diseases (e.g., Pancreatitis, Cancer of the Pancreas): Pancreatic cells contain GGT. When injured or diseased, these cells lyse and the GGT leaks into the bloodstream.
Epstein-Barr Girus (EBV) (Infectious Mononucleosis), Cytomegalovirus Infections, and Reye Syndrome: The pathophysiology is not clear. It may be associated with subclinical hepatitis that can occur with these infections.