GGT present in blood originates primarily from hepatobiliary system. Membrane-bound enzyme involved in glutathione metabolism and amino acid transport. The most sensitive parameter for diseases of the liver and/or biliary tract. Usually the first liver enzyme to rise after bile duct obstruction. Used to confirm hepatic origin of elevated ALP levels Not elevated in bone disease (in contrast to ALP)
Causes of increased blood GGT:
1- Biliary obstruction: GGT is markedly increased with obstructive jaundice (5 – 30 folds) Increase earlier (more sensitive) than ALP-Persists longer than ALP
2- Viral, toxic & alcoholic hepatitis: Increase is only 2 – 5 folds (less sensitive than ALT & AST)
3- Primary and secondary liver tumors: GGT is elevated earlier than other enzymes in liver neoplasm. Secondaries of other organ tumors in the liver can be early detected by elevated GGT. (arouse suspicious that the diseases is metastatic to liver)
4- Induction of GGT synthesis by these cells occurs without cell damage (isolated GGT ↑) by alcohol or drugs as anticonvulsants , phenobarbitone & phenytoin