GPT and Biliary tract obstruction: GGT levels increase markedly in the setting of bile duct obstruction, and the ratio of alanine aminotransferase (ALT) to GGT can guide the clinician in deciding between obstructive jaundice (low ALT/medium-high GGT) versus hepatitis (high ALT/low-medium GGT) during the investigation of a patient with jaundice [32].