Liver Function Tests Explained: ALT, AST, ALP, GGT, and Bilirubin
Mild elevations in liver enzymes are common and rarely dangerous — but they should never be ignored. Here is how clinicians think about them.
Hepatocyte injury: ALT and AST
ALT (alanine aminotransferase) is the most liver-specific enzyme; AST (aspartate aminotransferase) is also found in muscle and heart. Both rise when hepatocytes are injured — from fatty liver, viral hepatitis, alcohol, medication toxicity, or ischemia.
Pattern matters. ALT > AST is typical of non-alcoholic fatty liver and viral hepatitis. AST > ALT (ratio > 2) suggests alcohol-related injury. Very high values (thousands) suggest acute hepatitis, toxin exposure, or ischemic injury and warrant urgent evaluation.
Cholestasis: ALP and GGT
Alkaline phosphatase (ALP) and gamma-glutamyl transferase (GGT) rise together when there is impaired bile flow — gallstones, biliary strictures, medications, or primary biliary cholangitis. ALP alone (with normal GGT) usually points to bone rather than liver.
Bilirubin and albumin
Total bilirubin rises with cholestasis, hemolysis, or reduced hepatic uptake (Gilbert syndrome, a benign inherited variant, is common and harmless). Albumin and INR reflect the liver's synthetic function — low albumin or elevated INR suggests more advanced disease.
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Frequently asked questions
Do I need to fast for liver tests?+
Not required for accuracy, but usually done at the same time as fasting panels for convenience.
Can exercise raise my AST?+
Yes — intense exercise within 48 hours can transiently raise AST and CK. Rest before repeat testing.