Alanine Aminotransferase (ALT)

Reference Ranges

Reference ranges are typical adult values. Your laboratory may use slightly different intervals — always use your lab's reported reference.

What This Test Measures

ALT is an enzyme that catalyses a key step in amino acid metabolism and is highly concentrated in liver cells. When hepatocytes (liver cells) are damaged — by inflammation, medication toxicity, alcohol, or fat accumulation — they release ALT into the circulation. ALT is more liver-specific than AST, which also rises with cardiac or muscle damage.

When Is This Test Ordered?

• Hepatitis B and C screening and monitoring
• Non-alcoholic fatty liver disease (NAFLD) detection
• Medication monitoring (statins, methotrexate, antiepileptics)
• Alcohol use disorder assessment
• Unexplained fatigue or jaundice workup
• Routine liver function as part of a health check

ALT:AST ratio as a diagnostic clue

The ALT:AST ratio helps narrow the cause of liver enzyme elevation. An ALT:AST ratio above 2:1 strongly suggests alcoholic liver disease (because alcohol preferentially damages mitochondria that synthesise AST, and alcohol also depletes pyridoxine needed for ALT production). A ratio below 1:1 (ALT < AST) suggests viral hepatitis with AST rising alongside ALT.

In non-alcoholic fatty liver disease, the ratio is usually ALT > AST early on, but inverts as fibrosis advances — an AST > ALT in previously NAFLD-dominant elevation may signal progression to cirrhosis.

Factors That Can Affect Your Result

• Intense exercise can raise ALT by up to 3× through minor muscle damage
• Body mass index — fatty liver from obesity is the most common cause of chronic mild elevation
• Celiac disease causes ALT elevation through small bowel-derived ALT and liver involvement
• Hypothyroidism can mildly elevate ALT independently of liver disease
• Haemolysed blood samples give falsely elevated ALT

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