ALT (Alanine Transaminase)

ALT (Alanine Transaminase, also written ALAT or SGPT) is an enzyme concentrated in the liver. When liver cells are damaged, ALT leaks into the bloodstream, so it is the single most liver-specific enzyme on a standard Liver Function Test (LFT). It is often interpreted alongside AST, GGT and ALP to build a picture of liver health.

ALT is the first enzyme to rise in hepatocellular liver injury — including non-alcoholic fatty liver disease (NAFLD), viral hepatitis, drug-induced liver injury and alcohol-related liver damage. The AST/ALT ratio is particularly useful: a ratio below 1 typically suggests NAFLD, while a ratio above 2 is characteristic of alcoholic liver disease. In Australia, ALT is part of every routine LFT requested by GPs and specialists.

High ALT (above 40 U/L in men or 35 U/L in women) is the strongest blood-test signal of hepatocellular liver injury. The most common causes are non-alcoholic fatty liver disease (NAFLD) — now affecting roughly 1 in 3 Australian adults — alcohol-related liver damage, chronic viral hepatitis (B or C), and drug-induced liver injury (paracetamol, statins, antibiotics, anabolic steroids, some supplements). Mildly elevated ALT (40–100 U/L) is most often metabolic in origin, while ALT above 1,000 U/L typically indicates acute hepatitis, ischaemic liver injury, or paracetamol toxicity. Strenuous exercise, recent muscle injury and obesity can also produce mild elevations. The AST/ALT ratio helps narrow the cause: ratio <1 suggests NAFLD, ratio >2 with elevated GGT suggests alcohol-related disease.

Low ALT (below the reference range) is uncommon and rarely clinically significant on its own. It can occur with vitamin B6 (pyridoxine) deficiency, in some elderly or chronically malnourished patients, and occasionally with chronic kidney disease. A persistently low ALT in someone with metabolic risk factors does not rule out fatty liver, since ALT can sit within the normal range even when imaging shows steatosis.

In Australia, ALT is included in every routine Liver Function Test (LFT) ordered by a GP and is typically repeated annually as part of a general health check, every 6 months if metabolic risk factors are present (obesity, type 2 diabetes, hypertension), and every 3–6 months for anyone on hepatotoxic medication, anabolic steroids or testosterone replacement therapy. People with known fatty liver, hepatitis or elevated baseline ALT should follow their specialist's monitoring schedule.

Always interpreted alongside AST, GGT, ALP, total bilirubin and albumin (the standard Australian LFT panel). The AST/ALT ratio (De Ritis ratio) is particularly useful for distinguishing NAFLD from alcohol-related liver disease.