Alanine aminotransferase (ALT)

What it measures

ALT is an enzyme found mainly inside liver cells. When hepatocytes are injured, ALT leaks into the blood, so it is the most liver-specific of the routine transaminases and a key screen for hepatitis and drug-induced liver injury.

Specimen and preparation

• Specimen: Serum or lithium-heparin plasma
• Tube: Gold top
• Preparation: Avoid haemolysis; strenuous exercise shortly before sampling can raise values.

Reference range

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

• Acute viral hepatitis (often >10× upper limit)
• Drug or toxin injury (e.g. paracetamol overdose)
• Fatty liver disease (MASLD)
• Chronic hepatitis B or C
• Ischaemic hepatitis
• Muscle injury (mild rise)

Causes of low values

• Vitamin B6 (pyridoxal phosphate) deficiency
• Advanced chronic kidney disease

Method

Kinetic UV assay (IFCC, NADH oxidation)

Good to know

An ALT higher than AST suggests viral or fatty liver disease; compare with AST, ALP and bilirubin to classify the injury pattern. For synthetic function see the albumin and PT/INR entries.

Related tests