Alanine aminotransferase (ALT or SGPT) is an enzyme (protein) found mostly in the liver.
A low level of ALT in the blood has no clinical significance. An abnormally high level generally indicates that the liver cells are damaged and releasing their contents into the bloodstream. High ALT levels (300 units per litre (U/L) or more) are often caused by acute viral hepatitis. These levels generally go back to normal within 1 to 2 months, or in exceptional cases within 3 to 6 months. Extremely high ALT levels (often 3,000 U/L or more) indicate toxic impairment of the liver by a drug or poison, or a condition that decreases blood flow through the liver, destroying cells (ischemia). Moderately high levels (less than 4 times the normal) are found in most other liver diseases (such as chronic hepatitis, cirrhosis, bile duct obstruction, liver cancer) and a few muscular diseases, including myocardial infarction (heart attack).
The ALT test is more sensitive than the AST test in detecting mild liver impairments. In most liver diseases, except for chronic alcoholic hepatitis, ALT levels are higher than AST levels.