The normal range for AST is between 10 and 40 units per liter, and the normal range for ALT is 7 to 56 units per liter, according to eMedicineHealth. AST and ALT measurements are part of a routine liver enzyme test used to diagnose and monitor liver disease, explains WebMD.
AST stands for aspartate aminotransferase, and ALT is alanine aminotransferase, explains the American Academy of Family Physicians. These are enzymes normally found in liver cells and skeletal muscle cells. High amounts of AST and ALT are detected when the liver is damaged and they leak out of the cells into the blood. This frequently occurs in hepatocyte necrosis from acute hepatitis and toxic or ischemic liver injury.
Sometimes, AST and ALT levels are used as a ratio because their proportions have important diagnostic value, explains the Merck Manual. In acute alcoholic hepatitis, for example, the AST level is twice as high as ALT.
Liver injury is typically classified in three disease categories: hepatocellular, in which the injury involves liver cells; cholestatic, in which the disease involves the biliary system; and infiltrative, which involves amyloid replacement of liver cells and tumors, according to the Merck Manual. AST and ALT can be combined with a test for alkaline phosphate to help differentiate between hepatocellular and cholestatic disease.