Serum aspartate aminotransferase (AST; formerly SGOT) is an enzyme found in several organs and tissues, including liver, heart, skeletal muscle, and RBCs. AST elevation from nonhepatic sources is discussed elsewhere.

AST elevation originating from the liver is due to some degree of acute liver cell injury. Following onset of acute hepatocellular damage from any etiology, AST is released from damaged cells. The serum level becomes elevated in approximately 8 hours, reaches a peak in 24-36 hours, and returns to normal in 3-6 days if the episode is short lived. In mild injury, serum levels may be only transiently and minimally elevated or may even remain within reference limits. In acute hepatitis virus, AST levels frequently become elevated more than 10 times the upper reference range limit (about 75% of patients in one study and 100% in another) and typically rise more than 20 times the upper limit (about 45% of patients in the first study and 90% in the second). In fact, a serum AST more than 20 times normal usually includes acute hepatitis virus infection in the differential diagnosis. However, 1-2 weeks later the values fall toward normal, so that a test sample drawn in the subsiding phase may show moderate or possibly only mild abnormality. In extrahepatic obstruction there usually is no elevation unless secondary parenchymal acute damage is present; when elevations occur, they are usually only mild to moderate (<10 times the upper reference limit). However, when extrahepatic obstruction occurs acutely, AST values may quickly rise to values more than 10 times normal, then fall swiftly after about 72 hours. In cirrhosis, whether the AST level is abnormal and (if abnormal) the degree of abnormality seems to depend on the degree of active hepatic cell injury taking place. Inactive cirrhosis usually is associated with normal AST levels. In active alcoholic cirrhosis, AST elevation is most often mild to moderate, with the majority of AST values less than 5 times the upper range limit and over 95% of AST values less than 10 times normal. In active chronic hepatitis virus hepatitis, AST values are also usually less than 10 times normal. However, one group reported that about 15% of their patients had at some time values more than 10 times normal. However, some of these patients could have superimposed acute infection by non-A, non-B or delta hepatitis virus. In liver passive congestion, AST levels are elevated in 5%-33% of patients. About 80% of these patients have AST elevations less than 3 times normal. In severe acute congestive failure, liver hypoxia may be severe, and one study estimated that about 50% of these patients have AST elevation. In some of these patients AST levels may be higher than 3 times normal and occasionally may even exceed 20 times normal (about 1% of patients with substantial degrees of congestive failure). In some cases AST elevation due to acute myocardial infarction adds to AST arising from the liver. In infectious mononucleosis, AST levels are elevated in 88%-95% of patients, but only about 5% of elevations are greater than 10 times normal and about 2% are more than 20 times normal.

Some Etiologies for Aspartate Aminotransferase Elevation
Heart
Acute myocardial infarct
Pericarditis (active: some cases)
Liver
Hepatitis virus, Epstein-Barr, or cytomegalovirus infection
Active cirrhosis
Liver passive congestion or hypoxia
Alcohol or drug-induced liver dysfunction
Space-occupying lesions (active)
Fatty liver (severe)
Extrahepatic biliary obstruction (early)
Drug-induced
Skeletal Muscle
Acute skeletal muscle injury
Muscle inflammation (infectious or noninfectious)
Muscular dystrophy (active)
Recent surgery
Delirium tremens
Kidney
Acute injury or damage
Renal infarct
Other
Intestinal infarction
Shock
Cholecystitis
Acute pancreatitis
Hypothyroidism
Heparin therapy (60%-80% of cases)

There is a large group of common diseases with mild or moderate AST elevation (defined arbitrarily as elevated less than 10 times the upper reference limit); these include acute hepatitis in the subsiding or recovery phases, chronic hepatitis, active cirrhosis or alcoholic liver disease, liver passive congestion, drug-induced liver dysfunction (including intravenous [IV] or subcutaneous heparin), metastatic liver tumor, long-standing extrahepatic bile duct obstruction, infectious mononucleosis, cytomegalovirus (88%-95%; 2% more than 10 times normal), and fatty liver (40%; rare > 5 x normal). As mentioned earlier, in a few patients with active cirrhosis, liver congestion, infectious mononucleosis, and drug-induced liver injury, the AST may attain levels more than 20 times upper reference limit that are suggestive of acute hepatitis virus infection. AST levels are elevated in approximately 50% of patients with liver metastases, with most elevations less than 5 times the upper reference limit. One report found that obese patients had upper reference limits up to 50% higher than normal-weight persons.

In active alcoholic cirrhosis, liver congestion, and metastatic tumor to the liver, the AST level usually is considerably higher than the alanine aminotransferase (ALT; formerly SGPT) level, with an AST/ALT ratio greater than 1.0. Ratios less than 1.0 (ALT equal to or greater than AST) are the typical finding in acute hepatitis virus hepatitis and infectious mononucleosis. However, about 30% of patients with acute hepatitis virus infection and some patients with infectious mononucleosis have a ratio greater than 1.0; and ratios either greater than or less than 1.0 may occur in patients with AST elevation due to extrahepatic obstruction. The ratio tends to be more variable and less helpful when the AST value is greater than 10 times the upper reference limit. Some prefer to use an AST/ALT ratio of 1.5 or 2.0 rather than 1.0 as the cutoff point (i.e., an AST value more than twice the ALT value is more suggestive of alcoholic active cirrhosis than hepatitis virus hepatitis, especially if the AST value is less than 10 times normal). Although most agree that active alcoholic liver disease usually yields AST values significantly higher than ALT, there is disagreement in the literature on the usefulness of the ratio (especially the 1.0 value) for diagnosis of individual patients.