In medicine, various kinds of biochemical markers are used for diagnosis. Enzymes play a major role out of those biochemical markers. The type of enzyme also varies according to the disease and the organ involved. AST and ALT are two of such enzymes that are used to assess the liver function of an individual. The rise in AST and ALT levels varies according to the disease. Therefore, AST and ALT levels also give a clue as to the diagnosis as well as to the etiology. Both these enzymes are found in liver parenchyma. So, when there is liver parenchymal damage, the levels increase.
ALT, short for alanine transaminase, is a liver enzyme. It is also called serum glutamic pyruvic transaminase (SGPT). Even though it is widely measured to diagnose liver diseases, it is not specific to the liver. There are other organs in the body that also produce this enzyme. Therefore, it is measured along with AST to increase the specificity. If both enzymes are elevated, there is a high chance of the cause being liver disease. The reference range of ALT is 10-40 U/L.
AST is short for aspartate transaminase. It is also called serum glutamic oxaloacetic transaminase (SGOT). It is a liver enzyme that is not specific to the liver. It is also produced by the liver, heart, skeletal muscles, kidney and brain. AST levels are measured along with ALT levels to diagnose liver diseases. The reference ranges of AST are 14-20 U/L for males and 10-36 U/L for females.
ALT and AST
As mentioned earlier, both ALT and AST are not specific to the liver, but when taken together, they show a significant specificity of liver disease. Therefore, when you are suspected of having liver disease, your doctor will check both ALT and AST levels.
ALT and ALT levels are sometimes only mildly elevated. This is seen in conditions such as non-alcoholic fatty liver disease, alcoholic liver disease, medication associated liver disease, chronic viral hepatitis and hemochromatosis. In patients with chronic hepatitis B or C, the AST and ALT levels will be mildly elevated.
AST and ALT levels will be very high when there is extensive liver parenchymal injury. This usually occurs in acute settings. For example, when there is acetaminophene poisoning, the AST and ALT levels will be very high. When the levels are more than 1000 U/L, it is taken as a life threatening condition because the patient is at a very high risk of acute liver failure. ALT and AST levels are also very high in acute viral hepatitis. For example, in Hepatitis A, the liver enzymes will become very high. There are some instances where only one of these two enzymes is elevated. In alcoholic liver disease, the ALT levels will be twice as high as AST levels. This is a very important diagnostic clue.
There are many other liver enzymes that are very important to diagnosis. Alkaline phosphatase and Gamma Glutamyl Transferase are two of such enzymes. Physicians use these enzymes to differentiate liver diseases from one another. In conclusion, AST and ALT levels are very important to diagnose and to find out the etiology of liver diseases.