An enzyme is a protein that increases the rate of chemical reactions, like a tiny machine. AST stands for aspartate aminotransferase, and is an enzyme found in the liver as well as many other tissues. A typical blood test to evaluate liver damage, called a liver function test or LFT, measures levels of this enzyme and a number of other proteins in the blood. Since AST is found in many different cells, elevated AST is not specific for liver damage, but is interpreted against a person's medical history, physical exam findings and other laboratory results.
Alcohol and Medications
According to "Robbins and Cotran Pathologic Basis of Disease" by Dr. Vinay Kumar, since AST is found inside of cells, drugs and toxins that damage liver cells will release AST and other enzymes into the blood. Alcohol is one cause of elevated AST. In many problems affecting liver cells, the enzyme ALT will be higher than AST in the blood. However, in alcoholic liver damage AST is generally higher. Some medications, such as acetaminophen, also can cause liver damage, particularly in higher doses.
Viral Infections
Certain viral infections cause damage to liver cells. As noted by "Robbin's and Cotran Pathologic Basis of Disease," hepatitis A is a virus generally transmitted from fecal or digestive routes. It causes acute, short-term liver damage. Hepatitis B and C are transmitted from bodily fluids and are long-standing infections. Cytomegalovirus can infect anywhere in the body and can raise the level of AST. It tends to cause few if any symptoms unless a person has a weakened or immature immune system.
Bile Obstructions
One of the functions of the liver is to make bile, which aids in digestion in the intestine. The biliary ducts drain bile from the liver to the intestine. Bile duct obstruction with gallstones or tumors, or destruction of bile ducts by the immune system causes bile to back up into the liver, notes "Harrison's Principles of Internal Medicine" by Anthony Fauci, MD. This causes damage to the liver cells and the release of liver enzymes such as AST into the blood.
Liver Cancer
Liver cancer is another source of liver damage that can elevate liver enzymes in the blood. Primary liver cancers are those that develop from liver cells, such as hepatocellular carcinoma. Metastatic cancers are cancers that grow from other cells in the body and then spread. The liver is a common place for metastatic cancer, such as colon cancer, to seed.
Wilson's Disease
Inherited disorders and metabolic problems can also damage the liver and lead to elevated levels of AST. Wilson's disease is caused by a genetic defect that causes elevated levels of copper in the body. Elevated copper damages various organs, particularly the liver, causing the organ to release AST into the bloodstream.
References
- "Robbins and Cotran Pathologic Basis of Disease"; Vinay Kumar et al; 2009
- "Harrison's Principles of Internal Medicine"; Anthony S Fauci et al; 2008
