Liver enzymes, such as alanine aminotransferase, or ALT, and aspartate aminotrasferase, or AST, are normally contained within the healthy cells of the liver. However, when liver cells become inflamed or injured, liver enzymes leak into the bloodstream, resulting in elevated liver enzymes on blood tests. Elevated liver enzymes in the blood typically stem from liver damage, although they can suggest injury in some other tissues as well.
Acute Hepatitis
When liver enzymes in the blood are very high, with values of 10 to 100 times the normal level, it typically results from acute hepatitis, or sudden inflammation of the liver. Acute hepatitis often results from a viral infection, but it can also occur with exposure to drugs or other substances that are toxic to the liver. ALT is particularly sensitive to liver damage, more so than AST, and it can stay high from one to six months. Symptoms of acute hepatitis include dark urine, pale colored stools, nausea and vomiting, itching, yellow skin, fever, loss of appetite and fatigue.
Chronic Liver Disease
Elevated liver enzymes in the blood can suggest chronic, or long-term, liver disease, such as alcoholic hepatitis, non-alcoholic fatty liver disease, cirrhosis or liver tumors. With chronic liver disease, ALT levels are not as dramatically high as in acute hepatitis -- ALT and AST levels normally range from slightly elevated up to four times the normal value, and ALT is usually greater than AST. In the absence of overt disease, mildly elevated liver enzymes may also suggest liver dysfunction, such as detoxification dysfunction or liver congestion.
Bile Duct Obstruction
Elevated liver enzymes can suggest a problem in the bile ducts -- the tube-like structures that carry bile from the liver and gallbladder to the small intestine during digestion. Bile duct obstruction can result from gallstones, inflammation or liver damage. ALT and AST are typically elevated along with other blood markers, such as bilirubin and alkaline phosphotase.
Cardiovascular Problem
AST is present in not only the liver, but also in skeletal muscle, the kidneys, the heart and the lungs. When the tissues in these organs are damaged, AST leaks into the bloodstream. When blood levels of AST are elevated, it may be the result of liver damage, or the problem may originate from the cardiovascular system -- the heart, lungs and blood vessels. Possible cardiovascular problems include congestive heart failure, atherosclerosis or heart attack.
Muscle Injury
The liver enzymes ALT and AST are present to a certain extent in skeletal muscle. When ALT and AST levels become elevated in the blood, it may be the result of muscular injury. Damage to skeletal muscle cells can occur from weight training injuries, trauma to the muscles or muscle tissue disease like polymyositis.
References
- "Blood Chemistry and CBC Analysis - Clinical Laboratory Testing from a Functional Perspective"; Dicken Weatherby, N.D. & Scott Ferguson, N.D.; 2002
- Lab Tests Online: ALT
- Lab Tests Online: AST
- MedlinePlus: Hepatitis A
