A patient could have an abnormal liver function test for many reasons. In general, elevations in these laboratory tests are caused by damage to the liver. A number of infections, including those caused by the hepatitis viruses, cause these tests to be abnormal. Medications or other substances toxic to the liver can also cause abnormal results. Diseases originating in the liver, including non-alcoholic steatohepatitis, can cause abnormal results on liver function tests as well.
One reason for having an abnormal liver function test is infection. Viruses, including hepatitis A, hepatitis B, hepatitis C, and hepatitis E, can cause either acute or chronic changes in the liver function tests. For this reason, most patients with elevated liver tests will have their blood tested for evidence of infection with these viruses. Other viruses, including the Epstein-Barr virus (EBV), cytomegalovirus (CMV), and human immunodeficiency virus (HIV) can harm the liver. Bacterial infections and parasitic infections can also infect the liver, but viruses tend to more commonly infect this organ compared to other types of pathogens.
Many different medications or other ingested substances can cause an abnormal result on a liver function test. Alcohol is very toxic to the liver, and commonly affects the ability of the liver to do its job. Medications including acetaminophen, those in the statin class of medications, methotrexate, and chemotherapeutic agents are also common causes of elevations in these tests. Even natural remedies, including herbs and vitamins, could cause damage to the liver. For this reason, doctors investigating the reason why a patient has an abnormal liver function test need to carefully question the patient about any and all medications and other substances taken over the past months.
Other diseases originating in the liver could also cause an abnormal liver function test. Non-alcoholic steatohepatitis (NASH) develops in many patients in association with obesity, diabetes, or elevated blood cholesterol levels, and this condition causes mild elevations in liver function tests. This is one of the most common liver diseases that cause patients to have abnormal lab results. Rarer diseases such as Wilson's disease, alpha-1-antitrypsin disease, hemochromatosis, autoimmune hepatitis, and primary biliary cirrhosis could also cause elevations in these test results. Often, doctors might check for these diseases after some of the other, more common causes of liver function test abnormalities have been ruled out.
Patients who are severely ill could have an abnormal liver function test for other reasons. Decreased blood flow to the liver, either from heart failure or severe infection, causes a drastic increase in the level of the liver enzymes in the blood. For this reason, it is best to check the function of the liver when the patient is otherwise healthy in order to avoid noting an elevation that was only present due to an acute severe illness.