Liver function tests are biochemical measurements of blood levels of various substances to assess overall liver health. Although the term "liver function tests" is commonly used, it is a misnomer because none of these measurements directly assess liver function. The terms liver enzyme tests or liver chemistry tests more accurately reflect these laboratory measurements. There are many reasons for elevated liver chemistry tests, reflecting the diversity of medical conditions that may adversely affect the liver.
Liver Cell Injury or Disease
Liver cells contain high concentrations of proteins called enzymes, which help the liver carry out chemical reactions. Liver cell injury causes leakage of these enzymes into the bloodstream. As such, elevated liver enzymes may reflect liver cell injury. According to the American Gastroenterological Association (AGA)'s "Medical Position Statement: Evaluation of Liver Chemistry," three liver enzymes are the primary indicators of liver cell injury. These enzymes are alanine transaminase, also called alanine aminotransferase or ALT; aspartate transaminase, also called aspartate aminotransferase or AST; and lactate dehydrogenase, also called lactic acid dehydrogenase or LDH.
Common causes of liver cell injury include alcoholic hepatitis, nonalcoholic fatty liver disease, infectious hepatitis, liver toxins, and adverse reactions to medications. According to the Centers for Disease Control and Prevention, the hepatitis C and B viruses are the most common causes of infectious hepatitis in the United States. Nonalcoholic fatty liver disease is caused by an abnormal accumulation of fat in the liver cells. Obesity is a frequent cause of this condition. Rates of nonalcoholic fatty liver disease have increased in tandem with obesity rates in the United States. Many substances may prove liver-toxic, including ephedra, kavakava, anabolic steroids, PCP, and carbon tetrachloride. Medications that may be liver-toxic include acetaminophen, carbamazepine, glyburide, heparin, labetalol, nitrofurantoin, trazodone, and valproic acid.
Bile Duct Injury or Disease
Conditions associated with bile duct injury and disease are collectively termed cholestatic liver diseases. These diseases involve abnormalities with the liver's processing of bilirubin into bile, or the secretion of bile through the gallbladder into the intestine. Alkaline phosphatase and bilirubin are liver chemistry tests used to screen for or monitor cholestatic liver disease. The AGA reports cholestatic liver diseases that may precipitate elevations in bilirubin and alkaline phosphatase include primary biliary cirrhosis, primary sclerosing cholangitis, vanishing bile duct syndrome, and bile duct obstruction. Medications can also cause cholestatic liver injury, including allopurinol, indinavir, nevirapine, captopril, diltiazem, erythromycin, tolbutamide, estrogens, and trimethoprim-sulfamethoxazole.
Invasive Liver Disease
Infections and tumors are the primary types of invasive liver diseases. AGA advises an elevated alkaline phosphatase is often the only evidence of these disorders detectable with liver chemistry tests. Possible causes of invasive liver disease include tuberculosis with liver invasion, fungal infections, liver cancer, lymphoma, and metastatic tumors that have spread from elsewhere in the body.
References
- Gastroenterology: American Gastroenterological Association Medical Position Statement--Evaluation of Liver Chemistry Tests
- Cleveland Clinic Center for Continuing Medical Education: Approach to the Patient with Liver Disease--A Guide to Commonly Used Liver Tests
- Centers for Disease Control and Prevention: Viral Hepatitis
- Merck Manual for Healthcare Professionals: The Asymptomatic Patient with Abnormal Laboratory Test Results
- Mayo Clinic: Liver Function Tests, Definition
