The liver serves many metabolic functions, including the production of enzymes, which are chemicals that help reactions occur in the body. Inflammation or other conditions that affect the liver can cause these enzymes to leak out into the bloodstream. However, some other tissues, like the heart and muscle, may also contain some of these enzymes. Often further testing is needed to confirm a diagnosis. Elevated liver enzymes do not necessarily indicate a serious, chronic problem and your doctor may repeat the tests over time.
ALT and AST
Blood tests for liver function, also known as LFTs, most commonly include the enzymes alanine transaminase, or ALT and aspartate transaminase, or AST. ALT is also known as glutamic pyruvic transaminase. AST is also known as serum glutamic oxaloacetic transaminase. These enzymes are involved in the transferring of nitrogen from amino acids, the building blocks of proteins, to other compounds. Nitrogen must be removed from protein before further breakdown and oxidation for energy. The nitrogen combines with other small molecules, forming a compound called urea, which is excreted in the urine.
ALP, GGT, and LDH
Alkaline phosphatase, or ALP, and gamma-glutamyl transpeptidase, or GGT, are enzymes used to help differentiate between liver disease and bile duct or bone diseases. ALP and GGT may be elevated in liver and bile duct disease, but elevated ALP and normal GGT may indicate bone disease. Thus, the tests are often run in combination to help give a clearer picture. Elevated lactate dehydrogenase, or LDH, may also indicated liver problems, but this enzyme can be found in many other body tissues.
Other Blood Tests
Albumin, total protein and bilirubin may also indicate liver function. The liver makes proteins like albumin. Low albumin levels or total protein may indicate liver function problems or other conditions, such as malnutrition or kidney disease. Bilirubin is a breakdown product of red blood cells. It is sent to the liver for processing and excreted in the feces. Higher levels of bilirubin in the blood indicate the liver is not filtering it out properly and jaundice may occur.
Common Causes of Elevated LFTs
Hepatitis types A, B and C, nonalcoholic fatty liver disease, obesity, alcoholic hepatitis, autoimmune hepatitis, celiac disease, cirrhosis, gallbladder inflammation, heart attack and other conditions may cause elevated LFTs. Some over-the-counter pain medications and prescription medications, including statin drugs to help reduce cholesterol, can cause elevated liver enzymes. Other tests may be required to help determine the exact cause of elevated LFTs. Your doctor may also use LFTs periodically to determine if a treatment of a specific condition is working or if prescription drugs are affecting the liver.
References
- MedlinePlus: Liver Function Tests
- MayoClinic.com: Liver Function Tests
- "Marks' Basic Medical Biochemistry: A Clinical Approach, 3rd Edition"; Lieberman and Marks; 2008
