Liver Function Tests Health Article

Definition

Liver function tests, or LFTs, include tests for bilirubin, a breakdown product of hemoglobin, and ammonia, a protein byproduct that is normally converted into urea by the liver before being excreted by the kidneys. LFTs also commonly include tests to measure levels of several enzymes, which are special proteins that help the body break down and use (metabolize) other substances. Enzymes that are often measured in LFTs include gamma-glutamyl transferase (GGT); alanine aminotransferase (ALT or SGPT); aspartate aminotransferase (AST or SGOT); and alkaline phosphatase (ALP). LFTs also may include prothrombin time (PT), a measure of how long it takes for the blood to clot.

Purpose

Liver function tests are used to aid in the differential diagnosis of liver disease and injury, and to help monitor response to treatment.

Precautions

Bilirubin: Drugs that may cause increased blood levels of total bilirubin include anabolic steroids, antibiotics, antimalarials, ascorbic acid, Diabinese, codeine, diuretics, epinephrine, oral contraceptives, and vitamin A.

Ammonia: Muscular exertion can increase ammonia levels, while cigarette smoking produces significant increases within one hour of inhalation. Drugs that may cause increased levels include alcohol, barbiturates, narcotics, and diuretics. Drugs that may decrease levels include broad-spectrum antibiotics, levodopa, lactobacillus, and potassium salts.

ALT: Drugs that may increase ALT levels include acetaminophen, ampicillin, codeine, dicumarol, indomethacin, methotrexate, oral contraceptives, tetracyclines, and verapamil. Previous intramuscular injections may cause elevated levels.

GGT: Drugs that may cause increased GGT levels include alcohol, phenytoin, and phenobarbital. Drugs that may cause decreased levels include oral contraceptives.

Description

The liver is one of the most important organs in the body. As the body's "chemical factory," it regulates the levels of most of the main blood chemicals and acts with the kidneys to clear the blood of drugs and toxic substances. The liver metabolizes these products, alters their chemical structure, makes them water soluble, and excretes them in bile.

Liver function tests are used to determine if the liver has been damaged or its function impaired. Elevations of certain liver tests in relation to others aids in that determination. For example, aminotransferases (which include ALT and AST) are notably elevated in liver damage caused by liver cell disease (hepatocellular disease). However, in intrahepatic obstructive disease—which may be caused by some drugs or biliary cirrhosis—the alkaline phosphatases are most abnormal.