The liver plays a variety of important roles in the body: It stores fuel gleaned from food, makes needed proteins, and helps remove poisons and toxins. The liver also makes bile, a substance that helps in digestion.

Liver function tests are a group of blood tests that doctors order to find out whether the liver is functioning as it should or has suffered any kind of damage. These tests can help them diagnose viral infections (like mononucleosis or viral hepatitis) or liver damage from other health problems.

The Liver Function Tests

Sometimes liver function tests are referred to as a liver panel and they typically measure: aspartate transaminase (AST), alanine transaminase (ALT), alkaline phosphatase (ALP), albumin and total protein, and total and direct bilirubin.

Aspartate Transaminase (AST)

Aspartate transaminase is an enzyme that plays a role in processing proteins, and it's found in the liver, heart, muscles, and kidneys. When the liver is injured or inflamed, levels of AST in the blood usually rise.

Alanine Transaminase (ALT)

Alanine transaminase is an enzyme found only in the liver that plays a role in metabolism, the process that converts the fuel from food into energy the body needs. If the liver is injured in any way, ALT is released into the bloodstream.

Alkaline Phosphatase (ALP)

Alkaline phosphatase is an enzyme found in the liver, bones, intestines, kidneys, and other organs. Kids and teens normally have higher levels of ALP than adults, even when they're healthy, due to bone growth. But ALP levels can also increase when kids have viral infections or diseases of the liver.

Albumin and Total Protein

The levels of albumin (a type of protein the body makes) and total protein can help tell how well the liver is functioning. Sometimes when there's a problem with the liver, it can't make proteins as well, so the levels of total protein and albumin are lower.

Total and Direct Bilirubin

Bilirubin, produced by the normal breakdown of red blood cells, is the substance that gives bile its yellow-green color. Normally, bilirubin passes through the liver and is eventually excreted from the body. But if that doesn't happen due to a liver disease or other health problem, bilirubin levels in the blood can rise and the skin can take on the yellow discoloration known as jaundice.

Reviewed by: Steven Dowshen, MD
Date reviewed: September 2008
Originally reviewed by: Frederick A. Meier, MD