Understanding Liver Function Lab Tests

Key takeaways

• Liver function tests typically measure enzymes (ALT, AST, ALP, GGT) and proteins or substances made or processed by the liver (bilirubin, albumin, total protein).
• ALT is often considered a relatively liver-specific enzyme, while AST can also rise with muscle injury.
• Bilirubin is a yellow pigment produced when red blood cells break down; elevated levels can cause jaundice.
• Albumin and total protein reflect the liver's synthetic function, while clotting tests like PT/INR can also indicate how well the liver makes proteins.
• Mildly abnormal results are common and may not indicate serious disease; results are interpreted alongside symptoms, history, imaging, and other tests.

What Liver Function Tests Measure

Liver function tests, sometimes called a liver panel or hepatic panel, are blood tests that give a broad picture of liver health. They usually include enzymes such as alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), and gamma-glutamyl transferase (GGT). They also typically include bilirubin, albumin, and total protein, which reflect how the liver processes waste and makes important proteins.

These tests do not measure a single 'liver function' number. Instead, each marker provides a piece of information. Some markers indicate liver cell injury, others suggest bile flow problems, and others reflect the liver's ability to synthesize proteins. Because of this, healthcare providers interpret the pattern of results rather than any single value in isolation.

Enzymes: ALT, AST, ALP, and GGT

ALT and AST are enzymes found inside liver cells. When liver cells are damaged or inflamed, these enzymes can leak into the bloodstream, raising blood levels. ALT is found mainly in the liver, making it a fairly liver-specific marker. AST is present in the liver but also in muscle, heart, and other tissues, so an elevated AST alone does not always point to the liver.

ALP and GGT are often associated with the bile ducts. ALP is found in the liver, bile ducts, bone, and placenta, while GGT is found in the liver and bile ducts and can also be affected by alcohol use and certain medications. A provider may order these together to help determine whether an elevated ALP is coming from the liver or another source.

Bilirubin, Albumin, and Synthetic Function

Bilirubin is a yellow pigment created when old red blood cells are broken down. The liver processes bilirubin and helps excrete it in bile. When bilirubin builds up in the blood, it can cause jaundice, which is a yellowing of the skin and the whites of the eyes. Bilirubin tests can help distinguish between different causes of jaundice, such as increased red blood cell breakdown, impaired liver processing, or blocked bile flow.

Albumin is a protein made by the liver that helps keep fluid in the blood vessels and carries hormones, vitamins, and drugs. Total protein measures albumin plus other proteins in the blood. Because the liver makes albumin, low levels can occur with chronic liver disease, but they can also result from poor nutrition, kidney problems, or other conditions. Providers may also check prothrombin time (PT) or INR, since the liver produces clotting factors.

Interpreting Results and Next Steps

Results are usually reported with a reference range, which can vary by laboratory. A value outside the range may be flagged, but mild elevations are common and can be temporary. Causes of abnormal liver test results include fatty liver disease, viral hepatitis, alcohol use, medications, herbal supplements, autoimmune conditions, and inherited disorders. Sometimes the cause remains unknown despite testing.

When results are abnormal, a healthcare provider may repeat the tests, ask about medications and alcohol use, order additional blood tests, or recommend imaging such as an ultrasound. In some cases, a liver biopsy may be considered. Treatment depends on the underlying cause, and many liver conditions can be managed or improved with lifestyle changes, medication adjustments, or specific therapies.

Frequently Asked Questions

Sources

• MedlinePlus: Liver Function Tests
• National Institute of Diabetes and Digestive and Kidney Diseases: Liver Tests
• Centers for Disease Control and Prevention: Liver Disease
• Mayo Clinic: Liver Function Tests
• National Library of Medicine: Bilirubin Blood Test