Liver Function Testing: A Plain-Language Guide to ALT, AST, Bilirubin, and Related Markers

Key takeaways

• Liver function tests usually include enzymes such as ALT and AST, plus bilirubin, alkaline phosphatase, albumin, and total protein.
• ALT is generally considered a more liver-specific enzyme than AST, which is also found in muscle and other tissues.
• Bilirubin is a waste product from the breakdown of red blood cells; elevated levels can reflect liver, bile duct, or red blood cell issues.
• A single abnormal result does not diagnose a specific condition; clinicians interpret results alongside symptoms, history, medications, and other tests.
• Reference ranges vary by laboratory, so results should always be interpreted against the range provided by the testing lab.
• Many mild elevations are temporary and may relate to medications, alcohol use, intense exercise, or recent illness.

What Liver Function Tests Measure

Liver function tests, often abbreviated as LFTs, are a collection of blood measurements that provide different views of liver health. The most familiar are the enzymes alanine aminotransferase (ALT) and aspartate aminotransferase (AST), which are released into the bloodstream when liver cells are injured. Other markers include alkaline phosphatase (ALP), which relates to the bile ducts and bone, and bilirubin, a yellow pigment produced when red blood cells are broken down.

The panel also typically includes proteins made by the liver, such as albumin and total protein, along with measures like gamma-glutamyl transferase (GGT) in some cases. These tests do not measure liver function directly in the sense of how well the organ is filtering or metabolizing substances; instead, they offer indirect clues. A clinician combines these clues with a person's medical history, physical examination, imaging, and sometimes additional testing to build a complete picture.

ALT and AST: Enzymes That Signal Cell Injury

ALT is an enzyme found mainly in the liver, making it a relatively specific indicator of liver cell damage. When liver cells are injured, ALT leaks into the blood, and higher-than-usual levels can prompt further evaluation. AST is similar but is also present in skeletal muscle, the heart, the kidneys, and the brain, so an elevated AST is less specific to the liver on its own. The ratio of AST to ALT can sometimes offer additional context, though it is not definitive by itself.

Mild elevations in ALT or AST are common and can occur for many reasons, including fatty liver, alcohol use, certain medications, viral hepatitis, and even strenuous exercise. Importantly, the degree of elevation does not always correspond to the severity of an underlying condition. A clinician may repeat the test after a period of time, review medications and supplements, and consider additional testing such as hepatitis serologies or imaging before drawing conclusions.

Bilirubin and Alkaline Phosphatase: Bile Flow and Clearance

Bilirubin is a byproduct of red blood cell breakdown that the liver processes and excretes in bile. Total bilirubin can be divided into indirect (unconjugated) and direct (conjugated) fractions, and the pattern of elevation can help distinguish between different causes. Elevated bilirubin may occur with liver disease, bile duct obstruction, or conditions that increase red blood cell breakdown, such as certain anemias. Mildly elevated bilirubin can also be seen in Gilbert syndrome, a common and generally harmless inherited condition.

Alkaline phosphatase (ALP) is an enzyme present in the liver, bile ducts, bone, and other tissues. Higher ALP levels can reflect bile duct obstruction or other liver conditions, but they can also occur with bone disorders, during periods of growth in children, and in pregnancy. Because ALP is not liver-specific, clinicians often look at it alongside GGT or other markers to determine whether the source is likely hepatic or non-hepatic.

Albumin, Total Protein, and Synthetic Function

Albumin is a protein made by the liver that helps maintain fluid balance in the blood and transports various substances. Total protein measures albumin plus other proteins such as globulins. Because albumin is produced by the liver, low levels can sometimes indicate reduced synthetic function, but low albumin can also result from malnutrition, kidney disease, inflammation, or other conditions. For this reason, albumin is interpreted as part of a broader assessment rather than in isolation.

Prothrombin time (PT) and INR are additional tests that reflect the liver's ability to produce clotting factors. In more advanced liver disease, these values may be prolonged. Together with albumin and bilirubin, they are sometimes combined into scoring systems used by clinicians to assess liver function. These tools are intended for medical interpretation and are not a substitute for a comprehensive evaluation by a qualified healthcare professional.

Frequently Asked Questions

Sources

• MedlinePlus: Liver Function Tests
• MedlinePlus: ALT Blood Test
• MedlinePlus: Bilirubin Blood Test
• National Institute of Diabetes and Digestive and Kidney Diseases: Liver Tests
• Centers for Disease Control and Prevention: Viral Hepatitis Testing