Liver Function Tests: Understanding ALT, AST, Bilirubin, and Other Key Markers

Key takeaways

• Liver function tests are a panel of blood markers, not a single test, and typically include ALT, AST, ALP, bilirubin, albumin, and total protein.
• ALT and AST are enzymes that can rise when liver cells are injured, but elevations are not automatically a sign of serious disease.
• Bilirubin is a waste product from red blood cell breakdown; elevated levels can indicate liver, bile duct, or blood disorders.
• Albumin and total protein reflect the liver's synthetic function and can be low in chronic liver disease or other conditions.
• Reference ranges vary by laboratory, so results should always be interpreted by a clinician in the context of symptoms, history, and other tests.

What Are Liver Function Tests?

Liver function tests, often abbreviated as LFTs, are a set of blood tests that provide information about the liver's condition and function. Despite the name, not all of these tests directly measure how well the liver is working. Some, such as alanine aminotransferase (ALT) and aspartate aminotransferase (AST), measure enzymes released when liver cells are damaged. Others, such as albumin and bilirubin, reflect the liver's synthetic and excretory functions.

A typical liver panel includes ALT, AST, alkaline phosphatase (ALP), bilirubin, albumin, and total protein. Some panels also include gamma-glutamyl transferase (GGT) or prothrombin time. The combination of results helps clinicians distinguish between different patterns of liver injury, such as hepatocellular injury versus cholestasis, and decide whether further testing is needed.

ALT and AST: Enzymes That Signal Liver Cell Injury

ALT is an enzyme found primarily in the liver, making it a relatively specific marker of liver cell injury. AST is found in the liver but also in the heart, muscles, kidneys, and brain, so an elevated AST is less specific. Both enzymes can rise in conditions such as viral hepatitis, alcohol-related liver disease, fatty liver disease, and drug-induced liver injury. However, mild elevations are common and may be temporary or related to factors like strenuous exercise or certain medications.

The ratio of AST to ALT can sometimes offer clues about the underlying cause, but it is not diagnostic on its own. For example, an AST-to-ALT ratio greater than 2 is sometimes seen in alcohol-related liver disease, while ratios below 1 are more typical in fatty liver disease. Clinicians interpret these patterns alongside a person's history, symptoms, and other test results rather than relying on a single number.

Bilirubin and Alkaline Phosphatase: Excretion and Bile Flow

Bilirubin is a yellow pigment produced when red blood cells break down. The liver processes bilirubin and excretes it into bile. When bilirubin levels rise, it can cause jaundice, a yellowing of the skin and eyes. Elevated bilirubin can result from increased red blood cell breakdown, impaired liver processing, or blockage of bile flow. Tests often distinguish between indirect (unconjugated) and direct (conjugated) bilirubin to help identify the cause.

Alkaline phosphatase (ALP) is an enzyme found in the liver, bile ducts, bone, and other tissues. In the liver context, ALP tends to rise when bile flow is obstructed, a pattern called cholestasis. Because ALP is also present in bone, an isolated elevation may reflect bone conditions rather than liver disease. GGT is sometimes measured alongside ALP to help confirm whether an ALP elevation is coming from the liver or biliary system.

Albumin and Total Protein: The Liver's Synthetic Function

Albumin is a protein made by the liver that helps maintain fluid balance in the body and transports hormones, vitamins, and drugs. Total protein measures albumin plus other proteins, including immunoglobulins. Because the liver produces albumin, low levels can indicate reduced synthetic function, which may occur in chronic liver disease, malnutrition, or conditions that cause protein loss, such as kidney disease.

Unlike enzyme markers, albumin and total protein do not rise with acute liver injury; instead, they tend to fall slowly over time when liver function is chronically impaired. A low albumin level is therefore more suggestive of long-standing liver disease or another systemic condition. As with all liver panel components, albumin and total protein are interpreted together with other results and clinical findings.

Frequently Asked Questions

Sources

• MedlinePlus: Liver Function Tests
• National Institute of Diabetes and Digestive and Kidney Diseases: Liver Tests
• Mayo Clinic: Liver Function Tests
• Centers for Disease Control and Prevention: Viral Hepatitis Testing