Liver Function Tests: ALT, AST, Bilirubin, and GGT

Liver function tests are a group of blood tests that show how well the liver is working and whether its cells are injured. The panel usually includes ALT, AST, alkaline phosphatase, bilirubin, albumin, and sometimes GGT. No single value explains the liver. Clinicians read the combination to tell injury apart from blockage and to judge how much function remains.

What liver function tests measure

The liver performs hundreds of jobs. It processes nutrients, builds proteins, stores energy, and clears toxins, drugs, and waste from the blood. It also makes bile, a fluid that helps digest fat. Because the organ does so much, lab tests that look at liver health measure several different things at once: enzymes that leak from injured cells, substances that build up when bile flow is blocked, and proteins that drop when synthetic function falls.

A basic liver panel often comes bundled with a comprehensive metabolic panel. Your clinician may add viral hepatitis tests, an abdominal ultrasound, or a GGT test depending on the pattern. Because reference ranges vary, always compare your result to the range on your own report.

ALT and AST: markers of liver cell injury

Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) are enzymes found inside liver cells. When cells are damaged, these enzymes leak into the bloodstream and the blood level rises. ALT is the more liver-specific of the two. AST also exists in muscle, the heart, and red blood cells, so an elevated AST with a normal ALT can point away from the liver.

Mildly elevated enzymes are common and often temporary. They can follow a viral illness, a new medication, heavy alcohol use, or a period of intense exercise. Doctors usually repeat the test before investigating further unless the elevation is large or worsening.

Alkaline phosphatase and GGT

Alkaline phosphatase (ALP) is an enzyme concentrated in the bile ducts and in bone. A high ALP can signal a blocked or inflamed bile duct, but it can also come from bone conditions, especially in growing children. Gamma-glutamyl transferase (GGT) helps sort this out. GGT sits mainly in the liver and bile ducts, so a high GGT alongside a high ALP supports a liver or bile duct source.

GGT is sensitive but not specific. It rises with bile duct problems, some medications, and alcohol use, and it can be mildly high in people with excess body fat. A normal GGT with a high ALP makes a bone source more likely. Like the other markers, GGT means little on its own.

Bilirubin and what jaundice signals

Bilirubin is a yellow waste product made when old red blood cells are broken down. The liver processes it and sends it into bile so it can leave the body. When this process is disrupted, bilirubin builds up in the blood. At higher levels it tints the skin and the whites of the eyes yellow, a condition called jaundice.

Doctors separate bilirubin into unconjugated and conjugated forms. A rise in unconjugated bilirubin can reflect faster red blood cell breakdown or a problem with the liver's processing step. A rise in conjugated bilirubin usually points to bile flow being blocked or to liver cell injury. The split helps narrow the cause.

Albumin, total protein, and synthetic function

Albumin is the main protein the liver makes. It keeps fluid in the blood vessels and carries hormones and drugs through the body. When the liver is damaged for a long time or when nutrition is poor, albumin can fall. Total protein combines albumin with globulins, the proteins involved in immune function.

A low albumin does not prove liver disease. Kidneys can lose albumin in urine, and the gut or poor diet can also lower it. Still, a falling albumin with other abnormal liver tests suggests the liver is losing some of its ability to build proteins, which matters for long-term prognosis.

Patterns that point to different causes

Clinicians group results into patterns. When ALT and AST dominate, the picture is called hepatocellular, meaning injury is centered on liver cells. Viral hepatitis, medication injury, and fatty liver disease often fit this shape. When ALP, GGT, and bilirubin dominate, the picture is cholestatic, meaning the problem sits in bile flow. Gallstones and some drug reactions fit that shape.

Neither pattern names a single disease. A hepatocellular pattern could come from many causes, so the next step is usually targeted testing. An AST-to-ALT ratio above two can raise suspicion for alcohol-related liver injury, but it is only one clue among many and should never be treated as proof.

What can raise liver enzymes

Common causes of elevated liver enzymes include fatty liver disease, alcohol use, viral hepatitis, and reactions to medications or supplements. Acetaminophen, taken in large doses or combined with alcohol, can injure the liver. Herbal and dietary supplements are a growing cause of liver injury, partly because people assume natural products are always safe. Obesity, diabetes, and high cholesterol raise the risk of fatty liver disease, which is now a frequent reason for mildly abnormal results.

Less common causes include autoimmune hepatitis, inherited conditions such as hemochromatosis, and problems with the bile ducts. Because the list is long, your clinician will ask about alcohol, medications, weight changes, and family history before ordering deeper tests.

Preparation and when to talk to a provider

Some liver panels require fasting for eight to twelve hours, especially when glucose or lipids are included. Ask your provider whether yours does. Avoid alcohol for at least a day before the draw, since it can raise GGT and other markers. Tell the lab and your provider about every prescription, over-the-counter drug, and supplement you take, including acetaminophen and herbal products. Intense exercise in the day before testing can also lift AST, so take it easy.

Seek medical care promptly if you develop yellowing of the skin or eyes, dark urine, pale stools, severe abdominal pain, or confusion. These can be signs of serious liver or bile duct problems. Mildly abnormal results without symptoms often need only a repeat test, but anything persistent deserves a conversation with a qualified healthcare provider. This guide is educational and not a substitute for personal medical advice.