How to Read Your Liver Tests: ALT, AST, ALP, GGT, and Bilirubin

Liver tests are read as a pattern, not one number at a time. ALT and AST rising more than ALP points to injury to liver cells; ALP rising more than ALT and AST points to the bile ducts, a pattern called cholestasis. GGT tells whether a high ALP comes from the liver or from bone, and bilirubin, split into direct and indirect, tells where a yellow tinge comes from. The American College of Gastroenterology (ACG) puts a truly healthy ALT at 29 to 33 U/L for men and 19 to 25 U/L for women, lower than many labs’ upper limits.

This article is general information, not medical advice.

What Each Test Measures

• Alanine aminotransferase (ALT). An enzyme found mostly in liver cells, released when they’re damaged, so it’s the most liver-specific of the four enzymes.
• Aspartate aminotransferase (AST). Found in the liver but also in the heart and muscles, so a high AST alone may not come from the liver.
• Alkaline phosphatase (ALP). Found mostly in the liver, the bile ducts, and bone. It’s higher in pregnancy and in children and teens, whose bones are growing.
• Gamma-glutamyl transferase (GGT). Rises with ALP in diseases of the bile ducts and liver but not in bone disease, so it shows where a high ALP comes from. Alcohol raises it too.
• Bilirubin. A yellow pigment from the breakdown of red blood cells, cleared by the liver. Total bilirubin is split into direct (conjugated, processed by the liver) and indirect (unconjugated) bilirubin.

The ACG’s 2017 guideline (Kwo and colleagues) suggests calling them liver chemistries or liver tests rather than “liver function tests”, since most don’t measure function.

Reading the Pattern

The ACG guideline describes:

• Hepatocellular injury: ALT and AST raised more than ALP. Causes it lists to look for include viral hepatitis, fatty liver disease, alcohol, medicines, haemochromatosis, and autoimmune hepatitis.
• Cholestatic injury: ALP raised more than ALT and AST, with causes such as primary biliary cholangitis and primary sclerosing cholangitis, once the ALP is confirmed to come from the liver.
• Bilirubin: most bilirubin in the blood is unconjugated. A raised direct bilirubin implies liver disease or a blocked bile duct.

The guideline also notes that a raised ALT has been linked to higher liver-related mortality, and that ALT above the healthy ranges above should be looked into.

Harmless Reasons for a High Result

• Hard exercise. In a 2008 study by Pettersson and colleagues, an hour of weightlifting raised AST and ALT in healthy men who weren’t used to it, and they stayed raised for at least 7 days. ALP, GGT, and bilirubin didn’t change. If you’ve trained hard in the days before a test, say so.
• Gilbert syndrome. A common inherited condition, estimated by MedlinePlus to affect 3 to 7 percent of Americans, that raises unconjugated bilirubin, especially with fasting, illness, or intense exercise. It’s mild, and often found by chance on a routine test.
• Pregnancy and growth raise ALP.

Following It in Kaizora

Alanine Aminotransferase, Aspartate Aminotransferase, Alkaline Phosphatase, Gamma-Glutamyl Transferase, Serum Total Bilirubin, and Serum Direct Bilirubin are lab results on Labs, each logged with its time, laboratory, and printed range, so a result outside the lab’s range is flagged. A draw’s results share their time and laboratory, so the whole panel lines up, and charting ALT over years shows whether a high result was a one-off. Your workouts and strength training sets are on the same Timeline, so a hard session before a test is easy to spot. For what a lab’s range means, see What the Reference Range on Your Lab Results Actually Means.

The Short Version

• ALT and AST up more than ALP points to liver cells; ALP up more than them points to the bile ducts.
• GGT shows whether a high ALP is from the liver; direct bilirubin shows liver or bile duct disease.
• A healthy ALT is about 29 to 33 U/L for men and 19 to 25 U/L for women, per the ACG.
• Hard exercise, Gilbert syndrome, pregnancy, and growth can raise results harmlessly.

Sources

• Kwo PY, Cohen SM, and Lim JK, “ACG clinical guideline: evaluation of abnormal liver chemistries,” American Journal of Gastroenterology 112, no. 1 (2017): 18–35. doi:10.1038/ajg.2016.517
• Pettersson J et al., “Muscular exercise can cause highly pathological liver function tests in healthy men,” British Journal of Clinical Pharmacology 65, no. 2 (2008): 253–259. doi:10.1111/j.1365-2125.2007.03001.x
• MedlinePlus, Gamma-glutamyl Transferase (GGT) Test, Alkaline Phosphatase, and AST Test.
• MedlinePlus Genetics, Gilbert syndrome.