ALT

What it tells you

• ALT is a marker of liver injury, not of how well the liver works. Because it comes mainly from the liver, it points to the liver more clearly than AST does.
• In healthy adults without liver risk factors, the upper limit is about 29 to 33 U/L for men and 19 to 25 U/L for women. Guidelines say values above this should be looked at.
• Many lab limits are higher. Their reference groups include people with risk factors for fatty liver.
• A high ALT is graded by how far it sits above the upper limit. Under 2 times is borderline, 2 to 5 times mild, 5 to 15 times moderate, and over 15 times severe. Moderate and higher rises need prompt evaluation.
• A normal ALT does not rule out fatty liver or liver scarring. Guidelines use a score called FIB-4, built from age, ALT, AST and platelet count, to check for scarring.

What can throw it off

• Hard exercise, especially weightlifting, raises ALT. In one study of healthy men, it was still raised 7 days after a single hour of lifting.
• Muscle injury or muscle disease can raise it, along with AST. A muscle enzyme test called CK helps tell the two apart.
• ALT rises with body weight, and it runs higher in men than in women.
• Medicines, over-the-counter products, supplements and herbal products are common causes of a raised ALT.
• Alcohol can raise it. In alcohol-related liver disease, AST is usually higher than ALT.
• Thyroid disease and celiac disease can both raise liver enzymes.
• It falls when liver fat goes down with treatment.
• Upper limits differ between labs. A high result is usually repeated to confirm it before any workup.

Evidence

Limits

• A normal ALT does not clear the liver. Most people with fatty liver have a normal ALT, and it can be normal even with advanced scarring.
• It does not measure scarring, which drives long-term outcomes. Blood-based scores and liver stiffness scans do that better.
• It does not name the cause. Fatty liver, alcohol, viruses, medicines, iron overload and autoimmune disease all raise it.
• The healthy upper limit flags many people. Applied to US adults, it would call about 36% of men and 28% of women abnormal.
• Results swing with hard exercise and differ between labs. Compare results from the same lab, and confirm a high result before acting on it.

References

1. Kwo PY, et al. Am J Gastroenterol. 2017;112:18-35 (ACG guideline on abnormal liver chemistries).
2. Rinella ME, et al. Hepatology. 2023;77:1797-1835 (AASLD practice guidance on fatty liver disease).
3. Browning JD, et al. Hepatology. 2004;40:1387-1395.
4. Ma X, et al. BMC Gastroenterol. 2020;20:10.
5. Kim HC, et al. BMJ. 2004;328:983.