Explainer · updated 2026-09-09
ALT, explained in plain English
Key facts
- Typical adult reference interval in women: 8-41 U/L (HRI-HR)
- Typical adult reference interval in men: 9-59 U/L (HRI-HR)
- What ALT is: an enzyme inside liver cells that leaks into the blood when they are damaged; the most liver-specific of the routine liver tests (ACG 2017)
- What a raised ALT is linked to: higher liver-related mortality, so it gets followed up (ACG 2017)
- What a low ALT means: no liver meaning, though in older people it tracks frailty and loss of muscle (ACG 2017)
What it is
ALT stands for alanine aminotransferase. It is an enzyme that sits inside liver cells and leaks into the blood when they are damaged. It is the most liver-specific of the routine liver tests, which is why it usually leads when a doctor reads them. It is measured in the blood.
What the number is telling you
A raised ALT tells your doctor that liver cells are leaking. The size of the rise, the other liver results beside it and the trend over time all carry meaning, so the number is read as part of the liver panel rather than alone. One reading says little; time and trend matter.
What high or low can mean
High: A raised ALT means liver cells are leaking, and it is linked to higher liver-related mortality, so it gets followed up. Fat in the liver is the common reason for a mildly raised ALT, and the level falls as weight comes down. Alcohol is the other common driver, and cutting it back is the change that shows up here soonest.
Low: A low figure carries no liver meaning, though in older people it tracks frailty and loss of muscle.
What a typical result looks like
Typical adult reference intervals from the HRI-HR harmonized set, verified in Croatia and published in Biochemia Medica in 2026, run from 8 to 41 U/L in women and 9 to 59 U/L in men. The range on your own report is the one that counts.
Questions worth asking your doctor
- What do the other liver results beside ALT add to the picture?
- If my ALT is raised, is weight or alcohol the more likely driver?
- How does this compare with my previous results?
- How often should this test be repeated for me?
If a result worries you, ask your doctor.
Sources
- Verification of harmonized reference intervals in Croatia: is it time for a change? (2026)
- ACG Clinical Guideline: Evaluation of Abnormal Liver Chemistries (2017)
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