Liver Enzymes: What ALT and AST Actually Tell You
A doctor explains what raised ALT and AST mean on a liver blood test, the common causes of mild elevations, and which results need prompt follow-up.
See a doctor promptly if
- Yellowing of the eyes or skin, dark urine, or pale stools
- Liver enzymes more than ten times the upper limit of normal
- Raised enzymes with confusion, drowsiness, or easy bruising and bleeding
- Severe upper abdominal pain with vomiting and fever
What your ALT result means
| Result | In short | What it means |
|---|---|---|
| Under 35 U/L | Normal | Yes for most labs, though some now use lower cut-offs, around 25 for women and 33 for men. |
| 35 to 70 U/L | Mildly raised | Up to about twice the upper limit. Fatty liver and alcohol account for most results here, and a repeat in a few weeks is the usual next step. |
| 70 to 200 U/L | Moderately raised | Enough to look for a cause rather than repeat it. Medicines, viral hepatitis and alcohol are the first three considered. |
| 200 to 1000 U/L | Markedly raised | This level points to active liver injury and is followed up quickly, not at the next routine appointment. |
| Over 1000 U/L | Very high | It needs same-day assessment. A viral hepatitis, a medicine or a blood supply problem are the usual causes at this level. |
One result is a snapshot. Ranges differ a little between labs, and what a number means depends on the rest of you. Check it against the guidelines.
The short version
- ALT and AST are enzymes released when liver cells are irritated or damaged. A high level signals liver stress. It does not measure how well the liver is working.
- Mild rises up to about twice the upper limit are common. They are usually explained by fatty liver, alcohol, medicines, or recent heavy exercise.
- Levels more than ten times the upper limit need urgent assessment. So does any liver enzyme rise with jaundice.
Try the result lookupone number, free, nothing to installType the number you are holding and it lands in a band, with what usually happens next.
ALT and AST are enzymes that live inside liver cells. When those cells are irritated, inflamed or damaged, the enzymes leak into the bloodstream, so a raised level is a marker of liver cell stress, not of how well the liver is working. A liver can be badly scarred with near-normal enzymes, and a liver can have very high enzymes from a temporary insult and be entirely fine a month later.
Mild elevations are common. Most are explained without any dramatic diagnosis.
What each one is telling you#
ALT is found mainly in the liver, which makes it the more liver-specific of the two. AST is present in liver but also in muscle, heart and red blood cells. So it can rise from heavy exercise, a muscle injury, or an intramuscular injection.
The ratio between them is a rough clue. An AST that is roughly twice the ALT or higher raises the possibility of alcohol-related liver injury or a muscle source. An ALT higher than AST is the more usual pattern in fatty liver and viral hepatitis.
Two other enzymes usually appear on the same report. ALP and GGT rise more when the problem involves bile flow rather than liver cells. A raised GGT with a normal ALP often reflects alcohol or medicines.
The common causes of a mild rise#
Fatty liver is now the single most common cause worldwide. It travels with excess weight around the middle, prediabetes, high triglycerides and high blood pressure.
Alcohol, in quantities many people would not describe as heavy.
Medicines and supplements. Paracetamol in excess, some antibiotics, statins, anti-tuberculosis drugs, and a long list of herbal and bodybuilding supplements. Green tea extract and anabolic steroids are frequent culprits people do not think to mention.
Viral hepatitis, especially hepatitis B and C, which are common globally and often silent for years.
Celiac disease, thyroid disease and hemochromatosis account for a meaningful slice of otherwise unexplained elevations.
What a doctor usually does next#
For a mild, symptom-free rise: review alcohol, weight and medicines, then repeat the panel in four to eight weeks. If it persists, a standard non-invasive screen follows, usually hepatitis B and C testing, iron studies, glucose or HbA1c, lipids, celiac serology, thyroid function, and a liver ultrasound. In many health systems a fibrosis score such as FIB-4 is calculated from routine bloods to estimate scarring risk.
Very high levels, above roughly ten times the upper limit, show acute liver injury and are handled urgently.
What helps#
If fatty liver is the cause, weight loss of around seven to ten percent has the strongest evidence for improving liver inflammation. Reducing alcohol, treating diabetes and high triglycerides, and regular activity all help. Hepatitis B vaccination is worth checking if you have not had it.
The wider picture#
Liver enzymes are best read as part of a whole panel, and the in-depth guide to understanding your blood test results shows how they fit with kidney, blood count and metabolic markers.
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Common questions
Are liver function tests the same as ALT and AST?
Can exercise raise them?
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