What the ALT test shows
ALT (alanine aminotransferase) is an enzyme packed inside liver cells (hepatocytes), where it helps turn protein into energy. When those cells are inflamed or damaged it leaks into the blood, so the level in a sample is one of the most sensitive early signals of liver injury — MedlinePlus calls it a test used mainly to detect liver damage or disease. You may still see it under its older name, SGPT.
ALT is easy to confuse with the enzymes beside it. AST leaks from the same injured cells but also sits in heart and muscle, so it is less liver-specific; the AST/ALT ratio helps point to a cause. GGT and alkaline phosphatase rise mainly when bile flow is blocked, and bilirubin reflects how well the liver clears waste — ALT is the marker most specific to hepatocyte injury itself.
One caveat: ALT shows that liver cells are leaking, not why or how badly. The number does not track disease severity, so it is read with the other liver tests and your history.
ALT normal range
ALT is reported in units per litre (U/L); conventional and SI reports show the same figure, and only a few European labs convert to microkatals (1 µkat/L = 60 U/L). Typical adult orientation ranges:
| Group | Reference range, U/L (SI: µkat/L) |
|---|---|
| Men (adult) | ~10–50 (0.17–0.83) |
| Women (adult) | ~7–35 (0.12–0.58) |
| Children & teens | age-specific — read your lab’s range |
| Newborns & infants | higher than adults — read your lab’s range |
One nuance matters more than the standard band. Older cut-offs of 40–55 U/L miss real disease: the ACG now puts a “true healthy” upper limit far lower — about 29–33 U/L for men and 19–25 U/L for women — so a value called “normal” by an older range can still deserve a look. Ranges depend on the lab, sex and age, so read your result against your own report.
Why ALT is high
A high ALT means liver cells are leaking their contents. Causes, roughly by frequency:
- Metabolic (fatty) liver disease — the commonest cause. Fat building up in the liver — now called MASLD (metabolic dysfunction-associated steatotic liver disease, the 2023 rename of non-alcoholic fatty liver disease) — drives most mild ALT rises, alongside excess weight, type 2 diabetes and metabolic syndrome.
- Alcohol. Regular heavy drinking inflames the liver; here AST often rises higher than ALT, and an AST/ALT ratio above 2 is a classic clue.
- Medicines and supplements. Paracetamol (acetaminophen), statins, some antibiotics, anti-seizure drugs, NSAIDs and many herbal or bodybuilding supplements can injure the liver.
- Viral hepatitis. Chronic hepatitis B and C cause long-standing rises; hepatitis A and E cause sharp, high spikes.
- Less common but important: autoimmune hepatitis, haemochromatosis, Wilson disease, coeliac disease and thyroid disease.
- Muscle, not liver. A little ALT sits in muscle, so hard or unaccustomed exercise, injury or muscle disease can nudge it up — usually with a bigger jump in AST.
How high matters. Mild rises (under about five times the upper limit) usually reflect fatty liver or a medicine. Levels above 1,000 U/L point to a short list of serious problems — acute viral hepatitis, a drug or toxin such as a paracetamol overdose, or a liver starved of blood (“shock liver”), as StatPearls notes.
When is it urgent? A very high ALT with jaundice (yellow skin or eyes), confusion or easy bruising can signal acute liver failure and needs same-day emergency care; a suspected paracetamol overdose is an emergency whatever the number.
Why ALT is low
A low ALT is far less worrying than a high one and rarely a treatment target, but not always meaningless:
- Vitamin B6 deficiency. ALT needs vitamin B6 (pyridoxal-5’-phosphate) to work, so a shortfall can lower the measured value — MedlinePlus lists this among the causes of a low result.
- Frailty, ageing and low muscle mass. Studies link a persistently low ALT with older age, reduced muscle and, in some groups, poorer health outcomes.
- Chronic kidney disease, especially on dialysis.
Because a low ALT rarely changes management on its own, doctors weigh the whole picture rather than the single number.
What to test alongside
ALT is one line of a liver panel and means most in company:
- AST — the partner enzyme; the AST/ALT ratio helps separate alcohol, fatty liver and other causes.
- GGT — flags bile-flow problems and heavy drinking.
- Alkaline phosphatase — with GGT, points to the bile ducts rather than to liver cells.
- Bilirubin — shows how well the liver clears waste.
- Albumin and total protein — reflect the liver’s synthetic (manufacturing) work.
- Ferritin — often raised in fatty liver, and the screen for iron overload.
- HbA1c or glucose, plus total cholesterol and LDL cholesterol — the metabolic drivers behind most fatty-liver ALT rises.
- CRP — a general inflammation check.
What to do about an abnormal result
- Don’t self-treat or stop prescribed medicine on your own. Supplements sold for “liver detox” are themselves a common cause of injury.
- Confirm it. ALT rises and falls, so a single mild elevation is usually rechecked after a few weeks — avoiding alcohol and hard exercise beforehand.
- Check the obvious drivers: alcohol, any new medicine or supplement, and your weight, blood sugar and cholesterol.
- For a mild, persistent rise, your GP looks for fatty liver and metabolic risk, screens for hepatitis B and C, and may add iron and autoimmune tests.
- For a marked or stubborn rise, expect referral to a hepatologist or gastroenterologist, often with an ultrasound or fibrosis scan.
- Seek emergency care for a very high ALT with jaundice, confusion or easy bruising, or any suspected paracetamol overdose.
Mini-FAQ
What does a high ALT level mean?
A high ALT means liver cells are leaking the enzyme into the blood. The most common reasons are fatty (metabolic) liver disease, alcohol and certain medications. A mild rise is common and rarely an emergency, but the cause is still worth finding.
What is a normal ALT level?
Many labs report up to about 40–55 U/L as normal, but expert healthy thresholds are lower — roughly 29–33 U/L for men and 19–25 U/L for women. Always read your result against your own lab’s range.
What is the difference between ALT and AST?
Both enzymes leak from injured liver cells, but ALT is more specific to the liver, while AST is also found in heart and muscle. Doctors compare them as the AST/ALT ratio to help point to the cause.
Can exercise raise ALT?
Yes. Strenuous or unaccustomed exercise and muscle injury can raise ALT (and AST) for a few days. It is best to avoid a hard workout before the test and to repeat a borderline result once rested.
Is a mildly high ALT dangerous?
A mild rise is common and often reversible, as with fatty liver, but it should not be ignored because early liver disease is very treatable. A very high ALT with jaundice or confusion is a medical emergency.
How can I lower my ALT?
Treat the cause: losing excess weight, limiting alcohol and reviewing your medicines and supplements with a doctor often brings a fatty-liver ALT down. It is usually rechecked after weeks to months, and you should never stop a prescribed medicine on your own.
