ALT
Alanine Aminotransferase
What this test measures
ALT is an enzyme found mainly in your liver. When liver cells are damaged, ALT leaks into your blood. This test checks if your liver is healthy and working properly.
Understanding the numbers
Typical range: 0-35 U/L (some labs up to 56 U/L)
ALT is the most liver-specific enzyme. Ranges can vary between laboratories.
- Normal: 0–35 U/L
- Mildly raised: 35–100 U/L
- Moderately raised: 100–300 U/L
- Markedly raised: 300–500 U/L
Key numbers
- 35 U/L — Upper limit — above = abnormal liver
- 100 U/L — Moderate rise — investigate cause
- 300 U/L — Marked rise — urgent review
- 1000 U/L — Very marked — think acute hepatitis / drug / ischaemic
Upper limit varies by lab (many UK labs use 35 U/L; some cite up to 56). Mild rises (1-3× ULN) are most often fatty liver, alcohol or drugs. Marked rises (>300) or very marked (>1000) need urgent investigation for acute viral hepatitis, paracetamol toxicity, or ischaemic liver.
Ranges can vary slightly between laboratories — your report may show slightly different numbers. This page does not read or interpret your result; your GP looks at it alongside your history and other tests.
What a higher-than-normal result can mean
- Your liver cells may be damaged or inflamed
- Can be caused by fatty liver disease, which is very common
- Drinking too much alcohol can raise ALT
- Some medicines and infections can also cause high levels
When not to wait
Severe acute liver injury can progress to liver failure quickly.
- Yellow skin or eyes (jaundice) appearing rapidly
- Confusion, drowsiness or change in behaviour
- Vomiting blood or passing black tarry stools
- Severe tummy pain or tummy swelling
- Bruising or bleeding easily
- After a paracetamol overdose — even if you feel 'okay' initially
Call NHS 111 for urgent advice, or 999 if someone is seriously unwell.
Questions you could ask your GP
- What might be causing my raised liver enzymes?
- Do I need an ultrasound scan of my liver?
- Should I reduce my alcohol intake, and by how much?
- Could any of my medicines be affecting my liver?
Example stories
These are made-up examples to show how results are talked about in real appointments. They are not real patients.
Mildly raised ALT — fatty liver
Sanjay's ALT was 62 U/L. BMI 31, pre-diabetic, moderate alcohol. Ultrasound showed a fatty liver.
What the GP said: “This is non-alcoholic fatty liver disease. The liver blood tests are mildly abnormal because of fat buildup. The treatment is weight loss, better diet and less alcohol — the liver recovers well with these changes.”
What happened next: Sanjay lost 8 kg over a year, cut alcohol to weekends and his ALT was 28 U/L at the next check.
Mildly raised ALT is often fatty liver, and fatty liver is very reversible with lifestyle change. It's not 'just a number' — it's an early warning that the metabolic picture (weight, sugar, liver) is drifting.
Acute viral hepatitis
Felicity presented with flu-like illness, yellow skin and nausea. ALT 1240, bilirubin 120, INR 1.3. Hepatitis A IgM positive.
What the GP said: “This is acute hepatitis A — you caught it on a recent trip. I'm referring you to the liver unit today for supportive care and monitoring. Full recovery is the norm, but we need to watch for severe cases.”
What happened next: Felicity was managed in hospital for five days. Her ALT came down to 80 over six weeks and was normal by three months.
Very high ALT (over 1000) is an acute injury: viruses, drugs (especially paracetamol), or reduced blood supply to the liver. It warrants same-day hospital assessment — liver failure is rare but catastrophic.
Drug-induced liver injury
Aryan had been on flucloxacillin for a skin infection three weeks ago. ALT was 220 and bilirubin was raised. No other cause.
What the GP said: “Flucloxacillin can cause a drug-induced liver injury, sometimes weeks after stopping. We stop the suspicion, avoid it in future, and monitor your bloods. It usually settles fully but can take weeks.”
What happened next: Aryan's ALT fell steadily and was normal at 10 weeks. His records were flagged to avoid flucloxacillin.
Many medicines can injure the liver — often weeks after starting. Anti-TB drugs, flucloxacillin, some antifungals and even herbal supplements are examples. A good drug history is essential when liver tests go up.
Alcohol-related liver disease
Bernard drank 60 units a week. ALT was 95, AST 110, GGT 320, MCV 108.
What the GP said: “These numbers together tell us alcohol is affecting your liver and your red blood cells. Cutting alcohol is the single most effective thing here — I'll connect you with the alcohol team and recheck bloods in three months.”
What happened next: Bernard engaged with local services, stopped alcohol, and at three months all his liver tests had normalised.
In alcohol-related liver disease, AST is often higher than ALT and GGT is very raised. The liver recovers remarkably well from alcohol damage if intake is reduced early — later stages (cirrhosis) don't recover the same way.
Before you act on anything here
This page is general information, written to help you understand — it is not personal medical advice and it cannot tell you what is right for you. Talk to your own GP, pharmacist or nurse about your own health.
- Need help now but it's not an emergency? Call NHS 111 or use 111.nhs.uk.
- Emergency? Call 999 or go to A&E.