Glossary term
ALT
Also known as: Alanine Aminotransferase
Part of: Liver Function Tests
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- Written in plain English
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- Reviewed by Medipher Editorial Team
An enzyme found mainly in liver cells. It leaks into the blood when the liver is irritated or damaged.
Medipher explains medical wording in plain English — it does not diagnose or replace a clinician. If this is a medical emergency, call your local emergency number immediately.
Simple visual explanation
What is this?
ALT (alanine aminotransferase) is an enzyme found mostly inside liver cells. When liver cells are irritated or injured, small amounts of ALT leak into the bloodstream.
A raised ALT is one of the earliest signs of liver irritation, even if you feel completely well.
ALT is included in almost every Liver Function Test and is interpreted together with AST, ALP, bilirubin and albumin.
Why is it important?
ALT helps clinicians spot liver irritation early. It's very sensitive but not specific — meaning a raised ALT tells you something is happening, but not exactly what.
It's usually looked at with AST, ALP, bilirubin and albumin to build the fullest picture.
Where you'll see it: Liver Function Tests (LFTs).
How doctors use this result
Clinicians may use this result to:
- Screen for liver problems in people with risk factors.
- Investigate tiredness, discomfort or abnormal blood tests.
- Monitor known liver conditions and response to lifestyle changes.
- Check liver safety when starting medications that affect it.
What's the normal range?
| Adults | typically < 40–50 U/L |
Ranges vary by laboratory and by sex.
Mild elevations are very common and often normalise on a repeat test 4–8 weeks later, especially after lifestyle changes or a medication review.
- Lower than the usual range
- A lower result is also read in context rather than on its own, and is often simply repeated.
- Within the usual range
- A result inside the range printed by the laboratory is generally read as usual for ALT at the time of the test.
- Higher than the usual range
- Fatty liver, viral hepatitis, alcohol, certain medications or muscle injury.
Normal ranges are guides, not verdicts. Interpretation depends on your age, sex, symptoms, medical history, medications and other investigations — which is why your clinician looks at the whole picture rather than a single value.
Common reasons it may be high
Fatty liver
Fat build-up in liver cells causes low-grade irritation and slow ALT release.
Viral hepatitis
Viruses that infect the liver cause visible ALT rises, sometimes marked.
Regular alcohol use
Alcohol is processed by the liver and, in excess, damages liver cells over time.
Some medications
Certain painkillers, cholesterol drugs and antibiotics can irritate the liver, usually reversibly.
Muscle injury or intense exercise
ALT is also present in muscle, so vigorous exercise can raise it temporarily.
Does an abnormal result always mean something is wrong?
Not necessarily. A number that sits outside the printed range is a prompt for interpretation — not a diagnosis.
- A single result rarely gives the full picture — clinicians interpret it alongside your symptoms.
- Your medical history, medications and other blood tests all shape the meaning of one value.
- Small movements just outside the reference range are common and often not concerning.
- Repeat testing over time is frequently more useful than any single number.
What might happen next?
- A repeat test after 4–8 weeks.
- Additional tests for viral hepatitis or autoimmune liver conditions.
- An ultrasound of the liver in some cases.
- Lifestyle review — alcohol, weight, medications.
One result rarely tells the whole story. Clinicians look at your symptoms, medical history, medications and other tests together — and a single value that sits slightly outside the reference range is often normal for you. Repeat testing over time is frequently more useful than any one number. Medipher helps you understand what a result means in plain English, but it cannot diagnose medical conditions or replace advice from a doctor, nurse or pharmacist.
When should I speak to a healthcare professional?
Speak with your GP or another healthcare professional if:
- You've been asked to repeat the test.
- You're experiencing new or worsening symptoms.
- Your clinician has flagged the result as needing follow-up.
- You don't understand your results or feel unsure what to do next.
If you have severe symptoms — chest pain, severe breathlessness, fainting, heavy bleeding or feel very unwell — seek urgent help. If this is a medical emergency, call your local emergency number immediately. Medipher is designed to help explain medical information and is not an emergency service.
Frequently asked questions
Related guides
Commonly confused with
These tests sound similar to ALT, but each answers a different question.
- ASTAST is found in the liver but also in muscle and other tissues, so it is less specific to the liver than ALT.
- ALPALP relates more to the bile ducts and to bone, so it points at a different part of the picture than ALT.
- BilirubinBilirubin is a waste product the liver processes rather than an enzyme released from liver cells.
Learning collections
Follow a guided pathway that puts ALT in context, step by step.
Still unsure what your result means?
This page explains one medical term. If you have a blood test, GP letter or hospital report, Medipher can explain your entire document in plain English.
- what your results mean
- unfamiliar medical terminology
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Sources
General patient information referenced when writing this page.
- NHS — Blood testsGeneral background
- NICE Clinical Knowledge SummariesGeneral background
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About this page
- Author
- Omer Suliman
- Medical reviewer
- Clinical review pending
Evidence. Written from public patient-information sources including the NHS, NICE and the Association for Laboratory Medicine, and reviewed for plain-English accuracy before publication.
Educational content only. Medipher explains medical language in plain English. It does not diagnose, and it is not a substitute for advice from a doctor, nurse or pharmacist. Read the medical disclaimer.
Nearly there — keep going.
A short summary and recommended next reads are just below.
Continue learning
- Next upASTAn enzyme found in the liver, muscle and other tissues. It rises when these tissues are irritated or damaged.
- Next upBilirubinA yellow substance made when old red blood cells are broken down. The liver processes it and removes it in bile.
- Next upAbnormal (on a test report)A flag meaning a result differs from what the laboratory or test expected — not a statement about your health.
This glossary entry is general health information, not medical advice. Medipher explains medical wording in plain English and does not diagnose, provide treatment advice, or replace a GP, doctor, nurse or pharmacist. Always discuss your results with a qualified clinician.