Liver blood tests (ALT and GGT): what they measure and what can change them
ALT is an enzyme from liver cells; it rises when the liver is irritated by fat, alcohol, medicines or a virus.
General information about the test, from Hea's guide and the NHS · Last reviewed 5 November 2026
What it measures, and why GPs check it
ALT
ALT is an enzyme from liver cells; it rises when the liver is irritated by fat, alcohol, medicines or a virus.
It is part of liver function tests and one of the inputs of the FIB-4 score for liver scarring.
GGT
GGT is an enzyme that is abundant in the liver and bile ducts. It is a sensitive test: it can rise with alcohol, excess weight or some medicines, often with no symptoms.
British guidance puts it in the first set of liver blood tests, and it shows whether a raised ALP comes from the liver or from bone.
Levels in UK guidance
Ranges differ between labs — your report shows your lab's range.
ALT
- Reference ranges differ between laboratories and methods; British guidance asks the GP to look for a cause when a liver test is outside the laboratory's range, whatever the size of the rise and however long it has lasted. BSG 2018 R3, R4
- ALT or AST at 15 times the upper limit is on the list of results a laboratory phones a doctor about the same day. RCPath G158 v2 (2026)
GGT
- GGT most often rises with excess weight, alcohol or a medicine. British guidance says a raised GGT is not specific to liver disease, and asks the GP to look for the cause whatever the size of the rise. BSG 2018, 'What constitutes a liver blood test?' (GGT); R3, R4
- GGT does not come from bone. When ALP is raised as well, a raised GGT points to the liver or bile ducts and a normal GGT points to bone. BSG 2018, 'What constitutes a liver blood test?' (ALP, GGT); Lab Tests Online UK: Gamma-glutamyl transferase (GGT) test (2026)
- In people who drink regularly, alcohol in the 24 hours before the test can raise GGT for a short time; after stopping alcohol it takes a month or more to return to normal. Smoking and some medicines raise it too, and it is a little higher in men than in women. Lab Tests Online UK: Gamma-glutamyl transferase (GGT) test (2026)
What can shift the number
Before the test
- Alcohol in the day before. In people who drink regularly, alcohol in the 24 hours before the test can raise GGT for a short time; GGT mostly follows drinking over weeks. Lab Tests Online UK: Gamma-glutamyl transferase (GGT) test
- Hard exercise or cycling in the week before. Hard exercise can raise CK, ALT and AST: in one study they stayed raised for at least a week after an hour of weightlifting, while GGT and ALP did not rise. Pettersson 2008 (PMID 17764474); Lab Tests Online UK: Alanine aminotransferase (ALT); Aspartate aminotransferase (AST); Creatine kinase
Now or in the last 3 months
- Pregnant. In pregnancy the usual ranges for many tests are different: show these results to your midwife or GP. NICE NG201 Antenatal care (2021); NICE NG3 Diabetes in pregnancy (2015, updated 2020); RCOG Green-top Guideline 76: thyroid disorders in pregnancy (2025; PMID 40240075); NICE NG239 (2024) rec 1.3.2; BSG guidelines on the management of abnormal liver blood tests (2018); Lab Tests Online UK: Lipid profile; C-reactive protein (CRP); ESR
- An infection or illness in the 6 weeks before. An infection or other illness can raise liver enzymes and bilirubin and lower albumin for a while. BSG guidelines on the management of abnormal liver blood tests (2018); Lab Tests Online UK: Bilirubin; Albumin
- Started a new medicine or took antibiotics. Many medicines can raise liver tests, some antibiotics weeks after the course has ended: British guidance asks the GP to go through everything taken, prescribed or bought. BSG guidelines on the management of abnormal liver blood tests (2018), drug history; BNF: Flucloxacillin; Co-amoxiclav
Some medicines and supplements can change this result too. When you add your results, Hea asks which ones you take.
What a GP reads it with
ALT
Read together with: AST, GGT, ALP (alkaline phosphatase), Bilirubin, Albumin. BSG guidelines on the management of abnormal liver blood tests (2018); Lab Tests Online UK: Liver blood tests
A GP usually asks about alcohol, medicines, supplements and weight, reads ALT with the other liver tests, and may add tests for the cause, calculate FIB-4 or arrange a liver scan. British guidance does not simply repeat the same panel unless a passing cause is likely.
GGT
Read together with: ALP (alkaline phosphatase), ALT, AST, Bilirubin, Albumin. BSG guidelines on the management of abnormal liver blood tests (2018); Lab Tests Online UK: Gamma-glutamyl transferase (GGT) test
A GP usually asks about alcohol in a typical week, weight, and medicines and supplements, reads GGT with ALT, ALP and bilirubin, and may add tests for the cause, an ultrasound or a check for liver scarring.
Questions to ask your GP
- What could be the cause, and which further tests are needed?
- Should my other liver tests be looked at together?
- Could a medicine or supplement I take explain this?
- Could alcohol, my weight, or a medicine or supplement I take explain this?
- Which of my other liver tests should be read together with this one?
- Is a check for liver scarring worth doing, or more tests for the cause?
Safety first
If your lab or GP has flagged your result as urgent, follow their advice first.
Not for emergencies: call 999 or NHS 111.
Questions people ask
What is ALT in a blood test?
ALT, alanine aminotransferase, is an enzyme in liver cells. The blood test is used to pick up and monitor liver damage or inflammation. It is usually read with other liver tests, such as ALP, GGT and AST, which together are often called liver function tests.
Lab Tests Online UK: Alanine aminotransferase (ALT) test, Page updated 24 March 2026What can cause a high ALT?
ALT rises when the liver is damaged or inflamed. Causes include drinking a lot of alcohol, fatty liver linked to diabetes or raised triglycerides, viral hepatitis, and some medicines and 'natural' health products. Hard exercise or an injection into a muscle can raise it too, because muscle also releases ALT. Your doctor will want to know about everything you take.
Lab Tests Online UK: Alanine aminotransferase (ALT) test, Page updated 24 March 2026What is GGT in a blood test?
GGT, gamma-glutamyl transferase, is an enzyme measured to help find liver or bile duct problems. It also shows whether a raised ALP, another liver test, comes from the liver or from bone. GGT is very sensitive: it can rise without any symptoms, sometimes only for a while because of medicines or alcohol.
Lab Tests Online UK: Gamma-glutamyl transferase (GGT) test, Page updated 18 March 2026Does alcohol affect liver blood tests?
Yes. In people who drink regularly, alcohol in the 24 hours before the test can raise GGT for a short time, and your doctor may repeat the test. After stopping drinking, GGT takes at least a month, sometimes longer, to come back down. Smoking can raise GGT too.
Lab Tests Online UK: Gamma-glutamyl transferase (GGT) test, Page updated 18 March 2026How can I lower a high ALT or GGT?
Your GP looks for the cause first, asking about alcohol and checking your weight and waist. Fatty liver is very common and often linked to being overweight. For it, the NHS advises a healthy, balanced diet, regular exercise, losing weight if you are overweight, keeping alcohol within the recommended limit and not smoking.
NHS: Non-alcoholic fatty liver disease (NAFLD), Page last reviewed 31 July 2025Got your result? Start with this question in your AI.
Join Hea, add it to Claude or ChatGPT, and your chat opens with this question ready. Hea shows what the lab printed, your past results and what can shift a number; your GP decides what it means for you.
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Sources
Each line about the test above names the guideline it comes from, as Hea's guide cites it. The answers to the questions come from:
- Lab Tests Online UK: Alanine aminotransferase (ALT) test, Page updated 24 March 2026
- Lab Tests Online UK: Gamma-glutamyl transferase (GGT) test, Page updated 18 March 2026
- NHS: Non-alcoholic fatty liver disease (NAFLD), Page last reviewed 31 July 2025