Liver function tests: tests and normal ranges for ALT, AST, GGT and bilirubin

Liver function tests are a group of blood tests that most commonly measure ALT, AST, GGT, alkaline phosphatase (ALP) and bilirubin. They show whether liver cells are being damaged and whether bile is flowing properly, but rarely identify a specific disease on their own. The blood sample is taken in the morning, on an empty stomach, after at least a day without alcohol or strenuous exercise, and the result is always compared with the range provided by the laboratory. The result report is issued in Polish.

What are liver function tests and which tests do they include?

“Liver function tests” is a common name for a group of biochemical measurements taken from venous blood. They do not directly measure how well the liver is working, but rather signs of its activity and any damage. A typical liver test package includes five parameters:

• ALT (AlAT, GPT), or alanine aminotransferase. An enzyme found mainly in liver cells. When these cells are damaged, ALT enters the blood, making it a key marker of liver cell damage.
• AST (AspAT, GOT), or aspartate aminotransferase. It is found in the liver, but also in skeletal muscles and the heart. Raised AST with normal ALT more often points to the muscles or heart than to the liver.
• GGT (GGTP), or gamma-glutamyl transferase. It rises with damage to the liver and bile ducts, and also after alcohol consumption and with certain medicines. It is sensitive but not very specific.
• ALP, or alkaline phosphatase. It comes from the liver and bile ducts, as well as the bones. High ALP with high GGT suggests a liver source (impaired bile flow), while high ALP with other liver test results within normal ranges may be related to the bones.
• Total bilirubin. A pigment produced by the breakdown of old red blood cells, which the liver takes up and excretes in bile. Excess bilirubin in the blood causes jaundice.

Your doctor will often add albumin and prothrombin time (INR) to these tests. These two parameters are better than enzymes at showing whether the liver is still producing proteins effectively, including clotting factors.

When should you have liver function tests?

Testing is worthwhile both as a preventive check and when you have specific symptoms. The most common reasons for testing are:

• symptoms that may be related to the liver: chronic fatigue, loss of appetite, nausea, pain or discomfort beneath the right rib cage, itchy skin, dark urine, pale stools, yellowing of the skin or whites of the eyes, swollen legs;
• risk factors: overweight or obesity, type 2 diabetes, lipid disorders, regular alcohol consumption, a family history of liver disease, possible exposure to hepatitis viruses;
• taking medicines that may affect the liver (your doctor will then arrange monitoring tests at set intervals);
• monitoring an existing liver disease and assessing how well treatment is working;
• a routine health check, usually alongside a full blood count, glucose and a lipid profile.

How should you prepare for liver function tests?

Laboratory recommendations differ in their details, but the safest approach is to follow these rules:

• On an empty stomach. Blood is usually collected in the morning, after 8–12 hours without food. You can drink still water. Coffee, tea and juice are not recommended.
• Avoid a fatty evening meal. A large, fatty meal can temporarily raise ALP. On the other hand, prolonged fasting (more than a day) can raise bilirubin, so do not go too far in the other direction.
• Alcohol. Avoid it for at least 24 hours before the test. Some laboratories recommend 48 hours, as alcohol has a particularly strong effect on GGT.
• Exercise. Avoid strenuous exercise for a day before blood collection. Heavy exercise damages muscle fibres and can raise AST, which can easily be mistaken for a liver problem.
• Medicines and supplements. Do not stop taking them on your own. Tell your doctor about everything you take, including herbal preparations, as some of them affect test results.

How do you read the results? Normal ranges for ALT, AST, GGT, ALP and bilirubin

Reference ranges depend on the method, reagents, sex and age, so always compare your result with the normal range printed on your report. The table below shows typical adult ranges provided by Mayo Clinic and is intended as a guide.

It is worth knowing that the upper limits for ALT in many laboratories are quite high. The American College of Gastroenterology guidelines from 2017 state that, in healthy people without risk factors, ALT is usually up to 29–33 U/l in men and up to 19–25 U/l in women, and higher values should be assessed by a doctor. The European guidelines on fatty liver disease from 2024 consider ALT above 33 U/l in men and above 25 U/l in women to be raised.

A few patterns can help with interpretation:

• ALP in children, teenagers and pregnant women can be naturally higher. In children, this is due to bone growth.
• The AST to ALT ratio (known as the De Ritis ratio). A value of 2:1 or higher is a pattern typical of alcohol-related liver damage, while a value above 3:1 makes it even more likely. This is only a clue, not a diagnosis.
• An isolated rise in AST with normal ALT suggests a possible muscle or heart cause. Your doctor may then request a creatine kinase (CK) test.
• Mildly raised bilirubin (mainly indirect bilirubin), with normal enzymes and no signs of red blood cell breakdown, often indicates Gilbert’s syndrome. This affects a few per cent of the population and usually does not require treatment. In this condition, bilirubin may rise further after fasting or during illness.

One abnormal result does not mean that you have liver disease. However, the cause needs to be established, and sometimes this simply involves repeating the test after a few weeks.

Liver function tests and fatty liver disease (MASLD)

The most common cause of mildly raised ALT in adults is fatty liver disease associated with metabolic disorders. In 2023, international liver societies used a Delphi consensus process to replace the former name NAFLD (non-alcoholic fatty liver disease) with the term MASLD, meaning metabolic dysfunction-associated steatotic liver disease. The new terminology was adopted by the European EASL, EASD and EASO guidelines published in 2024.

According to these guidelines, MASLD is diagnosed when fat accumulation is found in the liver and at least one of five cardiometabolic risk factors is present:

• overweight or obesity (BMI of 25 kg/m² or above, or waist circumference above 94 cm in men and 80 cm in women of European origin);
• prediabetes or type 2 diabetes;
• triglycerides of 150 mg/dl or above, or lipid-lowering treatment;
• low HDL cholesterol (below 40 mg/dl in men, below 50 mg/dl in women) or treatment;
• blood pressure of 130/85 mmHg or above, or treatment for hypertension.

When someone has metabolic risk factors and also drinks more alcohol (20–50 g daily for women, 30–60 g daily for men), the guidelines use a separate category called MetALD.

An important practical point: normal ALT does not rule out fatty liver disease or even liver inflammation. This is why, in people with risk factors, doctors also assess the risk of fibrosis, most often using the FIB-4 score, calculated from age, AST, ALT and platelet count. A result below 1.3 means a low risk of advanced fibrosis; higher values indicate a need for elastography or a consultation with a liver specialist. If you have fatty liver disease, it is also worth checking your lipid profile and fasting glucose.

What should you do with your results, and when should you seek urgent medical advice?

See your GP if your results are slightly outside the normal ranges. They will usually start by taking a medical history (alcohol, medicines, supplements, body weight), repeating the tests and arranging further investigations: an abdominal ultrasound, anti-HCV antibodies, HBs antigen and, if necessary, tests for autoimmune and metabolic liver diseases.

Contact a doctor urgently, on the same day, or go to SOR (szpitalny oddział ratunkowy — hospital emergency department) if:

• you notice yellowing of your skin or the whites of your eyes, especially with dark urine, pale stools or itching;
• you develop confusion, excessive sleepiness, bleeding or easy bruising;
• your results show very high aminotransferases (many times above the normal range, especially ALT above 1000 U/l);
• your enzyme levels became high after taking a large dose of paracetamol or unknown supplements.

The ACG guidelines emphasise that moderately and markedly raised ALT or AST require immediate assessment, and acute hepatitis with prolonged prothrombin time or an altered level of consciousness requires urgent referral to a liver specialist.

Which tests should you have alongside liver function tests?

The package alone provides a general picture. To allow reliable interpretation of the results, it is worth adding:

• a full blood count (platelets are needed to calculate FIB-4, and haemolytic anaemia explains a rise in bilirubin);
• albumin and INR to assess the liver’s ability to produce proteins;
• HBs antigen and anti-HCV antibodies, because viral hepatitis can remain symptom-free for a long time;
• glucose, HbA1c and a lipid profile if MASLD is suspected;
• CK if AST is the main raised parameter.

If you are monitoring a single parameter, for example after changing medicines or cutting down on alcohol, you can have a separate ALT test, gamma-glutamyl transferase (GGT) test or bilirubin test. It is best to have repeat measurements at the same laboratory, so that the results are comparable.

Frequently asked questions

Do liver function tests need to be done on an empty stomach?

Yes, it is recommended that you go without food for 8–12 hours and have your blood collected in the morning. A meal, especially a fatty one, can affect ALP and make the sample cloudy, and the package is often carried out alongside tests that require an empty stomach.

How many days before the test should you avoid alcohol?

At least 24 hours, and preferably 48 hours.

Can exercise raise liver test results?

Yes, especially AST, which is present in muscles. Strenuous exercise the day before the test can produce a result suggesting a liver problem, even though the source is the muscles. It is therefore best to skip heavy training for a day before blood collection.

Do normal liver function tests rule out liver disease?

No. Fatty liver disease, and even fibrosis, can occur with normal ALT. In people with obesity, diabetes or lipid disorders, the doctor should also assess ultrasound findings and the FIB-4 score.

Sources

• Mayo Clinic, “Liver function tests”. link
• MedlinePlus, U.S. National Library of Medicine, “Liver Function Tests”. link
• MedlinePlus, “Alkaline Phosphatase”. link
• Kwo P.Y., Cohen S.M., Lim J.K., American College of Gastroenterology, “ACG Clinical Guideline: Evaluation of Abnormal Liver Chemistries”, Am J Gastroenterol, 2017. link
• EASL, EASD, EASO, “Clinical Practice Guidelines on the management of metabolic dysfunction-associated steatotic liver disease (MASLD)”, Journal of Hepatology, 2024. link
• Rinella M.E. et al., “A multisociety Delphi consensus statement on new fatty liver disease nomenclature”, 2023. link
• NHS, “Jaundice”. link
• Medycyna Praktyczna for patients, “Próby wątrobowe” (Liver function tests). link
• Diagnostyka, “Pakiet wątrobowy” (Liver test package), patient information material, 2024. link