Liver Function Tests (LFTs): ALT, AST, ALP, GGT and Bilirubin Explained

Slightly abnormal liver tests are very common, and most are not caused by serious liver disease. But some patterns do need further tests, and knowing them can help you ask the right questions.

Imagine a routine blood test showing your ALT is “slightly raised”. You drink only at weekends and feel well. Should you worry about your liver?

What LFTs measure

• ALT and AST: enzymes released when liver cells are damaged
• ALP: raised in bile duct problems and also bone conditions
• GGT: often raised with alcohol use and some medicines
• Bilirubin: high levels cause jaundice
• Albumin: a protein made by the liver; low levels can suggest long-term liver disease or poor nutrition

Common causes of abnormal results

• Fatty liver disease linked to weight, diabetes or alcohol
• Alcohol
• Medicines, including some painkillers and antibiotics
• Viral hepatitis
• Gallstones blocking the bile duct
• Muscle injury (can raise AST)

What happens next

• Repeat tests
• A liver screen for hepatitis viruses and other causes
• Ultrasound scan
• Assessment of liver scarring (fibrosis), for example with a FIB-4 score or elastography

Lab ranges vary

Reference ranges differ between laboratories and countries, and some use different units. Always compare your result with the range printed on your report and discuss it with your healthcare provider.

Patterns healthcare providers look for

Rather than looking at each number alone, healthcare providers look at the pattern. When ALT and AST are much higher than ALP, the problem is usually inside the liver cells themselves, for example from viral hepatitis, fatty liver, alcohol or medicines. This is called a hepatitic pattern.

When ALP and GGT are raised more than ALT, the problem is more likely to involve the bile ducts, such as a gallstone blocking the duct, or narrowing caused by inflammation or a tumour. This is called a cholestatic pattern. A raised ALP with a normal GGT may point to bone rather than liver, for example during growth in teenagers, after a fracture, or in some bone diseases.

The AST to ALT ratio

An AST level roughly twice the ALT level can suggest alcohol-related liver disease, while in fatty liver linked to weight and diabetes ALT is often higher than AST. These are clues, not diagnoses, and healthcare providers combine them with your history and other tests.

Liver function is more than LFTs

Despite the name, most “liver function tests” actually measure liver damage or bile flow rather than how well the liver works. The true markers of liver function are albumin, bilirubin and the INR (a clotting test), because a healthy liver makes proteins and clotting factors and clears bilirubin.

When albumin falls and INR rises in someone with liver disease, it suggests the liver is struggling to do its job. This needs specialist assessment.

Checking for liver scarring

Fatty liver disease is now one of the most common causes of abnormal LFTs worldwide. Most people with fatty liver do well, but some develop scarring (fibrosis) that can progress to cirrhosis.

Healthcare providers often use a simple score called FIB-4, calculated from age, AST, ALT and platelet count, to estimate the risk of significant scarring. People with a higher score may be offered a specialised scan called transient elastography (often known by the brand name FibroScan) or referral to a liver specialist.

What you can do to help your liver

• Keep alcohol within recommended limits, and have several alcohol-free days each week
• Lose weight gradually if you are overweight; even 5 to 10% can reduce liver fat
• Be active most days
• Manage diabetes, blood pressure and cholesterol
• Get vaccinated against hepatitis A and B if you are at risk
• Check with a pharmacist before taking herbal remedies or supplements, some of which can harm the liver

Warning signs that need urgent help

• Yellowing of the skin or eyes (jaundice)
• Very dark urine with pale stools
• Vomiting blood or passing black, tarry stools
• Confusion or unusual drowsiness in someone with liver disease
• Rapidly swelling tummy

Medicines and supplements that can affect LFTs

Many medicines can raise liver enzymes. Common examples include some antibiotics such as co-amoxiclav, anti-epileptic medicines, some antifungal tablets, methotrexate and tuberculosis treatment. Paracetamol at normal doses is generally safe, but taking more than the recommended amount, even over a few days, can cause serious liver damage.

Herbal and dietary supplements are an increasingly common cause of liver injury. Products marketed for weight loss, bodybuilding or “detox” are particular culprits. Always tell your healthcare provider about everything you take, including teas, powders and remedies bought online.

Viral hepatitis

Hepatitis B and C often cause no symptoms for years while slowly damaging the liver. A simple blood test can check for them, and effective treatments exist. Hepatitis C can usually be cured with a short course of tablets, and hepatitis B can be controlled with long-term medicine. If you have abnormal LFTs, your healthcare provider will usually test for these infections.

Questions to ask about abnormal LFTs

• Which results are abnormal, and what pattern do they show?
• Could any of my medicines or supplements be the cause?
• Do I need a liver screen, an ultrasound or a FIB-4 score?
• What lifestyle changes would help most?
• When should the test be repeated?

Alcohol and the liver

The liver can repair itself to a remarkable degree, but repeated heavy drinking can lead to fatty liver, inflammation (alcoholic hepatitis) and eventually cirrhosis. Often there are no symptoms until damage is advanced. If your LFTs are abnormal and you drink alcohol regularly, cutting down or stopping is one of the most powerful things you can do. Ask for support if you find it hard, and do not stop suddenly if you drink heavily every day, as withdrawal can be dangerous without medical help.

Frequently asked questions

Can paracetamol affect LFTs?

Normal doses are generally safe, but overdose can seriously damage the liver.

Is Gilbert’s syndrome dangerous?

Gilbert’s syndrome causes mildly raised bilirubin and is usually harmless.

Do I need to fast for LFTs?

Usually not, unless other tests are being done at the same time.

Can statins raise LFTs?

Statins can cause mild rises in ALT, which are usually not harmful. Your healthcare provider will advise if any change is needed.

How soon will abnormal LFTs be repeated?

It depends on the pattern and how abnormal they are. Mild changes are often repeated in a few weeks to months.

But my LFTs are normal. Is my liver definitely healthy?

Not always. Fatty liver and scarring can exist with normal LFTs, so risk factors matter too.

Related: Fatty liver disease · Normal lab values cheat sheet

Sources: British Society of Gastroenterology guidance on abnormal liver blood tests, American College of Gastroenterology, British Liver Trust, Lab Tests Online UK.