Liver function test (LFTs): what’s checked and what abnormal results mean

What an LFT actually checks

In the UK, the British Society of Gastroenterology recommends a standard liver panel of five lines: bilirubin, albumin, ALT, alkaline phosphatase (ALP) and GGT, together with a full blood count if you have not had one in the past year. Some laboratories add AST or total protein, so check the lines printed on your own results.

The name is misleading, and the same guideline says so. It deliberately calls them liver blood tests rather than liver function tests, because most abnormal results are enzymes rising from irritated cells, not a liver that is failing at its job.

The lines that do describe function are albumin, a protein your liver builds, and the clotting time (prothrombin time or INR), which depends on clotting factors the liver makes. High enzymes with a normal albumin say something quite different from normal enzymes with a low albumin.

An abnormal result is common, and in the UK it is still looked into

If one line on your results is flagged, you are in large company. At one Birmingham hospital trust, 30% of the 130 849 liver blood tests requested by GPs in 2016 had at least one value outside its range. In Tayside, a quarter of adults had liver tests over ten years, and about a third of them had at least one abnormal value.

What happens next is where UK guidance is distinctive. The British Society of Gastroenterology advises against simply repeating the same panel and waiting. Unless your doctor thinks the change is clearly temporary, it recommends looking for a cause whatever the size of the rise and however long it has lasted.

That search has a name, a liver aetiology screen: an ultrasound scan, tests for hepatitis B and C, a few antibody tests and immunoglobulins, and ferritin with transferrin saturation to look for iron overload. It sounds a lot, but it is routine, and most of it is a second set of blood tests.

The reason for that caution is the trend. Deaths from liver disease in the UK rose by around 400% between 1970 and 2010, while deaths from most other common conditions fell, and much of that disease is silent until late.

ALT or AST: which matters more?

ALT is the steadier pointer to the liver, because there is very little of it anywhere else. AST is also found in muscle, so a hard workout can lift it, and the standard UK panel recommended by gastroenterologists does not include it at all.

AST still has its uses. It can be the more sensitive sign in alcohol-related liver disease, and when both are measured, an AST higher than ALT (a ratio above 1) suggests that scarring may be advanced. UK gastroenterologists describe adding AST automatically, so that the ratio can be worked out, as clearly desirable.

One more detail worth knowing: the British Society of Gastroenterology notes that the upper limit printed for ALT may be too high. Hepatitis B guidelines already treat an ALT above 30 U/L in men and above 19 U/L in women as significant. A value printed as normal against a wider laboratory range can still be worth a question.

Why GGT is the most misread line

GGT has a reputation as an alcohol test. It is not one. A review of alcohol markers is explicit that liver enzymes lack specificity for alcohol, and that ALT and GGT also track insulin resistance and metabolic syndrome.

GGT has two real jobs. When ALP is raised on its own, GGT tells your doctor whether the ALP is coming from the liver or from bone: a raised GGT points to the liver. And in people who do drink, a raised GGT is linked with a higher risk of dying from liver disease and from other causes, which is why a high value is used to start a conversation about drinking.

So a raised GGT means something is making your liver work. Alcohol is one candidate; weight, several medicines, bile duct problems and fatty liver are others.

Fatty liver: common, and often invisible on the blood test

In a UK study of adults with abnormal liver tests in general practice, nearly 4 in 10 had a fatty liver on ultrasound, and a raised ALT was the strongest blood clue. The condition is now called metabolic dysfunction-associated steatotic liver disease (MASLD).

The catch runs the other way too. NICE notes that MASLD is present in more than half of adults with type 2 diabetes or metabolic syndrome, yet more than 80% of people with it have normal routine liver blood tests. A normal LFT does not rule fatty liver out.

That is why the liver lines are read next to blood sugar, weight and the cholesterol results, and why someone found to have fatty liver is offered a scarring score such as FIB-4, worked out from routine blood results, before anything more specialised.

What can throw off a liver function test?

Medicines come first. The Association for Laboratory Medicine points out that many over-the-counter medicines and herbal or dietary supplements can affect the liver, and that too much paracetamol, especially with alcohol, can cause serious damage. Bring a full list, including what you buy at the chemist.

Severe exercise, infections, injuries and pregnancy can also push one or more lines out of range. Bilirubin is read with your blood count, because it rises when red cells break down faster than usual, which says nothing about liver damage.

What happens after an abnormal LFT

Each range leaves about 1 in 20 healthy people outside it, so a single flagged value is a finding, not a diagnosis. Your GP will look at which lines moved, by how much, your medicines, your drinking and your weight, and usually arrange the aetiology screen rather than a simple repeat.

Common causes are checked first: fatty liver, alcohol, viral hepatitis and medicines, before rarer conditions such as autoimmune liver disease or iron overload. The order is your doctor’s call, guided by the pattern.