Liver enzyme explainer

Add ALT and AST with the limits on your report, and we will explain each one, starting with the number GPs look at first.

Your details

ALT 40, AST 32

Within your lab's range

FIB-4 1.05

ALT40 U/L (0.9 times your lab's upper limit)
AST32 U/L (0.8 times your lab's upper limit)
AST to ALT (De Ritis) ratio0.80, little meaning when both are in range
FIB-4 score1.05, below 1.3 (the low risk cut off for your age)

Both enzymes you gave limits for are within your lab's range. Your FIB-4 is in the range the British Society of Gastroenterology calls low risk of advanced scarring, for people being checked for fatty liver. The ratio and FIB-4 are clues doctors read alongside your history; neither can tell you what is going on by itself.

If a result worries you, ask your GP surgery or practice nurse to talk it through. Call 111 if you feel unwell and are not sure what to do.

The working

AST 32, ALT 40, age 52, platelets 250

ALT 40 against a limit of 45 is 0.9 times the limit, and AST 32 against 40 is 0.8 times, so both are within range. The ratio is 32 ÷ 40 = 0.80. FIB-4 is (52 × 32) ÷ (250 × √40) = 1,664 ÷ 1,581 = 1.05, below the 1.3 cut off the British Society of Gastroenterology uses for low risk of advanced scarring under 65.

What GPs look at

ALT first, then the pattern

The BSG guideline on abnormal liver blood tests starts with ALT, alongside bilirubin, albumin, ALP and GGT. A result is abnormal when it is outside your own lab's range, so we check AST and ALT separately against the limits on your report. If either is more than twice the limit, we say so plainly, because that is worth a prompt conversation with your GP. The old De Ritis ratio stays as a row, since a review by Botros and Sikaris (2013) shows simple rules like AST higher means alcohol are far too simple.

FIB-4

Why age and platelets matter

FIB-4 combines age, AST, ALT and platelets: (age × AST) ÷ (platelets × √ALT). The BSG recommends it as the first step for people with fatty liver (now often called MASLD), because a low score safely rules out advanced scarring for most people. It is a screening clue, so a GP reads it with your history and other tests.

Liver cells or bile flow

The R ratio and your trend

Add ALP and we work out the R ratio: ALT as a multiple of its upper limit, divided by ALP as a multiple of 130 U/L, a common adult upper limit. European liver guidance (EASL 2019) reads 5 or more as a hepatitic pattern, affecting the liver cells, 2 or less as cholestatic, affecting bile flow, and in between as mixed. ALT 90 against a limit of 45 with ALP 100 gives 2 ÷ 0.77 = 2.6, mixed. Add your last ALT to see the change; ALT can rise for a few days after hard exercise, alcohol or some medicines, so a repeat often settles it.

Read with care

What this cannot tell you

Diagnose liver diseaseNo enzyme, ratio or score can do that alone.
Judge AST aloneAST also comes from muscle, so a hard workout can raise it.
Know your lab's rangeUse the limits printed on your report.

Questions

Common questions

Which liver enzyme matters most?

GPs usually look at ALT first, alongside the other liver tests. AST is useful too, especially for FIB-4, but it also rises after hard exercise.

What is a normal AST to ALT ratio?

There is no single normal value that matters on its own. The ratio is mostly used when enzymes are raised, as one clue among many.

What does a FIB-4 score mean?

It estimates the chance of advanced liver scarring in people with fatty liver. Below 1.3 (or 2.0 from 65) is low risk; above 3.25 usually leads to a specialist opinion.

Should I worry about slightly raised liver tests?

Mildly raised results are common. Talk them through with your GP, who will decide whether more tests are needed. Results more than twice the limit are worth raising soon.

Sources

Last checked 21 October 2026. Spotted something wrong? Tell us and we will fix it.

Talk your results through

If a result worries you, book a chat with your GP or practice nurse, or ask a pharmacist. Call NHS 111 if you feel unwell and are not sure what to do, and 999 in an emergency.