Metabolic health
Fatty Liver Blood Test UK: ALT, AST, GGT & What the Numbers Mean
Reviewed by a qualified clinician · analysed at UKAS-accredited UK labs (ISO 15189)
Last reviewed June 2026
Every Helvy guide is written by our health editors, then checked by a qualified clinician before it goes live and re-checked as the science moves. We name clinical roles, not individuals, until each reviewer has agreed to be credited publicly. This is wellness guidance to help you understand your own data, not a diagnosis.
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Fatty liver is flagged by three liver enzymes, ALT, AST and GGT, read together with the AST:ALT ratio. Raised enzymes suggest the liver is under strain. A normal ALT does not rule fatty liver out: the fat can be there with every enzyme still in range. So the enzymes are read alongside HbA1c and your lipids, by a qualified clinician.
Non-alcoholic fatty liver disease (NAFLD) is the most common liver condition in the developed world. It is also largely silent. The liver enzyme ALT can stay mildly raised for years with no symptoms. That is why a blood test matters. It is often the only early signal.
The catch is that liver enzymes are an imperfect screen. They can look normal even when fat has built up. They can also rise for reasons that have nothing to do with the liver, such as recent intense exercise. So it helps to read ALT, AST, and GGT together, and to look at the AST:ALT ratio. That gives a far more useful picture than any single enzyme alone.
If you are holding a report and want the figures themselves, our page on the fatty liver blood test normal range lists the UK numbers for ALT and AST side by side.
1. What Fatty Liver Is
Fatty liver means exactly what it sounds like: excess fat stored inside liver cells. A small amount of fat is normal, but once it passes a threshold, the liver can become inflamed and, over years, scarred. Non-alcoholic fatty liver disease (NAFLD) is the term used when this happens without heavy alcohol intake being the cause.
NAFLD is common, and it is the leading cause of liver disease in the developed world. The drivers are the ones behind metabolic syndrome: excess weight, insulin resistance and a diet high in refined sugar.
Most fatty liver is silent. There is often no pain and no symptom at all until the condition is advanced. That is why blood markers, imperfect as they are, do useful work. A mildly raised liver function test is frequently the first hint that anything is wrong.
Raised triglycerides often turn up on the same report. Our guide to what a high triglyceride number means covers why NICE lists liver disease among the causes to rule out. ALP and albumin sit on the same panel, and ALP in particular shifts a long way with age.
Liver enzymes are also the numbers to watch if you are trying a supplement. Our guides to milk thistle and fatty liver and what the glutathione trial found go through both.
“Non-alcoholic fatty liver disease does not usually cause any symptoms.”
— NHS, Non-alcoholic fatty liver disease (NAFLD)
2. The Blood Markers: ALT, AST & GGT
Three liver enzymes do most of the work in screening for fatty liver. Each tells a slightly different part of the story.
ALT (alanine aminotransferase) →
ALT sits inside liver cells and leaks into the blood when the liver is inflamed or stressed. It is the most liver-specific of the three, and the one most often raised in fatty liver. Hard exercise can lift it for a day or two as well. We cover how high an ALT has to be to matter separately.
AST (aspartate aminotransferase)
AST is found in the liver but also in muscle and the heart, so it is less liver-specific than ALT. On its own it is a blunter tool, but its real value comes from comparing it to ALT. A pattern where AST rises relative to ALT can suggest more advanced liver change or an alcohol-related cause, which is why the two are read as a ratio.
GGT (gamma-glutamyl transferase)
GGT is sensitive but not specific. It rises with fatty liver, with alcohol, and with bile-flow problems, so it broadens the picture rather than pinpointing a cause. Raised GGT with a raised ALT supports a liver explanation. Raised GGT with a clear pattern of drinking points at alcohol instead.
3. The AST:ALT Ratio
The AST:ALT ratio is simply AST divided by ALT, and it adds information that neither enzyme gives alone. In early, uncomplicated fatty liver the ratio is usually below 1, because ALT tends to be the higher of the two. Both halves of the sum shift with age and sex, as ALT and AST levels by age sets out.
A ratio that climbs above 1, and especially above roughly 2, is a recognised flag that a clinician takes more seriously. It can suggest more advanced liver scarring. It can also point toward an alcohol-related pattern, where AST is often raised out of proportion to ALT. Either way the ratio is a signpost, not a diagnosis. It tells a clinician where to look next, not what is definitely wrong.
Like every marker here, the ratio is interpreted in context. A single slightly elevated ratio after a hard training week means something very different from a persistently raised ratio with other abnormal liver markers.
4. FIB-4 as Informational Context
You may see FIB-4 mentioned alongside fatty liver. It is not a blood test. It is a sum that combines your age, AST, ALT and platelet count to estimate how likely advanced scarring (fibrosis) is. NICE points to non-invasive scores of this kind as a way of deciding who needs further assessment. You can work out a FIB-4 score from numbers you already have.
It is context, not a verdict. A low score is reassuring about advanced fibrosis. An intermediate or high one is a prompt for a clinician to look further, often with a scan. The point is that the same enzymes that flag fatty liver also feed a fibrosis estimate.
5. Why a "Normal" ALT Can Still Hide Fatty Liver
This is the single most important thing to understand about liver enzymes: a normal ALT does not mean a healthy liver. A substantial proportion of people with fatty liver have enzyme levels that sit comfortably within the standard reference range. If you are holding a result and want the figures side by side, our guide to the fatty liver blood test normal range sets out the UK numbers and what they miss.
Part of the problem is the range itself. The upper limit many UK labs use for ALT was set decades ago, and by modern standards it is generous.
The published evidence increasingly suggests that an ALT in the upper part of the "normal" band can still reflect metabolic liver strain. That is most likely when weight, glucose and lipids all point the same way. It is why Helvy reports a tighter optimal range for ALT than the wide clinical range.
The practical takeaway: enzymes are a useful screen but not a rule-out. If your ALT is normal but you carry the metabolic risk factors for fatty liver, the markers should be read alongside your wider metabolic health picture rather than treated as an all-clear.
6. The Metabolic-Syndrome Link
Non-alcoholic fatty liver is widely regarded as the liver's expression of metabolic syndrome. The same drivers that raise blood sugar, blood pressure, and triglycerides also push fat into the liver: insulin resistance is the common thread.
When the body becomes resistant to insulin, the liver receives more fatty acids and converts more sugar into fat for storage. Over time that fat accumulates inside liver cells. This is why fatty liver so often travels with raised HbA1c, high triglycerides, central weight gain, and insulin resistance.
It also means the liver markers tell you most when they are not read in isolation. ALT, AST and GGT alongside HbA1c and a lipid panel give a clinician the joined-up picture a single enzyme never could.
There is good news here. Fatty liver is one of the more responsive metabolic problems: modest weight loss and less refined sugar can lower liver fat and bring enzymes down. Food matters too. Choline is the nutrient the liver needs to move fat back out, and most UK diets run short of it. Our choline and fatty liver guide covers what the feeding studies found.
7. Alcohol-Related vs Non-Alcoholic
Fat in the liver can come from metabolic causes (NAFLD) or from alcohol, and the two overlap. Blood markers cannot reliably separate them, which is why an honest account of what you drink is part of how a clinician reads the result.
There are patterns. A GGT raised out of proportion to ALT, or a ratio above 1, leans towards alcohol. A raised ALT with a ratio below 1 and metabolic risk factors leans towards NAFLD. These are tendencies, not proof, and plenty of people sit between the two.
Either way, the early stages are usually reversible. Less alcohol, less excess weight and better insulin sensitivity all lower liver fat. Testing is worth doing while that window is still open.
8. Reference Ranges
Reference ranges are wide and designed to flag disease, not to describe metabolic health. The ALT figures below are the ranges Helvy reports. AST and GGT ranges vary between laboratories and are quoted here as typical UK lab values for context; always read your result against the reference range printed on your own report.
| MARKER | TYPICAL RANGE | NOTE |
|---|---|---|
| ALT (Helvy clinical) | 0–41 U/L | Most liver-specific enzyme |
| ALT (Helvy optimal) | 10–26 U/L | Tighter band Helvy reports as optimal |
| AST | Typically up to ~40 U/L (lab-dependent) | Also found in muscle and heart |
| GGT | Typically up to ~50–60 U/L (lab-dependent) | Sensitive but not specific |
The ALT clinical range of 0–41 U/L and optimal range of 10–26 U/L are the bands Helvy reports for ALT. AST and GGT upper limits differ between laboratories and assay methods, so the figures above are indicative only. A qualified clinician interprets your numbers against your own report and your wider health picture.
If ALP is your raised number rather than ALT, the trail leads elsewhere. Our guide to a high ALP with normal liver function follows it.
9. When to See Your GP
A blood test result is information, not a diagnosis. Book a GP appointment if any of the following apply:
- Your liver enzymes are persistently raised on more than one test
- Your AST:ALT ratio is above 1, particularly if it is well above
- You have raised liver enzymes alongside high HbA1c, high triglycerides, or central weight gain
- You have yellowing of the skin or eyes, dark urine, pale stools, or persistent right-sided tummy pain
- You drink alcohol above the recommended limits and have any abnormal liver marker
- You have unexplained fatigue, nausea, or unintentional weight loss with abnormal liver results
The NICE guideline on abnormal liver blood tests (NG49) sets out how raised liver enzymes are assessed in the UK. Most fatty liver caught early is reversible, but the decision about what to do next, including whether further scans or scoring are needed, belongs with a qualified clinician who can read your results in full context.
10. Frequently Asked Questions
Can I have fatty liver with normal blood tests?
Yes. A substantial proportion of people with fatty liver have liver enzymes within the standard reference range. Enzymes are a useful screen but not a rule-out, which is why they are best read alongside your metabolic markers and risk factors rather than treated as an all-clear.
What does a raised ALT mean?
A raised ALT can indicate the liver is inflamed or under strain, and in the UK fatty liver is one of the most common reasons. ALT can also rise temporarily after intense exercise. A qualified clinician interprets it alongside AST, GGT, the AST:ALT ratio, and your wider health picture.
What is a normal AST:ALT ratio?
In early, uncomplicated fatty liver the ratio is usually below 1. A ratio that climbs above 1, and especially above roughly 2, is a flag a clinician takes more seriously, as it can point toward more advanced liver change or an alcohol-related pattern. It is a signpost, not a diagnosis.
Is fatty liver reversible?
In its early stages, usually yes. Modest weight loss, reduced refined-sugar intake, improved insulin sensitivity, and reduced alcohol can all lower liver fat and bring enzymes down. The value of testing is catching the problem while it is still in that reversible window. Discuss any abnormal result with a clinician.
Do I need to fast before a liver blood test?
Recent meals do not move liver enzymes much. A fasting morning sample is still often preferred, because the markers fatty liver is read alongside, HbA1c and your lipids, are more reliable fasted. Avoid hard exercise in the day or two before, as it can lift ALT and AST on its own.
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