METABOLIC HEALTH
Liver Function Test UK: What LFTs Measure, What Results Mean & When to Worry
Liver function tests — commonly called LFTs — are among the most frequently ordered blood tests in UK general practice. They are requested roughly 1 in every 5 GP blood requests, yet the results are often poorly explained and frequently misunderstood.
This guide explains every marker in a standard LFT panel, what abnormal results actually mean, why the British Liver Trust calls liver disease "the silent killer," and when you should see your GP.
1. What Are Liver Function Tests?
Liver function tests are a group of blood tests that measure enzymes, proteins and substances produced or processed by the liver. Despite the name, most LFTs actually measure liver damage rather than liver function — elevated enzymes indicate that liver cells are being injured and leaking their contents into the bloodstream.
Your liver performs over 500 vital functions: filtering toxins, producing bile for digestion, storing glycogen for energy, making clotting factors, metabolising drugs, processing haemoglobin, and regulating cholesterol. It is the only organ that can regenerate itself — but this remarkable resilience is also why liver disease is so dangerous. By the time symptoms appear, damage is often advanced.
The British Liver Trust reports that liver disease is now the third leading cause of premature death in the UK, with deaths increasing by 400% since 1970. Most liver disease is preventable, and early detection through blood testing is the single most effective intervention.
2. What Gets Tested: The Full LFT Panel
A standard UK LFT panel typically includes six markers. Each tells a different part of the story:
ALT (Alanine Aminotransferase)
The most liver-specific enzyme. ALT is found predominantly in liver cells (hepatocytes), so raised levels almost always point to liver injury. It is the primary screening marker for non-alcoholic fatty liver disease (NICE NG49).
NHS range: 0–41 U/L · Optimal: <30 U/L (male), <19 U/L (female)
AST (Aspartate Aminotransferase)
Found in liver, heart, muscle and kidney tissue. Less liver-specific than ALT, but the ratio of AST to ALT is clinically significant — an AST:ALT ratio above 2:1 strongly suggests alcoholic liver disease, while a ratio below 1:1 is more typical of fatty liver.
NHS range: 0–40 U/L · Optimal: <30 U/L
ALP (Alkaline Phosphatase)
Present in bile ducts, bone and intestine. Elevated ALP with normal ALT and AST often indicates a bile duct problem (cholestasis) rather than liver cell damage. ALP is also raised in bone conditions — growing children, Paget's disease, and healing fractures.
NHS range: 30–130 U/L · Context-dependent (age, sex, pregnancy)
GGT (Gamma-Glutamyl Transferase)
The most sensitive marker for bile duct problems and a strong indicator of alcohol use. GGT rises even with moderate regular drinking and is frequently used alongside ALT to distinguish alcoholic from non-alcoholic liver injury. It also correlates with cardiovascular risk independently of other factors.
NHS range: 0–60 U/L (male), 0–40 U/L (female) · Optimal: <30 U/L
Bilirubin
A yellow pigment produced when red blood cells break down. The liver processes bilirubin and excretes it in bile. Raised bilirubin causes jaundice (yellowing of skin and eyes). Mildly elevated bilirubin with otherwise normal LFTs is usually Gilbert's syndrome — a harmless genetic condition affecting roughly 5–10% of the UK population.
NHS range: 0–21 µmol/L · Gilbert's: typically 20–50 µmol/L
Albumin
The most abundant protein in blood, made exclusively by the liver. Albumin is a true measure of liver function— low albumin indicates the liver is struggling to synthesise proteins, which only happens in significant chronic liver disease, malnutrition, or kidney disease (nephrotic syndrome). It takes weeks to months to drop.
NHS range: 35–50 g/L · Optimal: 40–50 g/L
3. NHS Reference Ranges vs Optimal Levels
NHS reference ranges define what is statistically "normal" in the population — but normal is not the same as optimal. An ALT of 40 U/L sits within the NHS range, but research published in the BMJ suggests that the upper limit of normal for ALT should be lowered to 30 U/L for men and 19 U/L for women to improve detection of early liver disease.
| Marker | NHS range | Optimal | Notes |
|---|---|---|---|
| ALT | 0–41 U/L | <30 (M), <19 (F) | Most liver-specific enzyme |
| AST | 0–40 U/L | <30 U/L | AST:ALT ratio aids diagnosis |
| ALP | 30–130 U/L | 40–100 U/L | Also bone marker (context needed) |
| GGT | 0–60 U/L (M) | <30 U/L | Sensitive to alcohol + bile ducts |
| Bilirubin | 0–21 µmol/L | <17 µmol/L | Gilbert's = mild rise, harmless |
| Albumin | 35–50 g/L | 40–50 g/L | True liver function marker |
ALT
Most liver-specific enzyme
AST
AST:ALT ratio aids diagnosis
ALP
Also bone marker
GGT
Sensitive to alcohol
Bilirubin
Gilbert's = harmless
Albumin
True function marker
Important: A single abnormal result does not mean you have liver disease. Liver enzymes fluctuate with exercise, medications, supplements, recent illness and even time of day. A mildly raised ALT should be repeated after 4–6 weeks before further investigation. Persistently elevated enzymes warrant GP follow-up.
4. The ALT:AST Ratio — and Why It Matters
The individual numbers matter, but the ratio of AST to ALT is often more informative than either value alone. The British Society of Gastroenterology (BSG) guidelines use this ratio as a key diagnostic tool:
AST:ALT ratio < 1.0 — Typical of non-alcoholic fatty liver disease (NAFLD/MASLD). ALT is higher because hepatocyte injury dominates.
AST:ALT ratio > 2.0 — Strongly suggests alcohol-related liver disease. Alcohol suppresses ALT synthesis while AST rises from both liver and muscle damage.
AST:ALT ratio > 1.0 (rising over time) — May indicate progression from simple fatty liver to fibrosis or cirrhosis. As fibrosis advances, the ratio tends to invert.
GPs also use the FIB-4 score — a formula combining age, AST, ALT and platelet count — to estimate the degree of liver fibrosis without an invasive biopsy. NICE NG49 recommends FIB-4 as the first-line non-invasive assessment for anyone with suspected NAFLD.
5. NAFLD: The Epidemic Hiding in Plain Sight
Non-alcoholic fatty liver disease (NAFLD) — recently renamed metabolic dysfunction-associated steatotic liver disease (MASLD) — is now the most common liver condition in the Western world. A Lancet Gastroenterology & Hepatology meta-analysis estimated global prevalence at 32% of the adult population and rising.
In the UK, an estimated 1 in 3 adults has some degree of fatty liver. The British Liver Trust describes it as a "ticking time bomb" driven by obesity, type 2 diabetes, and metabolic syndrome. Most people have no symptoms.
NAFLD progresses through four stages:
1. Simple steatosis: Fat accumulation in >5% of hepatocytes. Usually harmless on its own. No inflammation.
2. NASH (steatohepatitis): Fat plus inflammation. Liver cells are being damaged. Affects roughly 10–20% of those with simple steatosis.
3. Fibrosis: Persistent inflammation causes scar tissue (fibrosis) to build around liver cells and blood vessels. Still potentially reversible.
4. Cirrhosis: Extensive scarring replaces healthy tissue. Liver function deteriorates. Largely irreversible. Risk of liver failure and hepatocellular carcinoma.
The critical insight: stages 1–3 are largely reversible with lifestyle intervention. A Lancet trial (Vilar-Gomez et al.) showed that 10% body weight loss resolved NASH in 90% of participants and improved fibrosis in 45%. This is why early detection through routine blood testing matters — catching fatty liver at stage 1 or 2 gives you the widest window to intervene.
6. Common Causes of Abnormal LFTs
Abnormal liver enzymes are extremely common — roughly 1 in 5 routine blood tests in UK primary care returns at least one raised LFT value. Most of the time, the cause is benign or easily addressable:
Non-alcoholic fatty liver disease (NAFLD)
The most common cause in the UK, affecting roughly 1 in 3 adults. Typically causes mildly raised ALT (1–3× upper limit of normal) with ALT > AST. Strongly associated with obesity, insulin resistance, type 2 diabetes and metabolic syndrome.
Alcohol
Even moderate regular drinking can raise GGT. Heavier drinking produces the classic AST:ALT ratio > 2:1 pattern with elevated GGT. The NHS estimates that alcohol-related liver disease accounts for over 60% of liver disease deaths in the UK.
Medications
Paracetamol (even at recommended doses in some individuals), statins, NSAIDs (ibuprofen, naproxen), antibiotics (co-amoxiclav, flucloxacillin), and some herbal supplements (green tea extract at high doses, kava) can all raise liver enzymes. Drug-induced liver injury (DILI) is a diagnosis of exclusion — timing relative to medication start is the key clue.
Intense exercise
Heavy weight training or endurance exercise can raise AST (and to a lesser extent ALT) for 48–72 hours due to muscle breakdown. This is a benign and temporary elevation. If you train hard, avoid testing within 48 hours of an intense session for the most accurate liver-specific results.
Viral hepatitis
Hepatitis B and C are important causes of persistently raised liver enzymes. The NICE PH43 recommends hepatitis B and C screening for anyone with unexplained persistently elevated ALT. Hepatitis C is now curable in >95% of cases with direct-acting antiviral therapy.
Gilbert's syndrome
Causes mild, fluctuating unconjugated bilirubin elevation with completely normal liver enzymes and albumin. Harmless. Affects 5–10% of the UK population. Often discovered incidentally on routine blood tests. No treatment needed.
Less common causes
Autoimmune hepatitis, haemochromatosis (iron overload), Wilson's disease (copper overload), primary biliary cholangitis, coeliac disease, and thyroid disorders can all cause abnormal LFTs. These are less common but important to exclude in persistently abnormal results.
7. GP Liver Test vs Helvy
GPs can and do order LFTs — but there are practical differences in what you get:
| NHS GP | Helvy (Performance panel) | |
|---|---|---|
| Markers | Standard LFTs (ALT, AST, ALP, GGT, bilirubin, albumin) | LFTs + full lipid panel + HbA1c + kidney + thyroid + vitamins + hormones |
| When you can test | When GP agrees to order it | Whenever you decide |
| Wait time | 1–4 week GP appointment + 1–2 weeks results | Order today, results in 5 working days |
| Range interpretation | NHS clinical ranges only | Optimal + clinical ranges, contextualised in app |
| Metabolic context | LFTs in isolation | Liver markers alongside HbA1c, insulin, cholesterol — the full metabolic picture |
| Cost | Free (if GP agrees to order) | From £149 (Performance panel) |
Markers
Wait time
Interpretation
Context
Cost
Neither replaces the other. If your GP is investigating a specific concern — persistent symptoms, medication monitoring, or follow-up on previous abnormal results — their testing is appropriate and free. Helvy is for proactive screening when you want a comprehensive metabolic baseline that includes liver health alongside the wider picture.
8. Interpreting Your Results: Four Common Patterns
Raw numbers on a lab report are meaningless without context. Here are the four patterns clinicians look for:
Pattern 1: Raised ALT, normal AST, normal GGT
Most common pattern. Usually indicates non-alcoholic fatty liver disease (NAFLD), especially if the patient is overweight or has metabolic risk factors (high HbA1c, insulin resistance, raised triglycerides). Retest after 4–6 weeks with lifestyle modifications. If persistently elevated, GP should calculate FIB-4 score.
Pattern 2: Raised AST > ALT with raised GGT
The classic alcohol-related liver injury pattern. AST:ALT ratio > 2:1 with elevated GGT is highly suggestive. Mean corpuscular volume (MCV) on a full blood count may also be raised. Requires honest assessment of alcohol intake and GP follow-up.
Pattern 3: Raised ALP with normal ALT and AST
Isolated ALP elevation suggests bile duct obstruction (cholestatic pattern) or bone disease rather than liver cell damage. GGT helps differentiate: if GGT is also raised, the source is biliary; if GGT is normal, the source is likely bone. Further investigation with ultrasound may be warranted.
Pattern 4: Isolated raised bilirubin, everything else normal
Almost always Gilbert's syndrome if the bilirubin is unconjugated (indirect) and mildly raised (20–50 µmol/L). Completely harmless, no treatment needed. Bilirubin levels fluctuate with fasting, exercise, stress and illness. Interestingly, some evidence suggests Gilbert's may be cardioprotective due to bilirubin's antioxidant properties.
9. Who Should Get Tested
The NICE NG49 guidelines recommend that GPs consider NAFLD in anyone with type 2 diabetes or metabolic syndrome. But liver disease is underdiagnosed precisely because it is rarely screened proactively. Consider testing if you:
- Are overweight or obese (BMI >25, particularly if you carry weight around your middle)
- Have type 2 diabetes, prediabetes, or insulin resistance
- Drink more than 14 units of alcohol per week (the UK Chief Medical Officers' guideline)
- Take medications known to affect the liver (statins, paracetamol regularly, NSAIDs, methotrexate)
- Have a family history of liver disease or haemochromatosis
- Feel persistently fatigued, nauseous, or have unexplained abdominal discomfort
- Have high cholesterol or triglycerides (liver produces cholesterol and processes fats)
- Want a proactive metabolic baseline — especially if over 40 and never had LFTs checked
10. Alcohol and Your Liver
The UK Chief Medical Officers recommend no more than 14 units per week, spread over 3 or more days, with alcohol-free days each week. One unit is roughly half a pint of 4% beer or a small (125ml) glass of wine.
The NHS identifies three stages of alcohol-related liver disease (ARLD):
Alcoholic fatty liver: Reversible within 2–4 weeks of abstinence. Affects almost all heavy drinkers. Usually no symptoms.
Alcoholic hepatitis: Inflammation caused by long-term alcohol misuse. Can be mild (reversible with abstinence) or severe (life-threatening). Symptoms: jaundice, abdominal pain, fever.
Cirrhosis: Permanent scarring. Develops in 10–20% of heavy drinkers over 10+ years. Damage is irreversible, but abstinence can prevent progression and improve outcomes.
The evidence on "moderate" drinking: A landmark Lancet study (GBD 2016 Alcohol Collaborators) analysing data from 195 countries concluded that the safest level of alcohol consumption is zero. There is no "safe" threshold, though risk at low levels is small in absolute terms.
11. Evidence-Based Ways to Improve Liver Health
The liver is remarkably resilient. Even if blood tests show early signs of fatty liver, the following interventions have strong evidence behind them:
Weight loss
The single most effective intervention for NAFLD. The Lancet trial showed 5% weight loss improves hepatic steatosis; 7% resolves NASH in most cases; 10% improves fibrosis. Any sustainable approach works — calorie deficit matters more than specific diet type.
Reduce or eliminate alcohol
Even in NAFLD (non-alcohol-related), alcohol adds an additional insult to an already stressed liver. For those with alcohol-related liver damage, complete abstinence is the most effective treatment. GGT typically normalises within 2–6 weeks of abstinence.
Exercise
Both aerobic exercise (brisk walking, cycling, swimming) and resistance training reduce liver fat independently of weight loss. A BMJ meta-analysis found that exercise reduced intrahepatic fat by approximately 3.5% on average, even without significant weight change. Aim for at least 150 minutes of moderate-intensity exercise per week.
Diet quality
- Reduce fructose and refined sugars — fructose is metabolised almost exclusively by the liver and drives hepatic fat accumulation
- Mediterranean diet pattern — olive oil, fish, nuts, vegetables — is the best-evidenced dietary pattern for NAFLD
- Limit ultra-processed foods (high in added fructose, refined oils, and emulsifiers)
Coffee
Coffee drinkers consistently show lower ALT, reduced risk of fibrosis, and lower rates of liver cancer. A BMC Medicine study of nearly 500,000 UK Biobank participants found 3–4 cups per day associated with a 21% lower risk of chronic liver disease compared to non-drinkers. The benefit appears to be independent of caffeine — decaf also shows protection.
12. Medications and Your Liver
The liver metabolises virtually every drug you take. Some medications require routine LFT monitoring:
Statins: Can cause mild transient ALT elevation (typically 1–3× ULN). Guidelines from NICE CG181 recommend LFTs before starting, at 3 months, and at 12 months. Statins are safe — and even beneficial — in NAFLD/NASH.
Methotrexate: Used for autoimmune conditions. Requires regular LFT monitoring (typically every 2–3 months) due to risk of hepatic fibrosis with cumulative doses.
Paracetamol: Safe at recommended doses (≤4g/day) for most people, but the margin between therapeutic and toxic doses is narrow. Chronic alcohol use reduces this margin further. Paracetamol overdose is the leading cause of acute liver failure in the UK.
GLP-1 agonists (semaglutide, liraglutide): Emerging evidence suggests GLP-1 agonists may actually improve NAFLD and NASH through weight loss and direct hepatic effects. Several clinical trials are underway. This is a rapidly evolving area.
Herbal supplements: Green tea extract (especially in concentrated capsule form), kava, black cohosh, and comfrey have all been linked to liver injury. If you take supplements, mention them to your GP when discussing abnormal LFTs — they are a frequently overlooked cause.
13. When to See Your GP
Book a GP appointment if any of the following apply:
- ALT or AST persistently elevated above the upper limit of normal on two tests at least 4 weeks apart
- ALT or AST more than 3× the upper limit of normal on a single test (seek urgent review)
- Jaundice — yellowing of skin or whites of eyes (may indicate significant bilirubin elevation requiring investigation)
- Low albumin (<35 g/L) — may indicate chronic liver disease or another serious condition
- Unexplained persistent fatigue, nausea, right upper abdominal pain, dark urine, or pale stools
- Spider naevi (small spider-shaped blood vessels on your skin), palmar erythema (red palms), or unexplained bruising — possible signs of chronic liver disease
The BSG guidelines recommend that GPs follow a structured pathway for abnormal LFTs: repeat after 4–6 weeks, exclude common causes (alcohol, NAFLD, medications), screen for hepatitis B and C, check autoimmune markers, calculate FIB-4 score, and refer for ultrasound if persistently abnormal. If FIB-4 suggests significant fibrosis, referral to hepatology is recommended.
14. Frequently Asked Questions
What is a normal ALT level in the UK?
The NHS reference range for ALT is 0–41 U/L. However, published research suggests tighter optimal thresholds: below 30 U/L for men and below 19 U/L for women. An ALT within the "normal" range can still reflect early fatty liver disease, which is why context (weight, metabolic markers, alcohol intake) matters more than any single number.
Can liver damage be reversed?
In most cases, yes — if caught early. Simple fatty liver and NASH (stages 1–2) are largely reversible with weight loss, dietary changes and reduced alcohol. Even early fibrosis (stage 3) can improve. Cirrhosis (stage 4) involves permanent scarring, but abstinence from alcohol and addressing the underlying cause can still prevent further progression.
Does exercise affect liver function test results?
Yes. Intense exercise (particularly heavy weight training or endurance sports) can temporarily raise AST and, to a lesser extent, ALT for 48–72 hours due to muscle damage. For the most accurate liver-specific results, avoid intense training for 48 hours before your blood test. Light exercise is fine.
What is Gilbert's syndrome and should I worry?
Gilbert's syndrome is a harmless genetic condition affecting 5–10% of the UK population. It causes mild, fluctuating elevation of unconjugated bilirubin with completely normal liver enzymes. It requires no treatment and no monitoring. Some research suggests it may even be slightly protective against cardiovascular disease due to bilirubin's antioxidant properties.
Do I need to fast before a liver function test?
Fasting is not strictly required for LFTs. Liver enzymes (ALT, AST, ALP, GGT) are not significantly affected by food. However, if your test also includes cholesterol or triglycerides (as in a comprehensive panel), a fasting morning sample gives the most accurate results. Water is always fine.
How often should I get my liver tested?
There is no universal NHS screening programme for liver disease, which is part of why it is so underdiagnosed. If you have risk factors (overweight, diabetes, regular alcohol, family history), annual LFTs are reasonable. If you're on medications that affect the liver (statins, methotrexate), your GP will set a monitoring schedule. For everyone else, including LFTs in a comprehensive blood test every 1–2 years is a sensible baseline.
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