Fatty Liver (NAFLD) Blood Test: Early Detection

Non-alcoholic fatty liver disease — now increasingly referred to as metabolic dysfunction-associated steatotic liver disease (MASLD) — affects an estimated 1 in 3 adults in the United Kingdom. That is roughly 20 million people walking around with excess fat in their liver, the vast majority of whom have no idea. NAFLD is the silent liver disease. It causes no symptoms in its early stages, and standard liver function tests often come back as "normal" even when significant fat accumulation is present.

This silence is what makes NAFLD dangerous. In most people it remains benign, but in approximately 20–30% of cases, it progresses to non-alcoholic steatohepatitis (NASH) — characterised by inflammation and liver cell damage. NASH can progress to fibrosis, cirrhosis, and ultimately liver failure or hepatocellular carcinoma. NAFLD is now the fastest-growing indication for liver transplantation in the UK.

Understanding which blood tests to monitor, and what the results actually mean, is essential for catching this condition before it becomes a serious problem.

Why Standard Liver Function Tests Are Not Enough

When most people think of a liver blood test, they think of the standard liver function test (LFT) panel that a GP might order. This typically includes ALT, AST, ALP, GGT, bilirubin, and albumin. The problem is that these tests were designed to detect liver cell damage and bile duct problems, not to detect fat accumulation.

It is entirely possible — and in fact common — to have significant hepatic steatosis (fatty liver) with completely normal standard liver function tests. Studies have shown that up to 79% of people with biopsy-confirmed NAFLD have ALT levels within the laboratory reference range. This means relying on standard LFTs to rule out fatty liver disease will miss the majority of cases.

The Key Blood Markers for NAFLD

ALT (Alanine Transaminase)

ALT is the most liver-specific of the transaminase enzymes and is considered the most sensitive blood marker for fatty liver disease. However, the standard laboratory reference ranges — often cited as up to 40 or even 55 U/L — are derived from population data that includes many people who themselves have undiagnosed NAFLD. This creates a circular problem: the "normal" range is inflated by the very condition it should be detecting.

Many hepatologists and liver specialists now advocate for lower ALT thresholds. Research published in the Journal of Hepatology and endorsed by several international liver societies suggests that the upper limit of normal should be 30 U/L for men and 19 U/L for women. Using these thresholds dramatically improves the sensitivity of ALT for detecting NAFLD. If your ALT is 35 and your laboratory says it is normal, it may still warrant further investigation in the context of metabolic risk factors.

GGT (Gamma-Glutamyl Transferase)

GGT is often thought of as a marker of alcohol consumption, but it is also elevated in metabolic liver disease independently of alcohol intake. In NAFLD, GGT elevation reflects oxidative stress in the liver and is associated with more advanced disease. Importantly, GGT is also an independent predictor of cardiovascular mortality — a connection that underscores the systemic nature of metabolic liver disease. The standard reference range is up to 60 U/L, but levels above 30 U/L in the context of metabolic risk factors are worth noting.

AST (Aspartate Transaminase) and the AST:ALT Ratio

AST is less liver-specific than ALT (it is also found in heart, muscle, and red blood cells) but provides important prognostic information when interpreted alongside ALT. In simple steatosis and early NASH, ALT is typically higher than AST, giving an AST:ALT ratio of less than 1. As fibrosis develops, this ratio reverses — an AST:ALT ratio greater than 1 suggests advanced fibrosis or cirrhosis. This simple ratio, calculable from a standard liver panel, provides meaningful staging information at no additional cost.

HbA1c and Fasting Glucose

The relationship between NAFLD and insulin resistance is so close that some researchers consider NAFLD the hepatic manifestation of metabolic syndrome. Insulin resistance drives fat accumulation in the liver, and hepatic fat accumulation worsens insulin resistance, creating a vicious cycle. HbA1c levels in the pre-diabetic range (42–47 mmol/mol) or diabetic range (48+ mmol/mol) significantly increase the risk of NAFLD progression to NASH and fibrosis. Monitoring HbA1c is therefore not just about diabetes risk — it is directly relevant to liver health.

Lipid Panel

Dyslipidaemia is almost universal in NAFLD. The typical pattern includes elevated triglycerides, low HDL cholesterol, elevated small dense LDL particles, and elevated non-HDL cholesterol. This lipid pattern is highly atherogenic, which partly explains why cardiovascular disease — not liver failure — is the leading cause of death in people with NAFLD. A comprehensive lipid panel (total cholesterol, LDL, HDL, non-HDL, triglycerides) should be part of routine monitoring.

The FIB-4 Score: Assessing Fibrosis Without a Biopsy

One of the most useful tools for assessing NAFLD severity is the FIB-4 score, a validated non-invasive fibrosis index. It is calculated using four readily available parameters: age, AST, ALT, and platelet count. No additional tests are needed beyond a standard blood panel.

The calculation is: FIB-4 = (Age × AST) / (Platelets × √ALT)

Interpretation:

• Below 1.30: Low risk of advanced fibrosis (negative predictive value >90%)
• 1.30–2.67: Indeterminate — further investigation recommended (such as transient elastography / FibroScan)
• Above 2.67: High probability of advanced fibrosis

The FIB-4 score is now recommended by NICE as the first step in assessing liver fibrosis in primary care. It is a free calculation that any clinician (or patient) can perform with a standard blood test result. If your FIB-4 score is in the indeterminate or high-risk range, referral for a FibroScan or specialist hepatology assessment is appropriate.

Risk Factors: Who Should Be Tested?

NAFLD is strongly associated with metabolic syndrome. You should consider testing if you have any of the following:

• Overweight or obesity (BMI above 25, or above 23 for South Asian ethnicity)
• Type 2 diabetes or pre-diabetes
• Elevated triglycerides or low HDL cholesterol
• Hypertension
• Polycystic ovary syndrome (PCOS)
• Obstructive sleep apnoea
• Family history of NAFLD or liver disease
• Incidental finding of fatty liver on ultrasound or scan

It is worth noting that NAFLD is not exclusively a condition of overweight individuals. An estimated 10–20% of people with NAFLD have a normal BMI — so-called "lean NAFLD." This is more common in people of South Asian descent and in those with specific genetic polymorphisms (particularly PNPLA3).

Monitoring Over Time

If you have been diagnosed with NAFLD or have risk factors, regular monitoring is important. The British Society of Gastroenterology recommends repeating liver blood tests and recalculating the FIB-4 score at intervals determined by the initial result and risk profile. For most people with NAFLD and a low FIB-4 score, annual monitoring is reasonable. If the FIB-4 score is in the indeterminate range, more frequent monitoring (every 6 months) and possible specialist referral are warranted.

Weight loss is the most effective treatment for NAFLD. A 5–10% reduction in body weight has been shown to reduce liver fat content, resolve NASH, and even reverse fibrosis. Monitoring your blood markers before and after lifestyle changes provides objective evidence of whether your interventions are working.

Testing Options at Lola Health

Our Liver & Kidney Function test (£81) includes ALT, AST, GGT, ALP, bilirubin, albumin, and full kidney markers — giving you the liver panel needed to calculate your FIB-4 score and assess your baseline liver health. For a more comprehensive metabolic assessment, our Core Health 45 test (£120) adds HbA1c, a full lipid panel, a complete blood count (including the platelet count needed for FIB-4), iron studies, and vitamins. Our Peak Insights 70 (£185) covers 70 biomarkers for the most thorough investigation.

Key Takeaways

• NAFLD affects 1 in 3 UK adults and causes no symptoms until advanced — blood testing is the primary detection tool.
• Standard liver function test reference ranges miss the majority of NAFLD cases. Consider using lower ALT thresholds: above 30 U/L for men, above 19 U/L for women.
• The AST:ALT ratio provides free prognostic information — a ratio above 1 suggests advanced fibrosis.
• The FIB-4 score (calculable from age, AST, ALT, and platelets) is recommended by NICE for non-invasive fibrosis assessment.
• NAFLD is closely linked to insulin resistance — HbA1c and lipid monitoring are essential alongside liver markers.
• Cardiovascular disease, not liver failure, is the leading cause of death in NAFLD — comprehensive metabolic monitoring is crucial.

Fatty liver disease is common, silent, and progressive in a significant minority of cases. The good news is that it is detectable with routine blood markers, assessable with simple scores, and — critically — reversible with lifestyle changes when caught early. Regular blood testing is the most practical way to stay ahead of a condition that, by design, will not tell you it is there.