Liver Function Tests (LFTs) Explained: ALT, AST, Bilirubin, and More

LFTs (liver function tests) are one of the most commonly requested panels in NHS primary care — yet many people receive the result and have no idea what they're looking at. That's because LFTs aren't one test: they're a panel of six or seven different measurements, each reflecting a different aspect of liver health. Some go up with liver damage; one (albumin) goes down. Understanding which is which makes the whole panel much less bewildering.

What Does the Liver Do?

The liver is the body's primary metabolic organ. It processes everything absorbed from your gut, detoxifies drugs and alcohol, produces bile for fat digestion, synthesises proteins (including clotting factors and albumin), stores glycogen for energy, and regulates cholesterol metabolism. When liver cells are damaged — by alcohol, fatty liver disease, infection, or medication — they leak enzymes into the bloodstream, which is what LFTs detect.

The Six Key Markers

ALT — Alanine Aminotransferase

ALT is the most specific marker of liver cell damage. It's found predominantly in liver cells, so when it's elevated it strongly points to hepatic (liver) pathology rather than another organ. It's the first marker to rise in most forms of liver disease — from fatty liver to hepatitis to medication side effects. Normal range: roughly 7–56 U/L, though ranges vary by lab and are slightly lower for women.

AST — Aspartate Aminotransferase

AST is less liver-specific than ALT — it's also found in heart muscle, skeletal muscle, and red blood cells. A raised AST alongside a raised ALT is consistent with liver disease. However, a raised AST with a normal ALT may point to heart or muscle damage. The AST:ALT ratio can provide useful diagnostic information — a ratio above 2:1 is classically associated with alcoholic liver disease.

GGT — Gamma-Glutamyl Transferase

GGT is highly sensitive to alcohol and is often raised in people who drink above recommended limits, even without other liver damage. It's also elevated in biliary disease, fatty liver, and as a side effect of certain medications (including some anticonvulsants and antibiotics). An isolated raised GGT, particularly in someone who drinks, is a useful early warning sign to prompt a conversation about alcohol and liver health.

ALP — Alkaline Phosphatase

ALP can be raised in liver disease — particularly in conditions affecting the bile ducts (cholestatic disease), such as primary biliary cholangitis or bile duct obstruction. However, ALP is also elevated in bone disease (Paget's disease, bone metastases, healing fractures), in normal pregnancy, and in growing children and teenagers. A raised ALP therefore needs to be interpreted in context of ALP isoenzymes, GGT, and the clinical picture.

Bilirubin

Bilirubin is a yellow pigment produced when haemoglobin is broken down. It's processed by the liver and excreted in bile. When bilirubin accumulates in the blood — due to liver disease, bile duct obstruction, or excessive red cell breakdown (haemolysis) — it causes jaundice: yellowing of the skin and whites of the eyes. Normal total bilirubin is below 21 µmol/L. Mildly elevated bilirubin (21–35 µmol/L) with otherwise normal LFTs may indicate Gilbert's syndrome — a harmless genetic condition affecting bilirubin processing, present in around 5% of the population.

Albumin

Albumin is a protein synthesised by the liver. Unlike the enzymes above, albumin is a marker of liver function (what the liver is producing) rather than liver damage. A low albumin suggests the liver is struggling to manufacture protein — seen in cirrhosis, chronic liver disease, severe malnutrition, nephrotic syndrome, and acute illness. Normal range: 35–50 g/L. Albumin falls slowly in chronic liver disease and is a useful prognostic marker.

Reference Ranges at a Glance

Test	Normal Range	Significance if Abnormal
ALT	7–56 U/L	Raised = liver cell damage. Most specific LFT marker for the liver.
AST	10–40 U/L	Raised = liver or muscle damage. AST:ALT >2 suggests alcohol.
GGT	8–61 U/L (men) · 5–36 U/L (women)	Sensitive to alcohol; also raised in biliary disease and drugs.
ALP	30–130 U/L	Raised in biliary disease or bone disease. Context is key.
Bilirubin	Below 21 µmol/L	Raised = jaundice risk. Gilbert's if isolated mild rise.
Albumin	35–50 g/L	Low = impaired synthetic function. Marker of chronic liver disease severity.

Common Causes of Raised LFTs

The most common cause of persistently raised ALT and/or GGT in UK primary care is non-alcoholic fatty liver disease (NAFLD) — now renamed metabolic dysfunction-associated steatotic liver disease (MASLD). This condition, caused by excess fat accumulating in liver cells (related to obesity, type 2 diabetes, and metabolic syndrome), affects approximately 1 in 3 adults in the UK to some degree. In many cases it's reversible with weight loss and dietary change.

Other common causes include alcohol, medication side effects (statins, methotrexate, some antibiotics, paracetamol in overdose, herbal supplements), viral hepatitis (hepatitis B and C — both of which can cause chronic infection without symptoms), autoimmune hepatitis, and haemochromatosis (excess iron accumulation).

What Happens After Abnormal LFTs?

A mildly raised ALT or GGT on its own, in someone with no symptoms and no alarm features, usually leads to a repeat test in 3 months to see whether it's persistent or transient. If persistently elevated, your GP will investigate further — typically with a liver ultrasound scan, hepatitis B and C serology, autoimmune markers, and iron studies.

A FibroScan (liver elastography) is increasingly used in NHS primary care to assess the degree of liver fibrosis (scarring) in people with fatty liver — it's painless, quick, and doesn't require a biopsy. If significant fibrosis is found, referral to hepatology is warranted.

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