ALT (Alanine Aminotransferase)

What is ALT?

Alanine aminotransferase (ALT) is an enzyme found primarily inside liver cells (hepatocytes), where it plays a role in amino acid metabolism. Under normal circumstances, very little ALT escapes into the bloodstream. When liver cells are damaged — by any cause — they become leaky, releasing ALT into the circulation, where it can be detected by a blood test.

ALT is the most liver-specific of the commonly measured liver enzymes. Unlike AST (aspartate aminotransferase), which is found in multiple tissues including heart and skeletal muscle, ALT is concentrated in the liver. This means that an isolated ALT elevation is a stronger indicator of hepatocellular damage specifically.

ALT is typically measured as part of a liver function test (LFT) panel alongside AST, alkaline phosphatase (ALP), gamma-glutamyl transferase (GGT), bilirubin, albumin, and total protein. Each enzyme provides different information about liver health: ALT and AST indicate cell damage, ALP and GGT point to bile duct disease, and albumin and bilirubin reflect how well the liver is performing its metabolic functions.

The degree and pattern of enzyme elevation guide the differential diagnosis. Very high ALT — hundreds or thousands of units — points towards acute hepatitis (viral, ischaemic, or toxic). Mild-to-moderate persistent elevation of ALT with or without GGT is the typical picture of non-alcoholic fatty liver disease (NAFLD) — now the most common cause of chronic liver disease in the UK.

Normal reference range

• Normal ALT: 7–40 U/L

Important: Reference ranges vary between laboratories and depend on individual factors including age, sex, pregnancy status, and medication. Always interpret your result in the context of a clinician review. The Vesey reviews every result before release.

What a high ALT may indicate

The magnitude of ALT elevation gives diagnostic clues, though the clinical context is always essential:

• Non-alcoholic fatty liver disease (NAFLD / MASLD) — the most common cause of mildly elevated ALT in the UK; associated with obesity, type 2 diabetes, and the metabolic syndrome
• Alcohol-related liver disease — ALT is typically elevated alongside GGT; an AST:ALT ratio above 2:1 suggests alcoholic hepatitis
• Viral hepatitis (B and C) — chronic hepatitis B and C are common causes of persistent ALT elevation; acute infection causes dramatic rises
• Drug-induced liver injury (DILI) — many prescription medications, over-the-counter drugs, herbal remedies, and supplements can cause ALT elevation; statins, NSAIDs, and antibiotics are common culprits
• Autoimmune hepatitis — an immune-mediated liver inflammation that can cause severe chronic damage if untreated
• Haemochromatosis — iron overload causes hepatocellular damage and elevated ALT
• Ischaemic hepatitis (shock liver) — dramatically elevated ALT (often above 1000 U/L) following severe hypotension or cardiac failure
• Coeliac disease — an underrecognised cause of mild, unexplained ALT elevation

What a low ALT indicates

ALT below the lower reference limit (below 7 U/L) is not clinically significant in most cases. Very low ALT in the context of advanced cirrhosis can paradoxically reflect a reduction in functioning liver cell mass rather than normal liver health — but this is an end-stage finding and will be apparent from other markers.

Get tested at The Vesey, Sutton Coldfield

ALT is measured within the following panels at The Vesey Private Hospital:

• Liver Health panel — full liver function including ALT, AST, GGT, ALP, bilirubin, and albumin
• Lifestyle Screen — broad health check including liver function
• Advanced Health Panels — comprehensive marker profiling including full LFTs

Results are reviewed by a clinician before secure release, typically within 24 hours. Appointments available Monday–Saturday in Sutton Coldfield.

Frequently asked questions

What is a normal ALT level?

Normal ALT is 7–40 U/L. Values vary slightly between laboratories. Persistent elevation — even mild — warrants investigation to identify the underlying cause, as chronic low-grade liver damage can progress silently over years.

What does a high ALT mean?

Elevated ALT indicates liver cell damage. Very high levels (above 1000 U/L) suggest acute viral hepatitis, ischaemic injury, or drug toxicity. Moderate elevation suggests chronic hepatitis or significant liver disease. Mild elevation is commonly due to fatty liver, alcohol, or medications.

What is the most common cause of mildly raised ALT?

The most common cause in the UK is non-alcoholic fatty liver disease (NAFLD/MASLD), closely associated with obesity, type 2 diabetes, and the metabolic syndrome. Alcohol consumption and medication side effects are also very common. Identifying the cause early allows intervention before significant liver damage develops.

Is ALT or AST better for detecting liver disease?

ALT is more liver-specific. AST is also found in heart, skeletal muscle, kidneys, and red blood cells, so it can be elevated by cardiac events or muscle injury. ALT rising in isolation is a stronger indicator of liver disease. The AST:ALT ratio provides additional diagnostic information — a ratio above 2:1 suggests alcoholic hepatitis.

Does exercise affect ALT levels?

Vigorous exercise can transiently raise ALT because muscles also contain this enzyme, though the effect is much more pronounced with AST and CK (creatine kinase). For the most accurate liver-specific baseline, avoid strenuous exercise for 24–48 hours before blood testing.