Alanine Aminotransferase (ALT) Test
The ALT test measures serum alanine aminotransferase, a cytosolic hepatocyte enzyme released during liver cell injury and the most liver-specific routine aminotransferase. It is the first-line marker of hepatocellular damage, interpreted with AST, ALP, GGT, and bilirubin and the AST:ALT ratio to classify liver injury, screen for steatotic liver disease, and monitor hepatotoxic therapy.
1
Biomarkers
Blood (serum)
Sample
1–2 business days
Turnaround
No
Fasting
What is the ALT test?
The Alanine Aminotransferase (ALT) Test measures the serum activity of a cytosolic hepatocyte enzyme released when liver cells are injured. It is the single most useful first-line marker of hepatocellular damage and a routine component of liver chemistry, ordered both to investigate suspected liver disease and to screen asymptomatic patients with metabolic risk factors. In the diagnostic landscape ALT rarely stands alone: it is interpreted alongside AST, ALP, GGT, and bilirubin to classify a liver abnormality as hepatocellular or cholestatic, and the AST:ALT ratio adds further discrimination — a ratio below one is typical of steatotic liver disease, while a ratio above two suggests alcohol-related injury. Practitioners order ALT to detect disease, to monitor known liver conditions and potentially hepatotoxic therapy, and to follow trends over time rather than to reach a diagnosis from a single value.
What does the ALT test measure?
Alanine aminotransferase is a cytosolic enzyme that catalyses transfer of an amino group from alanine to alpha-ketoglutarate, linking amino-acid and carbohydrate metabolism. Its concentration is highest in hepatocytes, with far smaller amounts in kidney and muscle, making it the most liver-specific of the routine aminotransferases. When hepatocyte membranes are injured, cytosolic ALT leaks into the circulation; its plasma half-life is on the order of a couple of days, so elevations persist somewhat longer than those of aspartate aminotransferase.
ALT is the first-line marker of hepatocellular injury. Because it is more liver-specific than AST, an isolated ALT elevation points strongly to the liver, and the pattern and magnitude of elevation — interpreted with AST, ALP, bilirubin, and the AST:ALT ratio — help separate hepatocellular from cholestatic processes and suggest the likely cause. Persistent mild elevation is a common trigger for evaluation of metabolic dysfunction-associated steatotic liver disease.
Why is the ALT test ordered?
- Investigation of suspected liver disease in patients with fatigue, right upper quadrant discomfort, jaundice, or dark urine
- Screening and monitoring of metabolic dysfunction-associated steatotic liver disease (MASLD) in patients with obesity, type 2 diabetes, or dyslipidaemia
- Baseline and serial monitoring of patients on potentially hepatotoxic medications (statins, methotrexate, isoniazid, antiepileptics, among others)
- Evaluation of incidental or persistent transaminase elevation found on a routine metabolic or wellness panel
- Work-up of suspected viral hepatitis, autoimmune hepatitis, or haemochromatosis, interpreted with the relevant specific tests
- Assessment of possible alcohol-related liver injury, where the AST:ALT ratio and GGT add context
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No
Collection method
Venipuncture at patient service centre
Turnaround
1–2 business days
Collection notes
Phlebotomy at the processing lab's draw site (LabCorp, Quest, or Boston Heart Diagnostics, depending on routing); no special preparation. Avoid vigorous exercise shortly before the draw, which can modestly raise ALT. Reject grossly haemolysed specimens.
Specimen requirements
Minimum 0.5 mL serum (gel separator acceptable). Separate from cells promptly; ALT is stable refrigerated for several days. Note the performing laboratory for serial comparability, as reference limits differ.
What can affect ALT results?
ALT is present in muscle, so vigorous exercise, rhabdomyolysis, or intramuscular injury can modestly raise ALT; correlating with creatine kinase helps identify a muscle source.
Red cells contain aminotransferases, so a haemolysed sample can spuriously alter results; grossly haemolysed specimens should be rejected and redrawn.
Hepatic steatosis raises ALT, frequently to the mild chronic elevations that prompt MASLD evaluation rather than reflecting acute injury.
Many drugs and some herbal or bodybuilding supplements cause dose-dependent or idiosyncratic hepatocellular injury; a full medication and supplement history is essential to interpretation.
ALT activity depends on pyridoxal-5'-phosphate as a cofactor, so B6 deficiency (seen in some patients with alcohol use or malnutrition) can lower measured ALT and mask true hepatocellular injury.
ALT shows modest within-day and between-day biological variation, so a small change between serial results may not be clinically meaningful.
What do ALT results mean?
Elevated
Hepatocellular injury: viral or autoimmune hepatitis, steatotic/steatohepatitic liver disease, drug-induced liver injury, ischaemic or toxic hepatitis (often very marked), and haemochromatosis or Wilson disease. Modest isolated elevations most often reflect MASLD; very high values (>10–15× the upper limit) suggest acute viral, ischaemic, or toxic injury.
Normal
A normal ALT does not exclude significant liver disease — cirrhosis and some chronic hepatitis can present with normal or near-normal transaminases. Interpret alongside the rest of the liver panel and clinical context rather than in isolation.
Reduced
Low ALT is usually not clinically significant but may be seen with pyridoxine deficiency and has been associated with frailty and sarcopenia in older adults. It can mask true injury when a cofactor deficiency is present.
Note: ALT must be read with AST, ALP, GGT, bilirubin, and albumin to classify the pattern of liver injury; the AST:ALT ratio aids aetiology. Magnitude, chronicity, and trend matter more than a single value, and reference limits differ between laboratories.
Frequently asked questions
Both are aminotransferases released in liver injury, but ALT is more liver-specific while AST is also found in heart and skeletal muscle. The AST:ALT ratio helps identify the cause — below one suggests steatotic liver disease, above two suggests alcohol-related injury.
How to access the ALT test
The ALT reference page is free and open to all practitioners. Create a free practitioner account to see pricing, save tests to your list, and generate requisition forms. Verification takes 1–2 business days.
Create Free AccountAbout this page
Published: October 7, 2026
Last reviewed: October 7, 2026
- Kwo PY, et al. ACG clinical guideline: evaluation of abnormal liver chemistries. Am J Gastroenterol. (2017)
- Rinella ME, et al. AASLD practice guidance on the clinical assessment and management of NAFLD/MASLD. Hepatology. (2023)
- Prati D, et al. Updated definitions of healthy ranges for serum alanine aminotransferase levels. Ann Intern Med. (2002)
- Newsome PN, et al. EASL–EASD–EASO clinical practice guidelines for the management of non-alcoholic fatty liver disease. (verify citation/PMID). (2024)
This content is for licensed healthcare practitioners only and does not constitute medical advice. Clinical decisions should be based on the full clinical picture and not on any single test result.
Related tests
gastrointestinal
AST
The AST test measures serum aspartate aminotransferase, an enzyme present in liver, heart, skeletal muscle, and other tissues. It is a sensitive but non-specific marker of cellular injury, interpreted with ALT to calculate the AST:ALT ratio, which distinguishes steatotic (ratio <1) from alcohol-related (ratio >2) liver disease, and with CK and troponin to separate hepatic from muscle and cardiac…
metabolic diabetes
CMP
The Comprehensive Metabolic Panel is a 14-analyte serum chemistry panel simultaneously evaluating renal function (creatinine, BUN, eGFR), hepatic function (ALT, AST, total bilirubin, albumin, total protein), metabolic status (glucose, calcium), and electrolyte-acid-base homeostasis (sodium, potassium, chloride, bicarbonate) from a single fasting serum specimen.
gastrointestinal
GGT
The GGT test measures serum gamma-glutamyl transferase, a hepatobiliary enzyme sensitive to cholestasis, alcohol, and enzyme-inducing drugs. Its principal use is confirming the hepatobiliary source of a raised alkaline phosphatase, since GGT is not present in bone. It is sensitive but non-specific and is interpreted alongside the wider liver panel.