ALT
The ALT test measures serum alanine aminotransferase, the most hepatocyte-specific aminotransferase, released during liver cell injury. It is the primary marker of hepatocellular damage, interpreted with AST, alkaline phosphatase, and bilirubin to classify liver injury pattern, screen for metabolic steatotic liver disease, and monitor hepatotoxic therapy.
1
Biomarkers
Blood (serum)
Sample
2–3 business days
Turnaround
No
Fasting
What is the ALT test?
The ALT test quantifies serum alanine aminotransferase activity to detect and monitor hepatocellular injury. It is ordered both as a component of broader liver-function screening and as a standalone marker when a practitioner suspects liver disease or wishes to track the hepatotoxic potential of medications. Because ALT is more liver-specific than aspartate aminotransferase, an isolated ALT rise directs attention to the hepatobiliary system rather than muscle or cardiac tissue. Practitioners interpret ALT in concert with AST, alkaline phosphatase, gamma-glutamyl transferase, and bilirubin to distinguish hepatocellular from cholestatic patterns and to assess severity. Chronic mild elevation — often discovered incidentally — is a common trigger for metabolic liver disease evaluation, while acute marked rises demand urgent investigation for viral, ischaemic, or toxin-mediated hepatitis. Serial ALT measurement tracks disease progression, treatment response, and medication safety over time.
What does the ALT test measure?
Alanine aminotransferase is a pyridoxal-5'-phosphate-dependent enzyme that catalyses reversible transamination between alanine and alpha-ketoglutarate, coupling amino-acid catabolism to gluconeogenesis. Its highest tissue concentration is in hepatocyte cytoplasm, with lower levels in kidney, skeletal muscle, and cardiac muscle. The cytosolic location and liver predominance make serum ALT a sensitive and relatively specific indicator of hepatocellular membrane integrity; when hepatocytes are damaged, ALT diffuses into the bloodstream, where it has a circulating half-life of approximately 47 hours.
ALT is the most hepatocyte-specific of the routinely measured aminotransferases and serves as the primary marker of hepatocellular necrosis or membrane injury. Elevation reflects liver damage rather than function, and the magnitude, pattern, and duration guide differential diagnosis: modest chronic rises suggest metabolic dysfunction-associated steatotic liver disease, while marked acute elevations point to viral hepatitis, drug-induced injury, or ischaemic insult. The AST:ALT ratio refines interpretation — a ratio less than one is typical of steatotic disease, whereas a ratio greater than two raises suspicion for alcohol-related hepatitis.
Why is the ALT test ordered?
- Patients presenting with symptoms or signs suggesting liver disease: fatigue, malaise, right upper quadrant pain, jaundice, pruritus, or unexplained dark urine
- Screening and longitudinal monitoring of individuals with metabolic risk factors for steatotic liver disease, including central obesity, type 2 diabetes, dyslipidaemia, or insulin resistance
- Baseline and interval monitoring of patients on potentially hepatotoxic medications — statins, methotrexate, azathioprine, amiodarone, certain antiepileptics, or anabolic supplements
- Evaluation of incidentally discovered transaminase elevation on routine health screening or pre-employment, insurance, or wellness panels
- Investigation of suspected viral hepatitis, autoimmune hepatitis, haemochromatosis, or Wilson disease, where ALT is part of the initial biochemical work-up
- Assessment of alcohol use disorder or suspected alcohol-related liver injury, interpreted alongside AST, GGT, and the AST:ALT ratio
- Follow-up of previously elevated ALT to determine whether injury is transient, persistent, or progressive
- Pre-operative assessment in patients with known or suspected liver disease to stratify perioperative risk
Sample collection and turnaround
Sample type
Blood (serum)
Fasting required
No
Collection method
Venipuncture at patient service centre
Turnaround
2–3 business days
Collection notes
Phlebotomy required. Patients should avoid vigorous exercise within 24 hours before the draw, as muscle-derived ALT can confound results. No fasting is necessary for ALT itself, though if drawn as part of a broader metabolic panel, fasting may be requested for glucose and lipids.
Specimen requirements
Minimum 0.5 mL serum; gel-separator tubes are acceptable. Separate serum from cells within two hours of collection to avoid in vitro haemolysis. ALT is stable refrigerated for up to one week. Grossly haemolysed specimens should be rejected and redrawn.
What can affect ALT results?
ALT is present in skeletal muscle; vigorous exertion, eccentric exercise, or rhabdomyolysis can raise circulating ALT modestly. Correlation with creatine kinase and myoglobin distinguishes muscle from liver sources.
Erythrocytes contain aminotransferases; haemolysed samples may yield spuriously altered results. Grossly haemolysed specimens should be rejected and the sample redrawn.
Chronic low-grade hepatocyte injury from lipid accumulation and lipotoxicity produces persistent mild ALT elevation, the biochemical hallmark of metabolic dysfunction-associated steatotic liver disease.
Many pharmaceuticals cause dose-dependent or idiosyncratic hepatocellular injury; over-the-counter herbal products, bodybuilding supplements, and traditional remedies are often overlooked causes. A complete medication and supplement history is essential.
ALT depends on pyridoxal-5'-phosphate as cofactor; B6 deficiency lowers measured enzyme activity, potentially masking true hepatocellular injury. This may occur in chronic alcohol use, malnutrition, or malabsorption.
ALT exhibits modest within-individual and day-to-day variation; small changes in serial results may fall within biological noise rather than signal true clinical change.
What do ALT results mean?
Elevated
Hepatocellular injury: metabolic dysfunction-associated steatotic liver disease or steatohepatitis, acute or chronic viral hepatitis (A, B, C, E), autoimmune hepatitis, drug-induced or herbal-supplement-induced liver injury, ischaemic hepatitis, haemochromatosis, Wilson disease, alpha-1 antitrypsin deficiency, and alcohol-related hepatitis. Mild elevation (less than twice the upper limit) most commonly reflects steatotic liver disease; moderate rises (2–10 times) suggest active viral or autoimmune hepatitis or drug injury; very high values (greater than 10–15 times) point to acute viral hepatitis, ischaemic injury, or severe toxin-mediated necrosis, including paracetamol overdose.
Normal
A normal ALT does not exclude significant liver disease. Advanced fibrosis, compensated cirrhosis, and some chronic hepatitis cases can present with normal or near-normal transaminases. Interpretation must integrate the full liver panel, clinical context, imaging, and — where indicated — non-invasive fibrosis assessment or biopsy.
Reduced
Low ALT is rarely of clinical concern but may reflect pyridoxine deficiency, which lowers cofactor-dependent enzyme activity and can mask hepatocellular injury. Very low values have been correlated with frailty, sarcopenia, and increased mortality in elderly populations, though causality is uncertain.
Note: ALT must be interpreted alongside AST, alkaline phosphatase, GGT, bilirubin, and albumin to classify the injury pattern as hepatocellular versus cholestatic and to gauge synthetic function. The AST:ALT ratio refines aetiology: less than one suggests steatotic disease, greater than two suggests alcohol-related injury. Magnitude, duration, and trend matter more than a single value. Reference ranges differ by laboratory, sex, and assay, so serial monitoring is best performed at the same laboratory.
Frequently asked questions
Both are aminotransferases released in hepatocellular injury, but ALT is more liver-specific whereas AST is also abundant in heart, muscle, and red blood cells. The AST:ALT ratio aids differential diagnosis — less than one typically indicates steatotic liver disease, greater than two suggests alcohol-related hepatitis.
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 nonalcoholic fatty liver disease. 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. J Hepatol. (2016)
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.