ALT (Alanine aminotransferase)
The most liver-specific transaminase. When ALT rises, the liver is the most likely source. Paired with AST and ratio for interpretation.
TL;DR. Normal adult: ~7-56 U/L. More liver-specific than AST. Interpret with AST/ALT ratio: <1 → NAFLD/viral/drug; >1 with modest rise → alcohol/cirrhosis; >2 → alcoholic hepatitis; >3 with high AST → muscle or cardiac. Most common cause of chronic mild elevation today is fatty liver.
Definition
ALT (alanine aminotransferase, SGPT) is an intracellular transaminase that catalyzes transfer of the amino group from alanine to α-ketoglutarate, producing pyruvate and glutamate. It is concentrated in hepatocyte cytoplasm. When hepatocytes are injured, ALT leaks into circulation. Because ALT is minimally present in non-hepatic tissue, it is the more liver-specific of the two standard transaminases, a rise in ALT almost always means the liver.
Reference range
| Range (U/L) | Interpretation |
|---|---|
| Male: 7-56; Female: 7-45 | Standard lab reference |
| Male: <30; Female: <19 (AASLD "healthy") | Stricter working threshold; flags early NAFLD |
| 1-2× ULN | Mild; workup if persistent |
| 2-5× ULN | Moderate; workup indicated |
| 5-25× ULN | Marked; acute hepatitis, drug injury |
| >25× ULN | Severe; acetaminophen, ischemic, fulminant viral |
What high ALT means
- Chronic: NAFLD / MASLD (most common), chronic viral hepatitis B or C, alcoholic liver disease, autoimmune hepatitis, hemochromatosis, Wilson's disease, α1-antitrypsin deficiency.
- Drug-induced: acetaminophen, statins, isoniazid, amiodarone, valproate, methotrexate, 17α-alkylated oral anabolic steroids (Anavar, Winstrol, Dianabol, Anadrol), green-tea extract supplements, kava, and many others.
- Acute: acute viral hepatitis (A, B, C, E), ischemic hepatitis ("shock liver"), acute bile-duct obstruction, drug overdose.
- Muscle: minor contribution; rhabdomyolysis can raise ALT but AST rises more.
What low ALT means
- Rarely clinically significant in isolation.
- Chronic vitamin B6 deficiency (B6 is a cofactor).
- Chronic kidney disease.
- Advanced frailty / sarcopenia in elderly, associated with increased mortality as a frailty marker, not a direct causal relation.
- No treatment indicated for isolated low ALT in a well adult.
AST/ALT ratio cheat sheet
| Ratio | Most likely |
|---|---|
| <1 (ALT > AST) | NAFLD, chronic viral hepatitis, drug-induced hepatitis |
| ~1 | Acute hepatitis of many causes |
| >1 with modest elevation | Alcoholic liver disease, cirrhosis of any cause, recent exercise |
| >2 | Alcoholic hepatitis (classical teaching) |
| >3 with high AST | Rhabdomyolysis, cardiac event, hemolysis, look extrahepatic |
When to test
- Any liver-function panel (CMP), ALT is always included.
- Before and during 17α-alkylated oral steroid cycles.
- Before and during any new hepatically-metabolized medication.
- Any new fatigue, jaundice, RUQ pain, dark urine, pale stool, or pruritus.
- Screening for NAFLD in metabolic syndrome, obesity, or type 2 diabetes.
Related tests
FAQ
- What is the normal ALT range?
- Approximately 7-56 U/L for adult men and 7-45 U/L for adult women in most US labs. Some expert groups (AASLD) propose lower 'healthy-population' upper limits: 29-33 for men, 19-25 for women. Values 1-2× ULN in the right clinical context may still indicate NAFLD and warrant follow-up.
- Why is ALT more informative than AST for liver health?
- ALT is concentrated in hepatocytes and is minimally present in other tissues; AST is widely distributed in liver, muscle, heart, kidney, brain, and red cells. A rise in ALT almost always reflects hepatocellular injury, whereas a rise in AST can come from muscle, heart, hemolysis, or liver. When they move together and ALT > AST, liver origin is likely; when AST moves alone or AST >> ALT, look harder at extrahepatic sources first.
- What does high ALT mean?
- Chronic causes: non-alcoholic fatty liver disease / MASLD (most common cause of mild chronic elevation worldwide), viral hepatitis (B, C), alcoholic liver disease, autoimmune hepatitis, hemochromatosis, Wilson's disease, alpha-1 antitrypsin deficiency. Drug-induced: acetaminophen, statins, isoniazid, amiodarone, valproate, 17α-alkylated oral anabolic steroids. Acute: acute viral hepatitis, ischemic hepatitis, acute bile-duct obstruction, drug-induced injury, muscle injury (minor contribution).
- What does low ALT mean?
- Low ALT is rarely clinically significant in isolation. Associations include chronic vitamin B6 deficiency, chronic kidney disease, and advanced frailty, low ALT in elderly patients has been associated with increased mortality, likely as a marker of sarcopenia and general decline. No intervention is indicated for isolated low ALT in a well adult.
- What is the AST/ALT ratio and what does it tell me?
- The AST/ALT ratio is a useful pattern-recognition tool. Ratio <1 (ALT > AST): typical of NAFLD, viral hepatitis, drug-induced injury. Ratio ~1: acute hepatitis of many causes. Ratio >1 with modest elevation: alcoholic liver disease, cirrhosis of any cause, recent muscle injury. Ratio >2: strongly suggests alcoholic hepatitis. Ratio >3 with high AST: think rhabdomyolysis or cardiac event.