Alanine Aminotransferase (ALT)
ALT is an enzyme that leaks into the blood when liver cells are damaged. It is the most liver-specific of the routine "liver function tests".
What it measures
ALT (also called SGPT) lives mostly inside liver cells. When hepatocytes are stressed or destroyed — by fatty infiltration, viral infection, alcohol, medication, autoimmune attack — they release ALT into the bloodstream. Because the kidney, heart and muscle contain very little ALT, an elevated ALT points fairly specifically at the liver. AST (its sibling) is found in muscle and heart too, so ALT alone is the more liver-focused signal.
What a high value can mean
- Non-alcoholic fatty liver disease (NAFLD) — by far the most common cause in developed countries.
- Viral hepatitis — A/B/C/E.
- Alcohol-related liver disease — classically AST > ALT, but ALT also rises.
- Drug-induced liver injury — acetaminophen overdose, statins (rare), antibiotics, supplements (especially green-tea extract, kava).
- Autoimmune hepatitis, hemochromatosis, Wilson's disease — less common, worth excluding when ALT is high without obvious cause.
What a low value can mean
- Vitamin B6 deficiency — ALT requires pyridoxal phosphate as cofactor.
- Severe end-stage liver disease — few remaining hepatocytes left to release the enzyme.
- Most low ALT readings are benign and don't need investigation.
When to discuss with a doctor
ALT >3× the upper reference limit, or any persistent elevation, warrants a primary-care visit and follow-up labs (AST, GGT, alkaline phosphatase, bilirubin, viral hepatitis serology, abdominal ultrasound). ALT >1000 U/L is acute liver injury and needs same-day evaluation. Mediora.AI flags critical values inside your report.