Alanine Aminotransferase
ALT· also: SGPT, Alanine Aminotransferase
Clinical Overview
ALT is an enzyme found predominantly in liver cells (hepatocytes). When liver cells are damaged or inflamed, ALT leaks into the bloodstream, causing elevated serum levels. ALT is the most liver-specific enzyme test and is the primary marker of hepatocellular injury.
Why This Test Matters
ALT is the gold standard for detecting acute and chronic liver injury. It is used to screen for viral hepatitis, monitor hepatotoxicity from medications, and evaluate fatty liver disease (NAFLD/MASH). ALT levels greater than 3–5 times the upper limit of normal (ULN) warrant further investigation. Serial ALT measurements track disease progression and treatment response.
Reference RangesWHO/IFCC standards
| Age Group | Sex | Reference Range | Unit | Notes |
|---|---|---|---|---|
| Adults (18–64) | Male | 7 – 56 | U/L | — |
| Adults (18–64) | Female | 7 – 45 | U/L | — |
Also reported in: IU/L.
What Causes Abnormal Results?
High ALT Causes
- Viral hepatitis (A, B, C, E)
- Non-alcoholic fatty liver disease (NAFLD/MASH)
- Alcoholic liver disease
- Drug-induced liver injury (statins, paracetamol overdose, antibiotics)
- Autoimmune hepatitis
- Ischemic hepatitis (shock liver)
- Celiac disease
- Thyroid disease (hypothyroidism)
Low ALT Causes
- Not clinically significant (low ALT is normal)
- End-stage liver disease (liver has lost mass, less enzyme present)
- Hemodialysis (enzyme cleared)
Signs & Symptoms to Watch For
How to Prepare for This Test
No fasting strictly required, but fasting for 8 hours reduces variability. Avoid heavy exercise 24–48 hours before testing (exercise releases ALT from muscle). Inform your doctor of all medications and supplements.
Factors That Can Affect Results
- Strenuous exercise (raises ALT from muscle)
- Obesity (elevates ALT through fatty liver)
- Hemolysis in sample (falsely elevates result)
- Medications: statins, paracetamol, NSAIDs, antibiotics, herbal supplements
- Hypothyroidism (raises ALT even without structural liver disease)
Related Topics
Frequently Asked Questions
What ALT level indicates serious liver damage?
ALT more than 3 times the upper limit of normal (>168 U/L in men, >135 U/L in women) is considered clinically significant and warrants investigation. In acute viral hepatitis, ALT can rise to 1,000–10,000 U/L. In ischemic hepatitis (shock liver), ALT can exceed 10,000 U/L — one of the highest elevations seen in clinical practice.
Is ALT or AST more important for liver disease?
ALT is more liver-specific because it is found primarily in the liver, while AST is also found in muscle, heart, and red blood cells. For liver disease, ALT is the preferred marker. The AST/ALT ratio is also useful: a ratio above 2:1 suggests alcoholic liver disease, while a ratio below 1 is typical of viral hepatitis and NAFLD. Both tests together provide a more complete picture.
Can ALT be elevated without symptoms?
Yes. Mildly elevated ALT (1–3× ULN) is often asymptomatic and discovered on routine blood tests. This is common in NAFLD, which affects up to 25% of the global population. Patients with fatty liver often feel completely well despite having elevated liver enzymes. Lifestyle modifications (weight loss, reduced alcohol) typically normalize ALT in NAFLD.