What are the normal ranges for Alanine Transaminase (ALT) and Aspartate Transaminase (AST) liver enzymes?

Normal Ranges for Liver Enzymes (ALT, AST)

The normal range for ALT is 29-33 IU/L (0.48-0.55 μkat/L) in men and 19-25 IU/L (0.32-0.42 μkat/L) in women, while AST reference intervals are 11-34 IU/L (0.18-0.57 μkat/L) for both genders. 1, 2

Gender-Specific Reference Ranges

• ALT normal ranges show significant gender differences, with higher upper limits in males (up to 59 U/L) compared to females (up to 41 U/L) 2
• AST reference intervals show minimal gender differences (approximately 1.7 U/L difference in upper limits), allowing for a single reference range for both genders 2
• Some studies suggest even lower upper limits for healthy ALT: 31 IU/L for males and 23 IU/L for females 3

Factors Affecting Reference Ranges

• Laboratory reference ranges for liver enzymes can vary significantly between facilities due to differences in:

  • Reference populations used to establish normal ranges 2
  • Analytical methods and equipment 2
  • Standardization practices 2

• Upper limits of normal (ULN) for ALT can range from less than 30 U/L to more than 70 U/L depending on the laboratory 4

Clinical Interpretation of Liver Enzyme Values

• Severity of aminotransferase elevations is typically classified as:

  • Mild: <5 times the upper reference limit
  • Moderate: 5-10 times the upper reference limit
  • Severe: >10 times the upper reference limit 1

• ALT is more specific for liver injury than AST due to its lower concentrations in non-hepatic tissues such as skeletal muscle and kidney 1

• AST elevations may indicate disorders involving other organs including heart, skeletal muscle, kidneys, brain, and red blood cells 1

Special Considerations

• In patients with nonalcoholic fatty liver disease (NAFLD), ALT values typically range between normal and 250 IU/L, usually less than 5× ULN 1

• In patients with elevated baseline liver enzymes (such as those with NASH), monitoring should focus on relative changes rather than absolute values:

  • For normal/near-normal baseline: ALT ≥3× ULN with symptoms or ALT ≥5× ULN without symptoms warrants further evaluation
  • For elevated baseline: ALT ≥2-3× baseline or ≥300 U/L requires additional monitoring 1

• Even liver enzyme values within the "normal" range may have clinical significance:

  • Higher levels of ALT and AST, even within normal ranges, have been associated with subclinical myocardial injury 5
  • "Unhealthy normal" ALT levels (above the healthy range but below traditional ULN) correlate with higher prevalence of metabolic syndrome and insulin resistance 3

Monitoring Recommendations

• When evaluating changes in liver enzymes, consider:

  • Baseline values and relative changes rather than just absolute values 1
  • Presence of symptoms (fatigue, nausea, vomiting, right upper quadrant pain) 1
  • Concurrent changes in other liver parameters (bilirubin, alkaline phosphatase) 1

• ALT and GGT changes can be useful markers for monitoring liver fat content changes, particularly following weight loss interventions 6

Common Pitfalls

• Relying solely on laboratory-provided reference ranges without considering gender differences 2, 3
• Failing to recognize that "normal" ranges may include individuals with subclinical liver disease 3
• Not accounting for regional variations in reference intervals, though these appear to be less significant for ALT and AST than for other liver enzymes like GGT 2