What is the normal range for Alanine Transaminase (ALT) levels?

Normal Range for Alanine Transaminase (ALT) Levels

The normal range for ALT is typically considered to be <40 IU/L, but more recent evidence suggests that optimal upper limits of normal (ULN) should be 21 IU/L for men and 17 IU/L for women. 1, 2

Gender-Specific Normal Ranges

• Men:

  • Traditional ULN: <40 IU/L 1
  • Updated ULN: <21 IU/L 2

• Women:

  • Traditional ULN: <40 IU/L 1
  • Updated ULN: <17 IU/L 2

Variations in Normal Ranges

Demographic Factors

• Gender differences: Upper reference ranges vary significantly between men (35-79 U/L) and women (31-55 U/L) 1

• Racial/ethnic differences: ALT levels show significant variations across different racial and ethnic groups 1:

  • White adolescents: 7.4% prevalence of elevated ALT (>30 U/L)
  • Mexican American adolescents: 11.5% prevalence of elevated ALT
  • Black adolescents: 6.0% prevalence of elevated ALT 3

• Age-related changes: Laboratory values may need adjustment for elderly patients, as many parameters change with advancing age 1

Laboratory Variability

• ULN values for ALT may vary substantially between laboratories, ranging from <30 U/L to >70 U/L 1

• When evaluating ALT levels, it's important to consider the specific reference range used by the testing laboratory 1

Clinical Significance of ALT Levels

Interpretation Thresholds

• Persistently normal ALT: Defined as ALT <40 IU/L on 2-3 occasions separated by at least a month over 6 months 1

• Mild elevation: Less than 5× ULN (typically <200 IU/L) 4

• Moderate elevation: 5-10× ULN

• Severe elevation: >10× ULN; values >1,000 IU/L are typically associated with acute ischemic injury, drug-induced liver injury, or acute viral hepatitis 5

Clinical Decision Points

• ALT >3× ULN: Consider interrupting potentially hepatotoxic medications and expedite evaluation 6

• ALT >5× ULN: Discontinue suspected hepatotoxic medications and consider referral to hepatology 6

• ALT >3× ULN with bilirubin ≥2× ULN: Discontinue hepatotoxic medications immediately and refer to hepatology (Hy's Law case) 6

Diagnostic Performance

Recent studies have evaluated the diagnostic performance of different ALT thresholds:

• Maximum correct classification:

  • Men: 29 IU/L (88% sensitivity, 83% specificity)
  • Women: 22 IU/L (89% sensitivity, 82% specificity) 7

• High sensitivity threshold (95% sensitivity):

  • Men: 24 IU/L (70% specificity)
  • Women: 18 IU/L (63% specificity) 7

• High specificity threshold (95% specificity):

  • Men: 44 IU/L (64% sensitivity)
  • Women: 32 IU/L (59% sensitivity) 7

Important Considerations

• Using the updated, lower thresholds would classify a substantial portion of the U.S. population as having abnormal ALT (36.4% of men and 28.3% of women) 7

• Even patients with persistently normal ALT by traditional standards may have progressive liver disease, particularly if ALT >23 IU/L 1

• When monitoring patients with baseline ALT elevation, consider using multiples of baseline rather than multiples of ULN 1

• Natural fluctuations in liver enzymes typically do not exceed 1.5-2× baseline in the natural course of liver diseases 6