What are the normal lab value ranges for liver function tests, including Alanine Transaminase (ALT), Aspartate Transaminase (AST), Alkaline Phosphatase (ALP), Gamma-Glutamyl Transferase (GGT), total bilirubin, direct bilirubin, and albumin?

Normal Laboratory Value Ranges for Liver Function Tests

The normal laboratory value ranges for liver function tests include ALT (<45 U/L for men, <34 U/L for women), AST (<40 U/L), ALP (<130 U/L), GGT (<30 U/L for men, <24 U/L for women), total bilirubin (<1.37 mg/dL), direct bilirubin (<0.41 mg/dL), and albumin (3.88-5.82 g/dL). 1

Standard Reference Ranges

Liver function tests are essential for detecting, assessing, and monitoring liver diseases. The standard reference ranges are:

• Alanine Transaminase (ALT)

  • Men: <45 U/L
  • Women: <34 U/L
  • Lower thresholds have been proposed: Men <30 U/L, Women <19 U/L 2

• Aspartate Transaminase (AST)

  • <40 U/L 1
  • AST:ALT ratio >2 suggests alcoholic liver disease
  • AST:ALT ratio <1 suggests metabolic-related fatty liver 1

• Alkaline Phosphatase (ALP)

  • 130-831 U/L (combined range for newborns and infants) 3
  • Adults: <130 U/L 1
  • Elevations >10× ULN are rare in primary biliary cholangitis 4

• Gamma-Glutamyl Transferase (GGT)

  • Men: <30 U/L
  • Women: <24 U/L 1
  • Important for confirming hepatobiliary origin of elevated ALP 4

• Total Bilirubin

  • <1.37 mg/dL 1, 3

• Direct Bilirubin

  • <0.41 mg/dL 3

• Albumin

  • 3.88-5.82 g/dL 1, 3

Interpretation of Abnormal Values

Classification of Liver Enzyme Elevations

• Mild: <5× ULN
• Moderate: 5-10× ULN
• Severe: >10× ULN 1

Clinical Significance

1. The magnitude of liver enzyme elevation does not necessarily correlate with clinical significance or prognosis 1
2. Clinical context and specific abnormal analytes are more important than the degree of elevation 1
3. Patients with significant liver fibrosis may have liver enzymes within normal ranges 1

Important Considerations

Laboratory Variability

• Reference ranges may vary significantly between laboratories due to differences in reference populations and analytical variation among commercial assays 4, 1
• When interpreting results, consider both absolute values and multiples of ULN 4

Age and Gender Considerations

• Age-specific reference ranges are important, particularly for ALP and albumin 1
• Gender-specific reference ranges are necessary for ALT, GGT, and total bilirubin 1
• Regional variations may affect total protein values 1

Special Populations

• Newborns and infants have different reference ranges, particularly for ALT, GGT, and ALP 3
• Patients who have undergone biliary-enteric anastomosis may have persistently elevated ALP (up to 166 IU/L) and total bilirubin (up to 1.3 mg/dL) despite excellent clinical outcomes 5

Common Pitfalls in Interpretation

1. Overlooking non-hepatic sources of enzyme elevations:

   • ALP may originate from bone, especially in post-menopausal women with osteoporosis 4
   • GGT and/or ALP fractionation should be used to determine hepatic origin 4

2. Using outdated reference ranges:

   • Traditional reference ranges may not be applicable to current populations 6
   • Reference intervals should be periodically updated based on population studies 6

3. Misinterpreting isolated GGT elevation:

   • Isolated elevation of GGT is a poor indicator of liver injury 4

4. Failing to consider the clinical context:

   • In NAFLD, ALT values typically range from normal to 250 IU/L (usually <5× ULN) 4
   • AST is typically lower than ALT in NAFLD, but with advanced fibrosis, AST may be higher 4

By understanding these reference ranges and their clinical context, healthcare providers can more accurately interpret liver function tests and make appropriate clinical decisions.