What labs are included in Liver Function Tests (LFTs)?

Components of Liver Function Tests (LFTs)

Liver function tests (LFTs) include alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), gamma-glutamyl transpeptidase (GGT), bilirubin, albumin, and prothrombin time/international normalized ratio (PT/INR). 1

Core Components of LFTs

Hepatocellular Injury Markers:

• ALT (Alanine Aminotransferase)

  • More specific for liver injury than AST
  • Normal levels: 29-33 IU/L in men, 19-25 IU/L in women 1

• AST (Aspartate Aminotransferase)

  • Less specific than ALT (also found in heart, skeletal muscle, kidneys, brain, and red blood cells)
  • Elevation patterns help distinguish liver disease types

Cholestatic Markers:

• ALP (Alkaline Phosphatase)

  • Elevated in cholestatic liver disease
  • Can indicate biliary obstruction 1

• GGT (Gamma-Glutamyl Transpeptidase)

  • Confirms hepatic origin of ALP elevation
  • Sensitive but not specific for liver disease 1

Liver Function Indicators:

• Bilirubin (Total and Direct)

  • Reflects liver's ability to take up, process, and secrete bilirubin
  • Elevated in various liver and biliary conditions 1

• Albumin

  • True marker of hepatocellular synthetic function
  • Decreased levels indicate chronic liver disease 1

• PT/INR (Prothrombin Time/International Normalized Ratio)

  • True marker of liver synthetic function
  • Prolonged in advanced liver disease 1

Additional Tests Often Included with LFTs

• Total Protein

  • Reflects both albumin and globulin levels 2

• Platelet Count

  • Surrogate marker for portal hypertension
  • Helps assess degree of fibrosis 1

Pattern Recognition in LFTs

Hepatocellular Pattern:

• Predominant elevation of ALT and AST
• Severity classification:
  • Mild: <5× upper limit of normal (ULN)
  • Moderate: 5-10× ULN
  • Severe: >10× ULN 1, 3

Cholestatic Pattern:

• Predominant elevation of ALP and GGT
• Suggests biliary obstruction or cholestatic disease 1, 3

Mixed Pattern:

• Elevation of both transaminases and cholestatic enzymes 3

Clinical Utility and Limitations

• LFTs are not truly "liver function tests" but rather markers of liver injury or cholestasis 1, 4
• True liver function is better assessed by albumin, PT/INR, and bilirubin 1
• When used in isolation, LFTs have limited diagnostic specificity 1, 4
• Most effective when interpreted as a panel rather than individual tests 4

Practical Considerations

• ALT and ALP represent the most efficient restricted panel when screening for liver disease in primary care 5
• Abnormal LFTs should be interpreted in the context of previous results and clinical condition 1
• The extent of abnormality doesn't necessarily correlate with clinical significance 1
• Genetic factors account for 20-77% of variation in LFT values 2

Common Pitfalls to Avoid

• Assuming all LFT abnormalities indicate liver disease (AST can be elevated in cardiac, muscle, and other conditions)
• Failing to consider the pattern of abnormalities (hepatocellular vs. cholestatic)
• Over-reliance on absolute values without considering trends
• Neglecting to assess true liver function (albumin, PT/INR) in addition to injury markers

Remember that LFTs should be interpreted as a panel rather than in isolation, and patterns of abnormality often provide more diagnostic information than individual test results.