Liver Function Tests: Interpretation and Limitations

Introduction to Liver Function Tests

The American Gastroenterological Association suggests that liver function tests (LFTs) are a panel of biochemical blood tests that primarily measure hepatocellular injury and cholestasis rather than actual liver function, with true functional assessment requiring measurement of synthetic capacity (albumin, INR) and metabolic clearance 1, 2

The American College of Radiology recommends that a standard LFT panel should minimally include ALT and ALP, as these identify the vast majority of adults with liver disease 5, 6
ALT (Alanine Aminotransferase) is most specific for liver injury due to low concentrations in other tissues; normal values are 29-33 IU/L in men and 19-25 IU/L in women 2, 4
AST (Aspartate Aminotransferase) is less specific than ALT as it's also present in heart, skeletal muscle, kidneys, brain, and red blood cells 2, 4
ALP (Alkaline Phosphatase) is elevated in cholestatic liver disease; requires GGT confirmation for hepatic origin 2, 5
GGT (Gamma-Glutamyl Transpeptidase) confirms hepatic origin of elevated ALP 5, 6

The American College of Radiology states that predominant elevation of aminotransferases (ALT/AST) >> ALP suggests a hepatocellular pattern 2, 4
Severity classification of hepatocellular pattern: Mild: <5× upper reference limit, Moderate: 5-10× upper reference limit, Severe: >10× upper reference limit 2, 4
AST:ALT ratio >2 suggests alcohol-induced disease; <1 suggests metabolic/NAFLD 4
The American Gastroenterological Association recommends that abdominal ultrasound is essential to distinguish extrahepatic (biliary obstruction) from intrahepatic cholestasis 5, 6

The American Gastroenterological Association notes that LFTs used in isolation are neither specific diagnostic tools nor effective exclusion tools for significant liver disease 3, 1
30% of routine LFTs show at least one abnormal value outside reference range 3, 1
Only 3.9% of patients with abnormal ALT/AST are diagnosed with significant liver disease within 5 years 3, 1

The American Gastroenterological Association recommends that when LFTs are abnormal without clear acute cause, fibrosis risk stratification is essential using validated scores 8
FIB-4 score <1.3 or NAFLD Fibrosis Score ≤-1.455 indicates low risk of advanced fibrosis 8
For patients >65 years, use higher cut-offs: FIB-4 <2.0 8

The American College of Radiology advises against assuming normal LFTs exclude liver disease: Liver disease develops silently; tests may be normal until complications of liver failure or portal hypertension develop 3, 1
Do not over-interpret albumin as liver-specific: Multiple non-hepatic conditions cause hypoalbuminemia 5, 6
Do not ignore the pattern: Hepatocellular vs. cholestatic patterns guide differential diagnosis more than absolute values 2, 4