How do I order a fractionated bilirubin test?

How to Order a Fractionated Bilirubin Test

Order a fractionated bilirubin (also called direct and indirect bilirubin) by requesting "bilirubin with fractionation," "direct and indirect bilirubin," or "conjugated and unconjugated bilirubin" from your laboratory—this is the essential first step when evaluating any patient with elevated total bilirubin to determine whether hyperbilirubinemia is conjugated or unconjugated. 1

Ordering the Test

• Request the test as "bilirubin fractionation," "direct/indirect bilirubin," or "conjugated/unconjugated bilirubin" when placing laboratory orders 2
• Most laboratories will automatically report total bilirubin only unless fractionation is specifically requested 2
• The test separates total bilirubin into its conjugated (direct-reacting) and unconjugated (indirect-reacting) components 2

When to Order Fractionated Bilirubin

• Order fractionation whenever total bilirubin is elevated to distinguish between unconjugated hyperbilirubinemia (hemolysis, Gilbert's syndrome) and conjugated hyperbilirubinemia (hepatocellular disease, biliary obstruction) 1
• Fractionation is particularly valuable when total bilirubin is <5 mg/dL, as this is the range where Gilbert's syndrome and early liver disease present 3
• In neonates and infants, order fractionation urgently if conjugated bilirubin may be >25 μmol/L to rule out serious liver disease 2

Accompanying Laboratory Tests

• Order a complete initial panel including: 1
  • Complete blood count with differential and peripheral smear (to evaluate for hemolysis)
  • Liver function tests: ALT, AST, alkaline phosphatase
  • GGT (to confirm hepatic origin of alkaline phosphatase elevation)
  • Albumin and prothrombin time/INR (to assess synthetic liver function)
  • Reticulocyte count (to evaluate for hemolysis)

Interpreting the Results

• Gilbert's syndrome is confirmed when conjugated bilirubin is <20-30% of total bilirubin in an asymptomatic patient with otherwise normal liver tests 1
• Conjugated hyperbilirubinemia (direct bilirubin >35% of total) indicates hepatocellular injury or cholestatic disease and requires further workup including abdominal ultrasound 1
• Unconjugated hyperbilirubinemia with normal liver tests and no hemolysis is virtually always Gilbert's syndrome and requires no treatment 2

Critical Pitfalls to Avoid

• Laboratory method variability is significant: Different laboratories may produce conflicting results, particularly at total bilirubin levels <5 mg/dL 3
• Direct bilirubin is not synonymous with conjugated bilirubin—it includes delta bilirubin (albumin-bound bilirubin with a 21-day half-life), which can cause persistent hyperbilirubinemia even after the underlying cause resolves 1, 4
• Delta bilirubin appears when hepatic excretion of conjugated bilirubin is impaired and becomes a larger component as jaundice subsides, delaying resolution 4
• Avoid extensive workup for isolated mild unconjugated hyperbilirubinemia with normal transaminases—this is almost always benign Gilbert's syndrome 1

Special Considerations

• In patients being evaluated for gene therapy or other hepatotoxic treatments, obtain ≥3 measurements over 6 months to capture fluctuating enzyme levels 2
• For patients with cholestatic disease, check vitamin K status as fat-soluble vitamin deficiencies can cause elevated INR that mimics liver dysfunction 1
• If fractionation shows conjugated hyperbilirubinemia, proceed with abdominal ultrasound to exclude biliary obstruction (98% positive predictive value for liver parenchymal disease) 1