What lab test checks bilirubin levels in the blood?

Bilirubin Testing: Laboratory Methodology

To check bilirubin levels in the blood, order a serum total bilirubin test, which should be fractionated into direct (conjugated) and indirect (unconjugated) components to guide diagnosis. 1, 2

Standard Laboratory Test Ordering

• Order "Total and Fractionated Bilirubin" or "Total and Direct Bilirubin" as the initial test when evaluating for liver disease, jaundice, or hemolysis 1, 2

• The fractionated bilirubin test measures:

  • Total serum bilirubin (sum of all bilirubin forms) 3
  • Direct bilirubin (conjugated + delta-bilirubin) 1
  • Indirect bilirubin (calculated as total minus direct) 1

Clinical Context for Test Interpretation

• Normal total bilirubin ranges from <1.2 mg/dL (<20 μmol/L), with values measured in either mg/dL or μmol/L depending on laboratory standards 3

• The American Gastroenterological Association recommends calculating the percentage of conjugated bilirubin relative to total bilirubin to determine diagnostic pathways 1

• When conjugated bilirubin is <20-30% of total bilirubin, suspect unconjugated hyperbilirubinemia (Gilbert syndrome, hemolysis) 1

• When direct bilirubin is >35% of total bilirubin, suspect hepatocellular injury or cholestatic disease 1

Laboratory Methodology

• The most widely used methods are:

  • Diazo transfer reaction (current gold standard) 4, 5
  • Bilirubin oxidase (BOD) enzymatic methods 6, 4
  • High-performance liquid chromatography (HPLC) for precise fractionation 4, 7
  • Direct spectrophotometry 4, 7

• HPLC provides the most accurate separation of all four bilirubin fractions but is impractical for routine clinical use 7

Common Pitfalls to Avoid

• Never rely on visual estimation of jaundice - always obtain objective bilirubin measurements, especially in darkly pigmented patients 1

• Do not order direct bilirubin alone without total bilirubin, as the percentage relationship is diagnostically critical 1, 2

• Recognize that direct bilirubin is not synonymous with conjugated bilirubin - it includes delta-bilirubin which has a 21-day half-life and can cause persistent hyperbilirubinemia even after the underlying cause resolves 1

• Screening direct bilirubin only when total bilirubin ≥1.0 mg/dL is acceptable and reduces unnecessary testing without missing clinically significant disease 8

Additional Testing Considerations

• Comprehensive liver panel should include: ALT, AST, alkaline phosphatase, GGT, albumin, and prothrombin time/INR alongside bilirubin 1, 2

• For unconjugated hyperbilirubinemia, add hemolysis workup: reticulocyte count, haptoglobin, LDH, and peripheral smear 1, 2

• For conjugated hyperbilirubinemia, obtain abdominal ultrasound to evaluate for biliary obstruction 1, 2