Lab Test for Unconjugated (Indirect) Bilirubin
Unconjugated bilirubin is measured by fractionating total bilirubin into its conjugated and unconjugated components, with the diagnosis confirmed when conjugated bilirubin represents less than 20-30% of total bilirubin. 1
Standard Laboratory Approach
Order a fractionated bilirubin panel that includes total bilirubin, direct (conjugated) bilirubin, and indirect (unconjugated) bilirubin—this is the standard clinical test available in most laboratories 1, 2
The unconjugated fraction is typically calculated by subtracting direct bilirubin from total bilirubin (Indirect = Total - Direct), though modern analyzers can measure both fractions directly 1
Most laboratories routinely report total bilirubin only, so you must specifically request fractionated bilirubin to distinguish between conjugated and unconjugated forms 1
Critical Terminology Distinction
"Direct" bilirubin is NOT synonymous with "conjugated" bilirubin—this is a common and clinically important error 1, 3
Direct bilirubin includes both conjugated bilirubin AND delta bilirubin (bilirubin covalently bound to albumin), which has a 21-day half-life and causes persistent hyperbilirubinemia even after resolution of the underlying cause 1
If the etiology of prolonged hyperbilirubinemia is uncertain, request a breakdown of the direct bilirubin fraction into conjugated and delta bilirubin components 1
Diagnostic Interpretation
Unconjugated hyperbilirubinemia is confirmed when conjugated bilirubin is <20-30% of total bilirubin in the absence of hemolysis 1
In Gilbert's syndrome (the most common cause of isolated unconjugated hyperbilirubinemia), total bilirubin is usually mildly elevated but rarely exceeds 4-5 mg/dL 1
The majority of measurable bilirubin should be conjugated even in individuals with significant liver disease, so if the majority is unconjugated (in the absence of hemolysis), the cause is virtually always Gilbert's syndrome 1
Advanced Testing Methods
High-performance liquid chromatography (HPLC) with alkaline methanolysis is the reference standard for specific measurement of unconjugated bilirubin, monoconjugates, and diconjugates, but this is primarily a research tool 4, 5, 6
Genetic testing for UDP-glucuronosyltransferase mutations can provide definitive confirmation of Gilbert's syndrome when the diagnosis is unclear 1, 2
Complete Evaluation Panel
When unconjugated hyperbilirubinemia is identified, obtain:
- Complete blood count with differential and peripheral smear to evaluate for hemolysis 2
- Reticulocyte count, haptoglobin, and LDH to assess for hemolytic causes 2
- G6PD testing, particularly in African American patients (11-13% prevalence), Mediterranean, or Asian descent patients 3, 2
- Liver function tests (ALT, AST, alkaline phosphatase, GGT) to exclude hepatocellular disease 1
Common Pitfalls to Avoid
Never rely on visual estimation of jaundice—always obtain objective bilirubin measurements, especially in darkly pigmented patients 2
Do not overinterpret isolated mild unconjugated hyperbilirubinemia as indicative of significant liver disease when other liver tests are normal 2
Remember that conventional diazo assays may yield unpredictably higher values than specific measurement techniques and include unidentified diazo-positive compounds distinct from bilirubin 4, 6