Normal Indirect Bilirubin Levels
In healthy adults, indirect (unconjugated) bilirubin normally comprises 70-90% of total bilirubin, with absolute values typically ranging from 0.2-0.6 mg/dL (3.4-10.3 μmol/L). 1, 2
Reference Range Components
- Total bilirubin in healthy adults ranges from 0.2-0.8 mg/dL 3
- Direct (conjugated) bilirubin should be 0-0.2 mg/dL 3
- Indirect (unconjugated) bilirubin is calculated as Total bilirubin minus Direct bilirubin, normally 0.2-0.6 mg/dL 2, 3
Normal Bilirubin Fraction Distribution
- In healthy individuals, unconjugated bilirubin represents approximately 84% ± 5% of total serum bilirubin, with a range of 72-90% 4
- The conjugated fraction in healthy adults averages 3.6% of total bilirubin 5
- This distribution pattern helps distinguish normal physiology from pathological conditions 4
Clinical Context for Interpretation
Gilbert Syndrome Threshold
- When unconjugated bilirubin exceeds normal limits but remains less than 20-30% of total bilirubin as conjugated, this suggests Gilbert syndrome rather than hepatobiliary disease 1, 2
- Total bilirubin in Gilbert syndrome rarely exceeds 4-5 mg/dL, with the unconjugated fraction comprising 95% ± 2% (range 90-99%) of total 1, 4
Pathological Thresholds
- Conjugated bilirubin >35% of total indicates true conjugated hyperbilirubinemia requiring evaluation for hepatobiliary disease 1, 2
- In neonates, conjugated bilirubin >25 μmol/L requires urgent assessment for liver disease 2
Important Distinctions
- The terms "direct" and "indirect" bilirubin are not interchangeable with "conjugated" and "unconjugated" 1
- Direct bilirubin includes both conjugated bilirubin and delta bilirubin (albumin-bound), which has a 21-day half-life and can cause prolonged hyperbilirubinemia 1
- For precise diagnosis in unclear cases, fractionation of direct bilirubin into conjugated and delta components may be necessary 1