Normal Total Bilirubin Range
The normal range for total serum bilirubin in adults is generally less than 1.0-1.2 mg/dL, with most healthy individuals having levels below 1.0 mg/dL.
Standard Reference Values
Total bilirubin levels are typically considered normal when less than 1.0-1.2 mg/dL in the general adult population 1.
In clinical practice and research settings, bilirubin levels below 1.5 mg/dL are commonly used as baseline normal values when assessing liver function 1.
The upper limit of normal (ULN) for total bilirubin is approximately 1.0-1.2 mg/dL, though this can vary slightly between laboratories 1.
Clinical Context and Variations
Disease-Specific Considerations
In patients with primary sclerosing cholangitis (PSC), total bilirubin levels are typically normal at presentation in the majority of patients, unless they have advanced disease, Gilbert's Syndrome, or hemolysis 1.
Clinical trials for cholestatic liver diseases typically require baseline total bilirubin levels ranging from <1.5× ULN to <3× ULN (approximately <1.5-3.6 mg/dL) for patient eligibility 1.
Gilbert's Syndrome Exception
Approximately 5-10% of the population has Gilbert's Syndrome, which causes intermittent mild elevations in unconjugated bilirubin 1.
In Gilbert's Syndrome, total bilirubin levels are usually mildly elevated but rarely exceed 4-5 mg/dL, with conjugated bilirubin representing less than 20-30% of the total 1.
Important Clinical Caveats
Laboratory Variability
Direct bilirubin measurements are not precise, and values can vary widely between laboratories, according to the American Academy of Pediatrics 2.
When interpreting bilirubin results, clinicians should be aware that "direct" and "conjugated" bilirubin are often incorrectly used interchangeably—direct bilirubin includes both conjugated bilirubin and delta bilirubin (albumin-bound) 1.
Prognostic Significance of Abnormal Levels
In severe sepsis, mortality increases significantly when bilirubin exceeds 1.0 mg/dL, with adjusted odds of mortality 3.85 times higher for levels between 1.1-2.0 mg/dL and 9.85 times higher for levels >2.0 mg/dL 3.
In acute calculus cholecystitis, serum bilirubin specificity for common bile duct stones is 60% at a cutoff of 1.7 mg/dL and 75% at 4.0 mg/dL 1.