Normal Range for Total Bilirubin in Healthy Adults
The normal reference range for total bilirubin in healthy adults is approximately 0.3–1.2 mg/dL (5–21 μmol/L), with slight variations by sex, age, and ethnicity. 1
Standard Reference Intervals
In healthy adults, total bilirubin typically ranges from 0.3–1.2 mg/dL (approximately 5–21 μmol/L), with values below 1.0 mg/dL considered definitively normal in most clinical contexts. 1, 2
Gender-specific differences exist: males tend to have slightly higher total bilirubin levels than females, with upper reference limits ranging from approximately 1.0–1.3 mg/dL in men versus 0.9–1.1 mg/dL in women. 1, 3
In elderly populations (≥65 years), reference intervals are narrower: 7.8–30.6 μmol/L (approximately 0.5–1.8 mg/dL) for males and 7.3–26.1 μmol/L (approximately 0.4–1.5 mg/dL) for females. 3
Key Clinical Context
Total bilirubin is one of only five standard laboratory tests (along with glucose, phosphorus, potassium, and uric acid) that does not show significant racial or ethnic variation in distribution among healthy populations. 1
Values between 1.0–1.5 mg/dL represent mild elevation and most commonly indicate Gilbert syndrome (affecting 5–10% of the population) when other liver tests are normal and the patient is asymptomatic. 4, 5
The conjugated (direct) bilirubin fraction normally comprises less than 20–30% of total bilirubin; when this threshold is exceeded, hepatobiliary disease should be investigated. 1, 4
Important Measurement Considerations
Reference measurement procedures establish a validated range of 5–525 μmol/L (approximately 0.3–30 mg/dL) with excellent precision (CV 0.5–1.4%), though clinical interpretation focuses on the lower end of this spectrum for healthy individuals. 6
In Chinese geriatric populations, established reference intervals are 6.6–21.8 μmol/L for males and 6.1–20.0 μmol/L for females, demonstrating geographic and demographic consistency with Western populations. 7
Critical Thresholds for Clinical Action
Total bilirubin ≥1.0 mg/dL (17 μmol/L) is the traditional screening threshold above which direct bilirubin fractionation should be performed to exclude conjugated hyperbilirubinemia. 2
In neonates, normal adult reference ranges do not apply; any total bilirubin ≥25 mg/dL (428 μmol/L) or ≥20 mg/dL in a sick infant constitutes a medical emergency requiring immediate intensive phototherapy. 8
Direct bilirubin concentration ≥10 μmol/L (approximately 0.6 mg/dL) should prompt consideration of conjugated hyperbilirubinemia, provided total bilirubin is also elevated above the reference interval. 9
Common Pitfalls
Do not assume that mild elevations (1.0–2.0 mg/dL) automatically indicate liver disease; Gilbert syndrome is the most common cause of isolated unconjugated hyperbilirubinemia in asymptomatic adults with otherwise normal liver function tests. 4, 5
Do not equate "direct bilirubin" with "conjugated bilirubin"—direct bilirubin includes both conjugated bilirubin and delta-bilirubin (albumin-bound), which has a 21-day half-life and can cause persistent elevation even after resolution of the underlying cause. 1, 4
Age-related changes must be considered: alkaline phosphatase increases by 20% between the third and eighth decades, but total bilirubin remains relatively stable across adult age groups when corrected for sex. 1, 3