What Bilirubin to Check in Newborns
Measure total serum bilirubin (TSB) or transcutaneous bilirubin (TcB) as the primary screening test in all newborns with jaundice, and obtain fractionated bilirubin (total and direct/conjugated) immediately if jaundice appears in the first 24 hours of life or persists beyond 3 weeks. 1
Primary Screening Approach
- Obtain TSB or TcB immediately for any infant with visible jaundice in the first 24 hours of life, as this is always pathologic and demands urgent evaluation. 1
- Never rely on visual assessment of jaundice severity, particularly in darkly pigmented infants, as this leads to dangerous errors—always obtain objective bilirubin measurements. 1
- For routine jaundice screening after the first 24 hours, TSB or TcB measurement is sufficient for initial risk stratification using hour-specific nomograms. 1
When to Fractionate Bilirubin (Measure Direct/Conjugated)
Measure fractionated bilirubin (total and direct/conjugated) in three specific scenarios: 1
- Jaundice appearing in the first 24 hours of life (always pathologic)
- Jaundice persisting at or beyond 3 weeks of age (to identify cholestasis)
- Any infant with clinical signs of illness or suspected liver disease
If TSB is at or below 5 mg/dL (85 μmol/L), a direct or conjugated bilirubin >1.0 mg/dL is considered abnormal and warrants further investigation. 2
Elevated direct (conjugated) bilirubin >25 μmol/L is never physiologic and demands immediate referral to pediatrics for assessment of possible liver disease. 3
Essential Laboratory Workup for Early or Pathologic Jaundice
When jaundice appears in the first 24 hours or TSB is significantly elevated, obtain: 1
- Blood type and direct antibody test (Coombs' test) for both infant and mother
- Complete blood count with peripheral smear and reticulocyte count (to evaluate for hemolysis)
- Fractionated bilirubin (total and direct/conjugated)
- G6PD level, particularly in African American, Mediterranean, or Asian descent infants
Critical Technical Considerations
- Do not obtain a venous sample to "confirm" an elevated capillary TSB level—this delays treatment initiation without providing useful clinical benefit, as almost all published data on bilirubin toxicity are based on capillary measurements. 2
- Direct-reacting bilirubin and conjugated bilirubin are commonly used interchangeably in clinical practice, though they are not technically identical—direct bilirubin includes both conjugated bilirubin and delta bilirubin. 2, 4
- Never subtract direct bilirubin from total bilirubin when making phototherapy decisions, as this can lead to dangerous underestimation of risk. 1, 4
Common Pitfalls to Avoid
- Failing to recognize that G6PD levels can be falsely elevated during active hemolysis—a normal level does not rule out G6PD deficiency in a hemolyzing neonate, and repeat testing at 3 months is necessary if strongly suspected. 2, 1
- Discharging an infant with jaundice in the first 24 hours without objective bilirubin measurement and a clear follow-up plan. 1
- Relying on visual assessment during or after phototherapy, as phototherapy "bleaches" the skin and makes both visual assessment and TcB measurements unreliable. 1