How to Check Bilirubin Levels in Pediatric Patients
Total serum bilirubin (TSB) measured from capillary blood is the definitive diagnostic test that should guide all clinical interventions for neonatal hyperbilirubinemia 1.
Algorithmic Approach to Bilirubin Measurement
Initial Screening (First 24 Hours and Beyond)
Use transcutaneous bilirubin (TcB) as your first-line screening tool for all jaundiced newborns. TcB devices provide non-invasive measurements and show strong correlation with TSB (correlation coefficients 0.961-0.966) 2. However, recognize that TcB measurements are screening tools only—they are not accurate enough to determine treatment decisions 1.
When to Obtain TSB (Blood Sample)
You must obtain a TSB measurement in these specific situations 1:
- TcB reading is within 3.0 mg/dL of the phototherapy treatment threshold
- TcB exceeds the phototherapy treatment threshold
- TcB is ≥15 mg/dL (257 μmol/L)
- Any jaundice appearing in the first 24 hours of life
- Jaundice appears excessive for the infant's age
Blood Sampling Technique: A Critical Pitfall to Avoid
Do not obtain a venous sample to "confirm" an elevated capillary TSB level—this is explicitly not recommended because it delays treatment initiation 3. Almost all published data on kernicterus and developmental outcomes are based on capillary blood TSB levels. While data on capillary versus venous differences are conflicting, the key point is that obtaining venous blood adds no clinical value and wastes precious time.
Measurement Methods: Practical Hierarchy
For Routine Screening (TcB ≤200 μmol/L)
- Use skin test devices (transcutaneous bilirubinometers)
- Measure on covered, unexposed skin (forehead/glabella preferred)
- Important caveat: Visual estimation of jaundice is unreliable, particularly in darkly pigmented infants 3
For Intermediate Values (TcB 200-250 μmol/L)
If you're already obtaining blood for other analytes using a blood gas analyzer or point-of-care device, you can include bilirubin in that analysis up to 250 μmol/L 2. However, be aware that these nonchemical photometric devices tend to underestimate bilirubin levels, especially at higher concentrations.
For High Values (TcB >200 μmol/L or Treatment Decisions)
Go directly to standard laboratory TSB measurement 2. This avoids repeated blood sampling and provides the most accurate result for treatment decisions.
Special Circumstances
During and After Phototherapy
TcB measurements are unreliable during phototherapy because phototherapy "bleaches" the skin 3. After phototherapy discontinuation:
- TSB should be measured to verify efficacy 1
- TcB can be used only if ≥24 hours have passed since phototherapy stopped 1
- Recent evidence shows TcB underestimates TSB during and after phototherapy with considerable variability 4
Monitoring Frequency
- Every 2 hours when TSB is at or near exchange transfusion threshold 1
- Timing otherwise guided by TSB trajectory (rate of rise) and infant age 1
- For rate of rise calculation: ≥0.3 mg/dL per hour in first 24 hours or ≥0.2 mg/dL per hour thereafter suggests hemolysis 1
Direct/Conjugated Bilirubin
When TSB is elevated or jaundice persists beyond 3 weeks, measure direct or conjugated bilirubin 3. If TSB ≤5 mg/dL (85 μmol/L), a direct bilirubin >1.0 mg/dL is abnormal 3.
Key Quality Considerations
All bilirubin measurements should be:
- Interpreted according to the infant's age in hours, not days 3
- Traceable to internationally endorsed reference measurement systems 5
- Performed using the same method when monitoring trends 6
The evidence strongly supports this hierarchical approach, with the most recent 2024 AAP guidelines 1 emphasizing that TSB remains the gold standard despite advances in transcutaneous technology. The critical principle is using TcB for screening efficiency while reserving TSB for all treatment decisions to prevent kernicterus and optimize outcomes.