What are the differences between transcutaneous (through the skin) bilirubin and serum bilirubin in diagnosing hyperbilirubinemia?

Differences Between Transcutaneous Bilirubin and Serum Bilirubin in Diagnosing Hyperbilirubinemia

Transcutaneous bilirubin (TcB) measurements are useful screening tools but cannot fully replace total serum bilirubin (TSB) measurements, particularly for levels above 15 mg/dL or in infants receiving phototherapy, as TcB primarily measures extravascular bilirubin while TSB measures intravascular bilirubin. 1, 2

Key Differences

Measurement Method

• TSB: Invasive method requiring blood sampling; gold standard for diagnosis and treatment of severe neonatal hyperbilirubinemia 3
• TcB: Non-invasive method using optical technology to measure bilirubin in the skin 2

Physiological Parameters Measured

• TSB: Measures intravascular bilirubin concentration in blood
• TcB: Primarily measures extravascular bilirubin in skin (>99% of measurement), with minimal contribution from blood vessels 2

Accuracy and Correlation

• TcB generally correlates well with TSB (r = 0.81-0.88) with measurements typically within 2-3 mg/dL (34-51 μmol/L) of TSB 4, 5
• TcB tends to be higher than TSB with a mean difference of approximately 0.44 mg/dL 5
• Accuracy decreases at higher bilirubin levels (>15 mg/dL or 257 μmol/L) 4

Clinical Applications

When TcB is Appropriate:

• Initial screening for hyperbilirubinemia
• TSB levels less than 15 mg/dL (257 μmol/L) 4
• Follow-up monitoring in non-phototherapy treated infants

When TSB is Required:

• Confirmation of elevated TcB readings
• Bilirubin levels approaching treatment thresholds
• Infants receiving phototherapy (TcB becomes unreliable) 4, 6
• Infants with hemolytic disease or G6PD deficiency 4
• When precise measurements are needed for clinical decision-making

Limitations of TcB

1. Phototherapy Effect: Phototherapy "bleaches" the skin, making TcB measurements unreliable in treated areas 4
2. Skin Pigmentation: Accuracy may vary across different racial groups 4
3. Blood Volume Fraction: The low blood volume fraction (0.1-0.75%) in measured skin areas limits TcB's ability to accurately reflect serum levels 2
4. Physiological Difference: TcB and TSB are fundamentally different parameters - one primarily measures extravascular bilirubin, the other intravascular 2

Recommended Cutoff Values for TSB Confirmation

TcB cutoff values requiring TSB confirmation based on postnatal age 5:

• 24 hours: TcB > 9 mg/dL (TSB threshold: 8 mg/dL)
• 36 hours: TcB > 12 mg/dL (TSB threshold: 10 mg/dL)
• 48 hours: TcB > 13 mg/dL (TSB threshold: 12 mg/dL)
• 72 hours: TcB > 15 mg/dL (TSB threshold: 15 mg/dL)

Common Pitfalls to Avoid

1. Relying solely on TcB for treatment decisions: TcB should be used as a screening tool, not for definitive treatment decisions in severe hyperbilirubinemia
2. Obtaining venous samples to "confirm" elevated capillary TSB: This only delays treatment without adding value 1
3. Using TcB in phototherapy-treated areas: Measurements should be taken from skin areas protected from phototherapy light 6
4. Delaying evaluation of conjugated hyperbilirubinemia: Always investigate promptly as this represents pathology 1

Practical Approach to Bilirubin Measurement

1. Use TcB as initial screening tool for jaundice
2. If TcB is below age-specific cutoffs, continue monitoring with TcB
3. If TcB approaches or exceeds cutoffs, confirm with TSB
4. For infants receiving phototherapy, rely on TSB or measure TcB on unexposed skin areas
5. For infants with risk factors (hemolysis, G6PD deficiency), use TSB for monitoring

By understanding these differences, clinicians can appropriately select the most suitable method for bilirubin assessment while minimizing unnecessary blood sampling and ensuring accurate diagnosis and treatment of hyperbilirubinemia.