What labs should be ordered for a Coombs (Direct Antiglobulin Test) positive newborn?

Laboratory Evaluation for Coombs Positive Newborns

For a Coombs (Direct Antiglobulin Test) positive newborn, essential laboratory tests should include total and direct bilirubin levels, complete blood count with differential and smear, reticulocyte count, blood type of both infant and mother, and serum albumin level.

Initial Laboratory Assessment

A positive Direct Antiglobulin Test (DAT/Coombs) indicates antibodies attached to red blood cells, suggesting potential hemolysis. This requires prompt laboratory evaluation:

Essential Tests:

• Total and direct bilirubin levels 1
• Complete blood count with differential and smear for red cell morphology 1
• Blood type (ABO, Rh) of infant and mother 1, 2
• Reticulocyte count to assess red cell production 1, 2
• Serum albumin level 1

Additional Tests Based on Clinical Presentation:

• G6PD screening if suggested by ethnic/geographic origin or poor response to phototherapy 1, 2
• ETCOc (end-tidal carbon monoxide) if available, to assess hemolysis rate 1
• Peripheral blood smear to evaluate for red cell morphology 1

Monitoring Protocol

The American Academy of Pediatrics recommends specific monitoring based on bilirubin levels:

• If TSB ≥ 25 mg/dL (428 μmol/L): Repeat TSB within 2-3 hours 1
• If TSB 20-25 mg/dL (342-428 μmol/L): Repeat within 3-4 hours 1
• If TSB < 20 mg/dL (342 μmol/L): Repeat in 4-6 hours 1

Special Considerations

For Isoimmune Hemolytic Disease:

Coombs positive newborns with ABO or Rh incompatibility require additional vigilance:

• More frequent bilirubin monitoring due to risk of rapid rise 1
• Lower threshold for intervention with phototherapy 2
• Consider intravenous immunoglobulin (0.5-1 g/kg over 2 hours) if TSB is rising despite intensive phototherapy or approaching exchange transfusion levels 1

Risk Assessment:

• Sixth-hour serum bilirubin measurement is particularly useful in predicting significant hyperbilirubinemia in Coombs positive infants 3
• A mean serum bilirubin level ≥4 mg/dL at six hours of life has 86.2% sensitivity for predicting significant hyperbilirubinemia 3
• A level of 6 mg/dL at six hours has 100% sensitivity for diagnosing severe ABO hemolytic disease 3

Clinical Pitfalls to Avoid

1. Don't subtract direct bilirubin from total bilirubin when making treatment decisions 1
2. Don't delay treatment if bilirubin levels are approaching exchange transfusion thresholds 1
3. Don't miss follow-up testing - Coombs positive infants require close monitoring even after discharge 1
4. Don't overlook albumin levels - Low albumin (<3.0 g/dL) increases risk of bilirubin encephalopathy 1
5. Don't rely solely on visual assessment of jaundice, as this can be unreliable, especially in darkly pigmented infants 1

Treatment Considerations

While not the focus of laboratory evaluation, treatment readiness is essential:

• Prepare for possible intensive phototherapy 1, 2
• Type and crossmatch blood if TSB ≥ 20 mg/dL in a sick infant or approaching exchange levels 1
• Calculate TSB/albumin ratio to assess risk of bilirubin encephalopathy 1

By following this comprehensive laboratory approach for Coombs positive newborns, clinicians can effectively monitor, predict, and manage potential hyperbilirubinemia, reducing the risk of kernicterus and improving outcomes.