What are the diagnostic criteria for Hemolytic Disease of the Newborn (HDN) in a newborn patient?

Diagnostic Criteria for Hemolytic Disease of the Newborn (HDN)

The diagnosis of HDN requires three essential components: documented blood group incompatibility between mother and infant, laboratory evidence of hemolysis, and a positive direct antiglobulin test (DAT/Coombs test) to confirm immune-mediated red cell destruction. 1

Core Diagnostic Requirements

1. Blood Group Incompatibility Documentation

• Rh (D) incompatibility: Rh-negative mother with Rh-positive infant 2
• ABO incompatibility: Most commonly group O mother with group A or B infant 3, 1
• Less common: Group A₂ mothers can rarely produce clinically significant anti-B causing HDN in group B infants, particularly when IgG anti-B titers exceed 256 4

2. Laboratory Confirmation of Hemolysis

The American Academy of Pediatrics recommends the following tests to confirm hemolysis 5:

• Reticulocyte count (elevated, indicating compensatory red cell production) 5
• Peripheral blood smear (showing spherocytes, fragmented cells, or nucleated RBCs) 5
• Haptoglobin (decreased due to binding free hemoglobin) 5
• Lactate dehydrogenase (LDH) (elevated from red cell destruction) 5
• End-tidal carbon monoxide corrected (ETCOc) provides direct measurement of heme catabolism rate and is more reliable than standard hemolysis markers 5, 1

Important caveat: Standard hemolysis markers have poor specificity and sensitivity; ETCOc measurement is the most reliable indicator when available 5

3. Direct Antiglobulin Test (DAT/Coombs Test)

• A positive DAT is essential for diagnosing immune-mediated HDN 5, 1
• The DAT differentiates immune from non-immune causes of hemolysis 5
• For ABO HDN specifically: A positive DAT combined with ABO incompatibility and significant hyperbilirubinemia establishes the diagnosis 1
• Critical pitfall: A negative DAT in a severely jaundiced ABO-incompatible infant should trigger an exhaustive search for alternative causes (G6PD deficiency, hereditary spherocytosis, other enzymopathies) using targeted gene panels if necessary 1, 6

Clinical Presentation Markers

Hyperbilirubinemia Assessment

• Measure total serum bilirubin (TSB) and plot on hour-specific nomogram 5
• Calculate bilirubin/albumin (B/A) ratio to assess neurotoxicity risk 5
• Severe elevation: TSB ≥25 mg/dL (428 μmol/L) constitutes a medical emergency 5

Additional Laboratory Studies

• Complete blood count with differential showing anemia and elevated reticulocytes 5
• Maternal antibody screen and typing 2
• For ABO HDN: Measure maternal IgG anti-A,B titers (titers ≥1024 indicate severe disease risk) 3, 7

Diagnostic Algorithm

1. Identify blood group incompatibility between mother and infant through blood typing 2, 1
2. Perform DAT on infant's blood - if negative, HDN diagnosis is suspect and alternative causes must be investigated 1, 6
3. If DAT positive, confirm hemolysis with reticulocyte count, peripheral smear, haptoglobin, LDH, and ETCOc if available 5
4. Measure TSB and calculate B/A ratio to determine severity and treatment urgency 5
5. For ABO incompatibility with negative DAT: Do not diagnose as ABO HDN; instead perform comprehensive evaluation for G6PD deficiency, hereditary spherocytosis, and other non-immune hemolytic causes 1, 6

Common Diagnostic Pitfalls

• Overdiagnosis of ABO HDN: Using ABO incompatibility alone as a default diagnosis for all hyperbilirubinemia without confirming positive DAT lacks diagnostic precision 1
• Assuming O-negative infant is protected: An O-negative newborn with negative DAT is at very low risk for HDN and requires only standard surveillance, not intensive monitoring 6
• Ignoring maternal blood subgroups: Group A₂ mothers can produce pathologic anti-B, so determine maternal A subgroup when evaluating unexplained HDN in group B infants of group A mothers 4
• Relying solely on standard hemolysis markers: These have poor sensitivity/specificity; ETCOc provides superior diagnostic accuracy 5, 1