A 3-day-old male neonate, born to a 28-year-old primigravida mother via normal vaginal delivery at term, presents with progressive jaundice. Birth weight was 3.2 kg. On examination, the infant is lethargic, with poor feeding and a weak cry. Serum bilirubin is 18 mg/dL (total), with direct bilirubin 0.4 mg/dL. Coombs test is negative. The mother's blood group is O positive and the infant's is A positive. Reticulocyte count is 8%. Which of the following is the most likely diagnosis?

Explanation

## Clinical Diagnosis: ABO Incompatibility ### Key Clinical Features **Key Point:** ABO incompatibility is the most common cause of hemolytic disease of the newborn (HDN) in developed countries, though it typically causes milder disease than Rh incompatibility. This case presents classic features of ABO incompatibility: - Mother O positive (has naturally occurring anti-A and anti-B IgG antibodies) - Infant A positive (A antigen on RBCs) - Early-onset jaundice (day 3 of life) - Negative Coombs test (IgG antibodies may not be detected in direct Coombs due to low RBC antigen density) - Elevated reticulocyte count (8%) indicating hemolysis - Negative direct bilirubin (unconjugated hyperbilirubinemia) - Maternal-neonatal blood group incompatibility ### Differential Features of HDN | Feature | ABO Incompatibility | Rh Incompatibility | Hereditary Spherocytosis | G6PD Deficiency | |---------|-------------------|-------------------|------------------------|----------------| | **Coombs Test** | Often negative or weakly positive | Strongly positive | Negative | Negative | | **Onset** | Day 1–3 (early) | Day 2–7 (variable) | Variable | Variable (trigger-dependent) | | **Severity** | Mild to moderate | Severe (hydrops in untreated) | Mild to moderate | Mild to severe | | **Maternal History** | O blood group | Rh negative, prior sensitization | Family history of anemia/jaundice | Family history, ethnic origin | | **RBC Morphology** | Normal | Normal | Spherocytes | Normal | | **Reticulocyte Count** | Elevated | Elevated | Elevated | Elevated | ### Pathophysiology **High-Yield:** ABO incompatibility occurs when: 1. Mother has blood group O (naturally occurring IgG anti-A and anti-B) 2. Infant has blood group A or B 3. Maternal IgG antibodies cross the placenta and bind to fetal RBCs 4. RBCs are hemolyzed in the fetal/neonatal circulation 5. Unconjugated bilirubin is released and accumulates ### Why Coombs Test May Be Negative **Clinical Pearl:** The direct Coombs (antiglobulin) test may be negative or weakly positive in ABO incompatibility because: - A and B antigens are present in low density on neonatal RBCs - IgG coating is sparse compared to Rh disease - The test may not detect the small amount of antibody present This contrasts with Rh incompatibility, where Coombs is typically strongly positive due to high-density D antigen and heavy IgG coating. ### Management Principles **Key Point:** Treatment depends on serum bilirubin level and age: - Phototherapy (threshold varies by postnatal age and risk factors) - Exchange transfusion if bilirubin exceeds exchange threshold - Frequent monitoring of bilirubin levels - Ensure adequate feeding to promote bilirubin excretion ### Clinical Significance **High-Yield:** ABO incompatibility: - Does NOT require antenatal intervention (unlike Rh disease) - Does NOT typically cause hydrops fetalis (unlike severe Rh disease) - Is usually milder than Rh incompatibility - May recur in subsequent pregnancies (unlike Rh disease, which worsens with each pregnancy) - Requires neonatal management but has excellent prognosis with appropriate treatment [cite:Nelson Textbook of Pediatrics 21e Ch 101]