Neonatal Alloimmune Thrombocytopenia Secondary to Maternal SLE
The most likely diagnosis is neonatal alloimmune thrombocytopenia (NAIT), not immune thrombocytopenia (ITP) secondary to maternal SLE, making the answer B: Neonatal alloimmune thrombocytopenia.
Clinical Reasoning
The key diagnostic features point definitively toward NAIT rather than the other options:
Severe isolated thrombocytopenia (18,000/μL) with normal coagulation studies excludes vitamin K deficiency bleeding (which would show prolonged PT) and hemophilia A (which would show prolonged aPTT) 1.
The platelet count of 18,000/μL meets ISTH diagnostic criteria for NAIT, which defines suspected cases as having a nadir platelet count below 100 × 10⁹/L at birth or within 7 days, with most affected infants having counts less than 50 × 10⁹/L 1.
While maternal SLE is mentioned, neonatal thrombocytopenia from maternal ITP/SLE is actually rare - accounting for only 3% of all cases of thrombocytopenia at delivery, and severe thrombocytopenia with clinical hemorrhage in neonates is uncommon in maternal ITP 1.
NAIT is the most common cause of severe thrombocytopenia (<30 × 10⁹/L) in term newborns, occurring in approximately 1 in 1,000 live births 2, 3.
Why Not the Other Options
Vitamin K deficiency (Option A) is excluded because PT and aPTT would be prolonged, not normal 4. The isolated thrombocytopenia with normal coagulation factors is inconsistent with this diagnosis.
Hemophilia A (Option C) is excluded because it presents with prolonged aPTT and normal platelet counts, the opposite of this clinical picture 4.
Congenital platelet function defect (Option D) would not typically present with such severe thrombocytopenia; these disorders usually have normal or near-normal platelet counts with abnormal function 1.
Immediate Management Required
Administer IVIG 1 g/kg immediately for platelet counts <20,000/μL with clinical hemorrhage (umbilical stump bleeding), with repeat dosing if necessary 1, 5, 4.
Perform transcranial ultrasonography urgently given the platelet count <50,000/μL to assess for intracranial hemorrhage 1, 5, 4.
Avoid all intramuscular injections including vitamin K until platelet count improves 1, 5.
Monitor platelet counts serially as they typically nadir between days 2-5 after birth 1, 5, 4.
Diagnostic Confirmation
To confirm NAIT diagnosis, the following testing should be performed 1, 5:
- HPA genotyping from mother, neonate, and father to identify platelet antigen incompatibility
- Maternal serum alloantibody testing using two different serological methods (bead-based multiplex assays, MAIPA, or flow-based assays)
- Crossmatch with paternal platelets to detect alloantibodies to low-frequency antigens
Critical Pitfall
Do not assume this is neonatal ITP from maternal SLE without confirmatory testing - while the maternal history may suggest this, NAIT is far more likely given the severity of thrombocytopenia and clinical bleeding 1, 2. Neonatal thrombocytopenia from maternal SLE may persist for months and requires long-term monitoring, but the acute presentation with severe thrombocytopenia and bleeding is more consistent with NAIT 1, 4.