An unusual case of severe neonatal thrombocytopenia.

Neonatal alloimmune thrombocytopenia (NAIT) occurs when maternal antibodies are directed against antigens on fetal and paternal but not maternal platelets. Most cases of NAIT arise when platelets of the father (and the fetus) express the human platelet antigen (HPA)1a and the mother's platelets expresses HPA-1b. A female patient presented with congenital severe thrombocytopenia and received 4 platelet transfusions, on days 2, 7, 16, and 28. This appeared to be a case of NAIT; however, extensive serologic evaluation by 2 reference laboratories failed to reveal the offending platelet antigen. Consistent with NAIT, the condition resolved by 6 weeks of age. By patient day of life 42, the platelet count had increased without additional need for transfusions, and by 16 weeks, the patient had a completely normal platelet count of 437,000/microL. The patient's platelet count remains normal at a 2-year follow-up. This case is reported as an instructive atypical case of NAIT, in which the relevant platelet antigen could not be identified.