The first reported case of neonatal alloimmune thrombocytopenia due to low-frequency human platelet antigen-6b antibodies in the United Kingdom

BACKGROUNDNeonatal alloimmune thrombocytopenia (NAIT) is a potentially serious clinical condition caused by maternal alloantibodies directed to human platelet antigens (HPA), inherited from the father and expressed on fetal/neonatal platelets. We report a case of an otherwise well, full term child, with a profound thrombocytopenia (33 x 109/L). There was no bleeding or obvious explanation for the low platelet count. Samples were sent for the investigation of NAIT. METHODSerological investigations were performed on maternal serum taken at day (D)+4 and D+78. The platelet immunofluorescence test (PIFT) and monoclonal antibody immobilization of platelet antigens (MAIPA) assays were performed with a panel of HPA typed donor platelets and against paternal platelets in a crossmatch. HPA 1-6, -9 and -15 and HLA genotyping was performed by in-house PCR-sequence based typing (SBT) and next generation sequencing (NGS). RESULTSHPA antibody screening of D+4 maternal serum indicated that platelet-specific antibodies were absent. HPA genotyping of the father and child revealed the presence of the low frequency HPA antigen (LFHPA), HPA-6b, which was absent in the mother. Maternal samples were crossmatched against paternal platelets and were positive by PIFT and glycoprotein (GP) IIb/IIIa and HLA class I in the MAIPA assay. The infant required no platelet transfusion support as the thrombocytopenia resolved spontaneously. DISCUSSIONWe conclude that the positive crossmatch reaction was due to anti-HPA-6b alloantibodies. This case further emphasizes the importance of platelet crossmatching and HPA genotyping of LFHPA in cases where there is a high clinical suspicion of NAIT but initial screening is negative.