Antenatal screening for human platelet antigen-1a: results of a prospective study at a large maternity hospital in Ireland

Objective Fetomaternal mismatch for human platelet antigen (HPA)-1a accounts for approximately 85% of cases of neonatal alloimmune thrombocytopenia. The purpose of the study was to determine the prevalence of the HPA-1a negative platelet phenotype in a cohort of pregnant women in Ireland, the rate of alloimmunisation to HPA-1a in HPA-1 mismatched pregnancies and the associated incidence of neonatal alloimmume thrombocytopenia. Design A prospective case-control study. Setting The antenatal clinics of a large maternity teaching hospital. Population or sample Pregnant women, regardless of parity, presenting at the antenatal clinics. Methods An enzyme-linked immunosorbent assay (ELISA) designed for simultaneous HPA-1a typing and antibody detection was used. Further analysis for HPA-1a alloantibodies was performed using commercial ELISA's (GTI PakPlus and Pak1) and the monoclonal antibody immobilisation of platelet antigens assay. Confirmation of serological typing was by the polymerase chain reaction technique using sequence-specific primers (PCR-SSP). Main outcome measures The presence of the HPA-1a negative phenotype and its association with the development of maternal anti-HPA-1a and infant thrombocytopenia. Results Eighty-four of 4090 consecutive women enrolled in the study tested positive for HPA-1a in the screening ELISA. Confirmatory genotyping was performed on 67 women (representing 80% of the cohort), and 54 women (representing 3272 non-selected pregnancies), were homozygous for the HPA-1b allele (1.7%). Three of 34 (9%) women who delivered HPA-1a positive babies had detectable anti-HPA-la and all three babies had neonatal alloimmune thrombocytopenia, for an overall incidence of 1:1100 non-selected pregnancies. Conclusions The observed prevalence of 1.7% for the HPA-1a negative platelet phenotype is as expected from studies in other countries. While we have demonstrated the practicability of antenatal HPA-1a screening, further research is warranted to investigate maternal parameters predictive of severe fetal thrombocytopenia in HPA-1a alloimmunised pregnancies.