Large-scale HPA-1 epidemiological study in scotland: an up-date

Abstract

Neonatal alloimmune thrombocytopenia (NAIT) in the Caucasian population is mainly due to a mismatch in the HPA-1 marker between mother and foetus. About 10% of pregnant HPA-1b women develop anti-HPA-1a antibodies (ab) which may lead to NAIT with possible intracranial haemorrhage and death or life long morbidity. The ab development is significantly correlated with HLA-DRB3*0101. Although the frequency of homozygous HPA-1b varies between 1·5 and 4·5%, the frequency of HPA-1 NAIT detection in our laboratories was very low suggesting either that the epidemiology was different in Scotland or that the disease was being under-reported. In August 1999 we began a two year study of HPA-1 NAIT involving 25 000 pregnant women in Scotland. The aim of the study is to determine the frequency of HPA-1b homozygosity, monitor ab levels during pregnancy, and assess the cost effectiveness of routine screening across Scotland. The HPA-1 antigen and antibody assays consist of internationally validated ELISAs (Bessos et al. Transfusion, 38, 73S, 1998). The laboratory and clinical data are being collated using a Microsoft Access database. Of 11,500women screened to date in 3 Scottish regions, 216 are HPA-1b homozygous (1·9%): 136/6841(2%) in Edinburgh; 67/3973 (1·69%) in Aberdeen; and 13/686 (1·9%) in Glasgow. So far, 11/145 (about 8%) consented HPA-1b women are ab positive, with ab titres ranging from 1 : 2 to 1 : 256, but staying low or decreasing during pregnancy. Two have had previously affected babies and are being managed according to current guidelines. One woman with no previously affected children but with an ab titre declining from 1 : 256 to 1 : 32 during pregnancy, delivered recently a severely thrombocytopenic baby whose platelet counts rose to 137 × 109/L 3 days later. The mother's ab titre two weeks postdelivery rose back to 1 : 128. Due to observed interregional differences, our results caution prudence in the interpretation of regional vs. national epidemiological studies. Moreover, the overall HPA-1b frequency in Scotland appears to be considerably higher than earlier indications, suggesting that NAIT may be under-reported.