Risk of HPA-1a-immunization in HPA-1a-negative women after giving birth to an HPA-1a-positive child.

Abstract

BACKGROUNDFetal/neonatal alloimmune thrombocytopenia (FNAIT) is the platelet counterpart of hemolytic disease of the newborn. Most severe cases of FNAIT are caused by antibodies against human platelet antigen‐1a (HPA‐1a). HPA‐1a–negative women giving birth to an HPA‐1a–positive child are at risk of becoming HPA‐1a–immunized, particularly women who are HLA‐DRB3*01:01–positive. The aim of the study was to estimate the risk of HPA‐1a–immunization in both HPA‐1a–negative/HLA‐DRB3*01:01–positive and HPA‐1a–negative/HLA‐DRB3*01:01–negative women after delivery of an HPA‐1a–positive child.STUDY DESIGN AND METHODSA literature search was conducted, which identified 10 prospective FNAIT studies. The risk of becoming HPA‐1a–immunized postpartum was calculated by Bayes' theorem. The results of HLA‐DRB3/4/5 typing of 212,472 European Caucasians from the National Marrow Donor Program were used as estimate of the frequency of the HLA‐DRB3*01:01 allele.RESULTSIn HPA‐1a–negative/HLA‐DRB3*01:01–positive women, the risk of HPA‐1a–immunization after delivery of an HPA‐1a–positive child was estimated to 12.7% (95% confidence interval, 8.6%–16.8%) as compared to 0.5% (95% confidence interval, 0.1%–0.9%) in women who were HPA‐1a–negative/HLA‐DRB3*01:01–negative. Potential differences between nulliparous and multiparous and the role of one versus two doses of HLA‐DRB3*01:01 could not be determined.CONCLUSIONIn HPA‐1a–negative/HLA‐DRB3*01:01–positive women, the risk of HPA‐1a–immunization is 12.7% after delivery of an HPA‐1a–positive child, which is 25 times higher than in HPA‐1a–negative/HLA‐DRB3*01:01–negative women. Thus, the risk of HPA‐1a–immunization in high‐risk pregnancies is in the same range as the risk of RhD immunization in RhD‐negative women after delivery of a RhD‐positive child without RhD prophylaxis.