Population Prevalence and Incident HPA ‐1a Alloimmunisation in Pregnancy: A Prospective, Multi‐Ethnic, Natural History Study

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Abstract
Objective: Foetal‐neonatal alloimmune thrombocytopaenia (FNAIT) is a rare disease, most commonly caused by maternal alloantibodies to human platelet antigen (HPA)‐1a that can result in severe bleeding. The present study was designed to determine the frequency of higher risk for HPA‐1a alloimmunisation and development of FNAIT among a racially and ethnically diverse cohort of pregnant women. Design: Longitudinal, prospective, non‐interventional, natural history study. Setting: Conducted across 28 sites in North America and Europe. Population: Pregnant women (≥ 18 years) with no history of FNAIT. Methods: Blood samples were collected at 10–14 weeks' gestation and a sequential screening algorithm applied: maternal HPA‐1a/1b genotyping, human leukocyte antigen [HLA]‐DRB3*01:01 allele status, anti‐HPA‐1a antibody status and foetal HPA‐1 genotype. Main Outcome Measures: Number of women at higher risk for HPA‐1a alloimmunisation (i.e., those who are HPA‐1b/b, HLA‐DRB3*01:01 positive, anti‐HPA‐1a antibody negative and carrying an HPA‐1a‐positive foetus) that alloimmunise. Results: Of the 14 114 participants screened, 1.7% were HPA‐1b/1b genotype, which varied according to race and geography. Of 24 women at higher risk for FNAIT, five were lost to follow‐up with no post‐partum test for alloimmunisation. At Week 10 post‐partum, 2/19 higher risk participants (11%; 95% confidence interval [CI]: 1.3%, 33.1%) demonstrated anti‐HPA‐1a antibodies, while 17 remained negative. Pre‐existing anti‐HPA‐1a antibodies were identified in 18 other HPA‐1bb, DRB3*01:01 positive women at screening. Conclusions: This prospective study confirms the rarity of a population at higher risk for HPA‐1a alloimmunisation and FNAIT, describes the risk among Black and Asian women, and establishes a rigorous methodology to assess for risk of HPA‐1a immunisation.

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