A study recently published in BJOG: An International Journal of Obstetrics & Gynaecology highlighted how uncommon it is for women to be at high risk for HPAHuman platelet antigen Proteins found on the surface of platelets. Incompatibility between maternal and fetal human platelet antigens can trigger FNAIT.-1a alloimmunizationAlloimmunization The process by which the mother becomes sensitized to fetal platelet antigens and begins producing alloantibodies. This is what causes FNAIT., an immune response that can lead to fetal and neonatal alloimmune thrombocytopenia (FNAIT).
The researchers also found that Black and Asian women had a lower prevalence of risk factors for alloimmunization.
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Among 14,114 women screened, researchers found only two new alloimmunizations and no cases of FNAIT. They collected blood samples at 10 to 14 weeks’ gestation and used a series of tests to identify women at increased risk. The first step was to identify those with the HPA-1b/1b genotype, meaning they lacked the HPA-1a antigen and could therefore develop antibodies against it during pregnancy.
Overall, 1.7% of participants had the HPA-1b/1b genotype, which means they were HPA-1a negative and potentially at risk for alloimmunization. The prevalence varied by race, affecting 2.0% of White women, 1.1% of Black women and 0.7% of Asian women.
“Black women in Europe and North America have a lower — but still significant — risk of HPA-1a alloimmunisation and FNAIT compared with White women,” the authors wrote.
The prevalence was even lower among Asian participants. Although 11 Asian women had the HPA-1b/1b genotype, none also carried HLA-DRB3*01:01, a genetic variant associated with increased risk of HPA-1a alloimmunization. As a result, none met the study’s criteria for higher risk.
Read more about FNAIT causes and risk factors
After the full screening process, only 24 women, or 0.17% of the entire cohort, met all the criteria for higher risk. In addition to being HPA-1a negative, they carried the HLA-DRB3*01:01 allele, had no pre-existing anti-HPA-1a antibodies and were carrying an HPA-1a-positive fetus.
Five were lost to follow-up. Of the remaining 19 women, two developed anti-HPA-1a antibodies by 10 weeks postpartum. Both delivered full-term infants with normal platelet counts.
The authors noted that the relatively small number of women identified as high risk limited more detailed comparisons across racial and ethnic groups. The study did not assess maternal-fetal mismatches involving platelet antigens other than HPA-1a.
Still, the researchers said the study provides a screening approach that could be used in future research and offers a more diverse estimate of HPA-1a alloimmunization risk than previous studies, which largely focused on Northern European populations.
