Maternal red blood cell alloimmunization prevalence in the United States.

Hemolytic disease of fetus and newborn (HDFN) is a life-threatening disease mediated by maternal alloimmunization to red blood cell (RBC) antigens. Studies of maternal alloimmunization prevalence in the United States (U.S.) lack national data. This study describes prevalence and trends in alloimmunization in pregnancy in the U.S. RBC antibodies (abs) were identified in a large, nationwide, commercial laboratory database from 2010-2021. The cohort comprised pregnancies for which the year of lab collection and patient's state of residence were available. Data were normalized based on U.S. Centers for Disease Control and Prevention estimates of live births and weighted by year and U.S. Census Division. Cochrane-Armitage tests assessed temporal trends of alloimmunization. Of 9,876,196 pregnancies, 1.5% (147,262) screened positive for RBC abs, corresponding to an estimated prevalence of 1,518/100,000 pregnancies. Of identified RBC abs, anti-D comprised 64.1% (586/100,000 pregnancies). Prevalence of other high-risk RBC abs for HDFN included anti-K (68/100,000) and anti-c (29/100,000). Incidence of all three high-risk abs increased from 2010-21 (all p<0.001). Among almost 10 million pregnancies in the US, comprising an estimated 14.4% of all pregnancies, 1.5% screened positive for RBC abs. Almost three-quarters (74.3%; 683/100,000) of RBC abs identified were high-risk for HDFN. Though prevalence of anti-D is difficult to interpret without the ability to distinguish alloimmunization from passive immunity, it remains problematic in HDFN, ranking second only to anti-K in critical titers. Given the sequelae of HDFN, new initiatives are required to reduce the incidence of alloimmunization in patients of reproductive potential.