Characterization of a population with pregnancy potential and anti-K alloimmunization in the United States

Maternal alloantibody formation against fetal blood cell antigens can lead to hemolytic disease of the fetus and newborn (HDFN), and anti-K alloimmunization has the potential to result in severe HDFN. Despite the benefits of prophylactic K-matched red blood cell transfusions demonstrated in other high-income countries, the United States lacks such a national policy for patients with pregnancy potential (PWPP). This study aims to characterize a population of PWPP impacted by anti-K alloimmunization in a large US academic medical system.

Retrospective review was undertaken to identify all cases of anti-K alloimmunization among PWPP between August 1, 2021, and July 31, 2024, in the University of California, San Francisco (UCSF) network. Demographic, clinical, and transfusion data were collected from electronic medical records and the blood bank information system and reviewed by an interdisciplinary physician team.

Over the 3-year study period, 150 anti-K results were reported for 69 unique PWPPs. The median age was 38 years (range: 11-50). Most PWPPs were not chronically transfused (32/39; 82% with ≤6 lifetime RBC units) and lacked identifiable alloimmunization-associated comorbidities (45/69; 65%). Thirty-seven (63%) of the PWPPs had red blood cell transfusions administered only at facilities outside of the UCSF network. Five of fifteen (33%) pregnancies following incident anti-K alloantibody identification resulted in healthy live births.

Our results support reconsideration of a national policy of prophylactic K-matching or K-negative transfusion for people with pregnancy potential as a way to prevent further alloimmunization and improve clinical care in this population.