Rate of RhD‐alloimmunization after the transfusion of multiple RhD‐positive primary red blood cell‐containing products

Introduction Early transfusion reduces mortality in bleeding patients. In this setting, RhD‐positive blood products might be transfused. This study determined the association between the RhD‐alloimmunization rate and the number of RhD‐positive products transfused. Methods RhD‐negative patients between 13 and 50 years who were transfused with ≥1 RhD‐positive red blood cell (RBC) or whole blood units between January 1, 2000 and December 31, 2019 in a healthcare network were identified. Study patients had to have had at least one antibody detection test performed ≥14 days after the index RhD‐positive transfusion and not receive RhIg. Patients were stratified into groups that received 1, 2, 3–5, 6–10, 11–20, and >20 RhD‐positive transfusions and the RhD‐alloimmunization rate was determined for each group. Results There were 335 patients included; 52/335 (15.5%) were females. Overall, there were 117/335 (34.9%, CI: 29.8%–40.3%) recipients who became RhD‐alloimmunized. There was no significant dosage effect in the RhD‐alloimmunization rates as the exposure to RhD‐positive units increased from one RhD‐positive unit to more than 20 RhD‐positive units (p = .270 for non‐parametric trend test). In an exploratory analysis, patients who received 100% of their RhD‐positive transfusions within 72 h of the index transfusion had a significantly higher rate of RhD‐alloimmunization compared to those who were transfused over a longer period of time (42.3% vs. 21.4%, respectively; p = .001). Conclusion These results suggest that there may not be an increased RhD‐alloimmunization risk with transfusing multiple RhD‐positive units after one RhD‐positive unit has been transfused. These findings need confirmation in larger studies.