Background Patients receiving red‐blood‐cells may form antibodies against the alloantigens expressed by red‐blood‐cells, with the risk of serious morbidity and the need for extensive phenotype‐matching in subsequent transfusions. The incidence of alloimmunization is considered variable for specific patient groups and for first time antibody formation. We therefore studied the cumulative incidence of the first formed alloantibody as a function of red‐blood‐cells exposure.
Methods We performed a new‐user cohort among all previously non‐transfused non‐alloimmunized patients that received non‐extended matched (ABO and RhD) red‐blood‐cells transfusions from January 2005 to December 2009 in our university medical centre. Alloimmunization incidences were estimated by Kaplan–Meier survival‐analysis.
Results A total of 3002 previously non‐transfused patients received 31 103 red‐blood‐cell units. A first time alloantibody forming event was experienced by 54 (1·8%) patients. The cumulative incidence of alloimmunization was 1·0% at 5 units, 2·4% at 10 units, 3·4% at 20 units and 6·5% at 40 units of red‐blood‐cells transfused.
Conclusion The risk to develop a first red‐blood‐cells alloantibody increases up to the 40th transfusion and is similar for men and women. More data are needed to examine the risk after 40th transfusion.