R-FACT study

Risk factors for alloimmunization after red blood cell transfusions


Individuals exposed to red blood cell (RBC) alloantigens through transfusion, pregnancy or transplantation may produce antibodies against the alloantigens expressed by RBC. Although the incidence of these events is debated and ranges between percentages of 1-6% in single transfused patients and up to 30% in poly-transfused patients, they can pose serious clinical problems such as delayed hemolytic reactions as well as logistic problems e.g. to obtain timely and properly matched transfusion blood for patients in which new alloantibodies are detected. Extensive matching of donors for those patients who are defined to have a high alloimmunization risk would be a feasible step forward.

It is already known that the risk of a recipient to develop antibodies depends on dose and route of administration as well as the immunogenicity of the antigen. Moreover, it is believed that genetic and particular clinical conditions (e.g. underlying diseases, medication use, state of immunity) may contribute to the risk of immunization. Those latter factors however are ill defined. 

Research objective 

The aim of the project is to examine the association between clinical, environmental and genetic related characteristics of the recipient of erythrocyte transfusions and the risk of development of antibodies against RBC alloantigens exposed to during this transfusion episode.


Currently a retrospective matched case-cohort study is performed in several large red blood cell using hospitals in The Netherlands. The study cohort comprises consecutive red blood cell transfused patients at the study centers from January 2005 onwards.
Cases are defined as first time ever irregular red blood cell antibody formers with no prior history of red blood cell transfusions and alloimmunization before the study period.
Controls are all consecutive transfused patients who had received their first and subsequent red blood transfusions at the study center with no prior history of red blood cell transfusions and alloimmunization.
Matching. For every case patient, two control patients are randomly sampled from the source population. These controls are matched to the case on the cumulative number of red cell units received up until alloantibody formation. To account for inter-hospital differences nationwide, we will also match the cases and controls on the site / study center.
Implicated period. This period is defined as the period (in days) between the date of the Nth transfusion immediately preceding the first positive alloantibody screen and 30 days earlier. To optimize the likelihood that case patients are primary immunizations and not secondary booster immunizations, a mimimal ‘lag period’ of 7 days between the first positive screening and the preceding implicated transfusion is required.  A similar implicated period around the Nth is defined for controls.

Data acquirement, measurements and handling 

Transfusion cohort data is acquired from the hospital blood transfusion services and onsite patient records. After selection of control and case patients, potential clinical risk factors and conditions during the implicated period will be scored using the patient medical record. Secondly, environmental and lifestyle factors e.g. vaccination status, previous pregnancies, level of education and current profession will be inventoried via a patient information questionnaire. Thirdly, blood sampling of consenting patients is carried out to assess genetic risk factors, HLA typing and immune activity markers. 

Statistical analysis 

Logistic regression models will be used to assess the association between the risk to develop antibodies and potential risk factors, adjusted for other risk factors and for the number of exposures to the antigen.


Comparing immunized and non-immunized patients with a similar transfusion history will generate relative risk and relative protective factors related to the formation of alloantibodies. By identifying these risk and protective factors, we hope to become better capable in determining those transfusion recipients who should be more extensively matched for red blood cell products. Furthermore, we hope that this study brings us a better understanding of the mechanisms underlying the development of alloantibodies to erythrocyte transfusion.

A detailed protocol information can be found here.