We report three cases of massive ex vivo coagulation in a leukocyte depleted red cell concentrate during transfusion. Molecular blood typing indicated that patient blood was present in the filter housing of the infusion set and/or in the blood bag itself. Together with the history of events, this suggested that the observed clotting was caused by backflow of patient blood. Therefore, we recommend maintaining the blood bag above heart level during the entire transfusion procedure, until the infusion set is removed.

(BELG J HEMATOL 2017;8(5):195–7)