False-positive result of sterility testing after a supposed adverse transfusion reaction

We report on the case of a woman who suffered from chills and fever when she was transfused with two units of red blood cell concentrate. Immunohaematological evaluation of the adverse reaction was inconspicuous, as well as sterility testing of one unit, but bacteriological evaluation of the second unit showed growth of Stenotrophomonas maltophilia. The sample for sterility testing of this unit had been drawn from the distal end of the transfusion device that was still connected to the blood bag. Later, two samples were drawn from the blood bag itself and were found to be sterile, whereas another sample drawn from the distal end of the transfusion device also showed growth of S. maltophilia. We conclude that reflux of the patient's blood into the transfusion device has caused a false-positive result of sterility testing of the second unit when the sample was drawn from the distal end of the device.