Abstract
Allogeneic white blood cells in red blood cell units and platelet concentrates have been involved in a variety of transfusion reactions, including HLA alloimmunization,the transmission of cell-associated viruses, and immunosuppressive effects. These observations and the emergence of the new variant Creutzfeldt-Jakob disease in the United Kingdom have led the Paul Ehrlich Institute to mandate for Germany ‘universal’ leukocyte reduction for all cellular blood components in 2001. This decision has raised the need for a simple, stable and accurate routine method for the enumeration of residual leukocytes. An important limitation of the traditionally used Nageotte chamber and of the standard hematology analyzer is their insensitivity with respect to the expected low leukocyte contamination. The detection of these ‘rare events’presents a technical challenge. Flow cytometry is an ideal technology in phenotyping and quantifying cellular events, and nowadays the implementation of single-platform assays and standard gating protocols allows for the accurate determination of residual leukocytes in blood components by DNA staining with acceptable reproducibility and high throughput.