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.