Routine quantitation of white cells as low as 0.001 per microL in platelet components

A method is described for routine quantitation of very low numbers of white cells (WBCs) in platelet components, using flow cytometry and dual nucleic acid stains. The WBC quantitation is based on the detection of nucleated cells labeled with propidium iodide and thiazole orange, with chicken red cells added as an internal counting standard. Platelet concentrates containing 0.001 to 1500 WBCs per microL (2 x 10(2)-3 x 10(8) WBCs/component) and 600 to 2800 x 10(3) platelets per microL were analyzed for the number of contaminating WBCs. The method was found to be linear throughout the 7 log10 range and to have sufficient reproducibility and sensitivity for routine analysis of WBC-reduced platelet concentrates.