Flow cytometric determination of residual white blood cell levels in preserved samples from leukoreduced blood products

BACKGROUND: In preparation for a proposed consolidated testing service, Canadian Blood Services undertook the evaluation of a commercial test kit for the enumeration by flow cytometry of residual white blood cells (rWBCs) present in preserved samples recovered from leukoreduced (LR) blood and platelet products. STUDY DESIGN AND METHODS: The stability of preserved WBCs, the equivalency of WBCs used for spiking, test method precision, specificity, reliability, accuracy, and sensitivity were investigated. For comparative purposes, WBC counts were also determined by Nageotte as well as by flow cytometry. RESULTS: WBCs were stable up to 4 weeks at room temperature for all components by either method. Within methods, no differences were observed due to the source of WBC used for spiking purposes. By either method, test precision was acceptable (< 20% coefficient of variation) and of similar reliability at a target value of 10 +/- 5 WBCs per mu L. The flow cytometric method was shown to be more specific and accurate than the Nageotte method. Sensitivity by either method was 0.1 WBCs per mu L. On average, Nageotte counts were lower than those observed by flow cytometry. CONCLUSIONS: These results demonstrate that WBCs in WBC stabilizing solution-treated samples from LR blood components were stabilized up to 4 weeks at room temperature and that rWBC determinations made with a WBC enumeration kit by flow cytometry have the required precision, specificity, reliability, and accuracy in the relevant test range. This validated WBC stabilization and flow cytometric counting method is considered acceptable as part of a quality control program for leukoreduced blood products.