MULTICENTER EVALUATION OF THE 3-PERCENT PARAFORMALDEHYDE METHOD FOR WHITE CELL COUNTING IN LEUKOCYTE-REDUCED RED-BLOOD-CELLS

Background/Aim: The 3% paraformaldehyde (PFA) method is a simple techn ique for counting residual white blood cells (WBC) in leukocyte-deplet ed red blood cells (RBC). Preliminary data suggested that its sensitiv ity is at least equal to PCR and flow cytometry. We report the results of a multicenter study conducted by the BEST Working Party to determi ne precision and accuracy of the 3% PFA method. Study Design: In the 7 participating laboratories, 5 sets of samples containing nominal conc entrations of 200, 100, 50, and 10 WBC/ml were prepared by diluting wh ole blood into 'WBC-free' RBC. Ten milliliters of each sample were pro cessed using the 3% PFA method, which is based on erythrocyte lysis an d WBC concentration into 5% of the original sample volume; a Nageotte chamber is used to count concentrated WBC. Results: The precision of t he technique varied according to the nominal concentration, ranging fr om a CV of 12% at 200 WBC/ml to 57% at 10 WBC/ml. The technique measur ed fewer than the nominal WBC concentrations (mean of all laboratories , -12.4%); underestimation was probably due to cell loss during sample manipulation. Overall accuracy was however acceptable, because statis tical considerations establish that the actual WBC concentration would unlikely exceed 2 times the estimated count. Conclusions: The 3% PFA method is suitable for the enumeration of residual WBC at concentratio ns greater than or equal to 50/ml. It represents a useful tool for eva luation of high performance filters by reference laboratories.