White cell‐reduced red cells prepared by filtration: a critical evaluation of current filters and methods for counting residual white cells

White cell (WBC) reduction, red cell (RBC) recovery, and filtration time were determined in 1‐day‐old standard and buffy coat‐depleted RBCs filtered in the laboratory through six commercial filters for WBC reduction. Residual WBCs were counted with a Burker chamber (BC), with a Nageotte chamber (NC), and by flow cytometry (FC). Results show that BC counts were 0 in several cases in which WBCs were detected with NC and FC, which indicated that the traditional BC method is too insensitive in use with currently available filters. Calibration curves performed by FC and with NC with samples containing known concentrations of WBCs from 1000 to 1 per microL showed that both FC and NC detected, on average, 67 percent of WBCs present in the samples (efficiency). However, the efficiency of FC showed small variability (61–70%) at different WBC levels, whereas the variability with NC was large (39–91%). This greater variability prevented the correction of NC counts by using a single factor and indicated difficulty in NC standardization. Therefore, because our main aim was to compare different filters rather than to define absolute levels of WBC contamination, uncorrected FC and NC counts were chosen to be reported. True WBC counts per unit should not exceed values that can be obtained by dividing uncorrected counts by the lowest efficiencies (61% for FC and 39% for NC). Uncorrected NC and FC counts were below 2 × 106 per unit in all units processed through three of the filters and below 5 × 106 per unit in all units processed through the other three. Removal of the buffy coat from RBCs before filtration generally but not invariably caused slightly higher WBC removals. Median RBC recovery in CPD RBCs containing the buffy coat, filtered through four of the filters, was greater than 90 percent, which was significantly higher than that shown by the other two filters. Buffy coat removal prior to filtration involved an additional median loss of 12.4 percent of RBCs. Median filtration time ranged from 5 to 23 minutes per unit. Although NC appears to be less expensive than FC, its performance in experimental and routine conditions requires further evaluation and thorough standardization. Because of the continuous development of more effective filters, more sensitive methods for WBC counting in WBC‐reduced blood components are needed.