A flow cytometric technique to accurately measure post-filtration white blood cell counts

Leukocyte-depleted blood products are being used with increasing frequency in hopes of preventing or delaying platelet alloimmunization. However, accurately monitoring the efficiency of leukocyte (WBC) removal is a difficult problem because electronic cell counters are not accurate at very low WBC numbers and hemocytometer counts are tedious and time consuming. A simple flow cytometric technique was developed which accurately and rapidly measures extremely low WBC counts. Using a propidium iodide solution which causes DNA to fluoresce, the residual WBC count was measured in 42 units of blood products after leukocyte depletion using a commercial filter. There was a significant correlation with simultaneous hemocytometer counts, r = 0.672, but residual WBC could be identified in every unit using the flow cytometer, whereas in 19% of the units no WBC were seen using the hemocytometer. Extremely low counts, as low as a single WBC/4 microliters could be reproducibly obtained. In addition, serial dilution studies yielded a correlation coefficient of 0.997. Because most clinical laboratories now have access to flow cytometers, this technique can be widely used.