Evaluation of red blood cell filterability test: influences of pore size, hematocrit level, and flow rate

To improve understanding of the basic principles of the red blood cell filtration test, we studied the influence of pore size, flow rate, and hematocrit level. Suspensions of washed red blood cells in Ringer's solution with hematocrit values of 5%, 10%, and 15% were pumped at flow rates of 0.42 and 0.82 ml/min through polycarbonate filters with pore sizes of 2.6 +/- 0.2, 4.5 +/- 0.6, and 6.9 +/- 0.8 micron (mean +/- SD). The initial pressure generated at the filter was normalized for pressure reading obtained with cell-free Ringer's solution at the same flow rate and used for the calculation of the relative resistance of one red blood cell in a pore. This measurement was independent of the flow rate and the hematocrit level, but it varied inversely with the pore size: 175.1 +/- 62.5, 5.2 +/- 1.2, and 2.4 +/- 0.8 for the 2.6, 4.5, and 6.9 micron pores, respectively. Plugging of filter pores as evidenced by progressive pressure rise at constant flow was a prominent feature for 2.6 micron filters, but was not significant for 6.9 micron filters. At the flow rates studied, less than 1% of the filtered red blood cells showed morphologic changes or underwent hemolysis. We found that 2.6 micron filters are most sensitive in detecting altered red blood cell filterability induced in vitro and occurring in vivo. These results may help to define an optimum filtration test for clinical investigations.