Basic principles for evaluation of less deformable erythrocyte subpopulations with the Microfiltrometer

The Microfiltrometer consists of a filtration system for diluted erythrocyte suspensions, through a filter containing 30 cylindrical micropores, 5 microm in diameter, under the influence of a driving pressure. A feeding sinusoidal alternating current of 40 kHz, 300 microA is delivered to the filter. The change in impedance is collected for each temporary flow of erythrocytes through a given micropore. Two main parameters are measured for individually explored erythrocytes: the entry time tau in the micropore and the maximal variation of impedance deltaZ occurring for the transitory flow. The slope deltaZ/tau defines the velocity of pore blockage. A "Microfiltrometer Deformability Index" (MDI) is established by using this slope. When MDI > or = 1, the erythrocyte is considered to be deformable and, conversely, when MDI < 1, the erythrocyte is considered to be undeformable. Using this procedure, less than 2% undeformable erythrocytes in healthy blood samples are identified, with a specificity of 99% and a sensitivity of 97.5%.