Coincidence error during measurement of cellular osmotic properties by the electrical sensing zone method

Coincidence is a phenomenon that occurs when electrical sensing zone instruments fail to temporally resolve two or more particles passing through the sensing zone in close proximity. We have investigated the potential for coincidence errors to confound the estimation of cellular osmotic properties. A mathematical model was developed to predict the magnitude of coincidence error as a function of the instrument sensing volume, the cell concentration, and the degree of cell aggregation. The model was validated in a representative instrument (Coulter counter model Z2, with a 100-micron aperture tube), for which the sensing volume was estimated to be approximately 2 nL. Furthermore, we measured the degree of cell aggregation in trypsinized cultures of MIN6 cells, and used these data to estimate the effect of coincidence on MIN6 cell volume measurements. Finally, we simulated water transport experiments, and determined the sensitivity of estimates of the osmotically inactive volume and the membrane water permeability to coincidence error. Our results revealed that coincidence can result in significant overestimation of these two parameter values for high cell concentrations and for suspensions containing cell aggregates.