Electrical cell counting process characterization in a microfluidic impedance cytometer

Umer Hassan, Rashid Bashir

Particle counting in microfluidic devices with coulter principle finds many applications in health and medicine. Cell enumeration using microfluidic particle counters is fast and requires small volumes of sample, and is being used for disease diagnostics in humans and animals. A complete characterization of the cell counting process is critical for accurate cell counting especially in complex systems with samples of heterogeneous population interacting with different reagents in a microfluidic device. In this paper, we have characterized the electrical cell counting process using a microfluidic impedance cytometer. Erythrocytes were lysed on-chip from whole blood and the lysing was quenched to preserve leukocytes which subsequently pass through a 15 μm × 15 μm measurement channel used to electrically count the cells. We show that cell counting over time is a non-homogeneous Poisson process and that the electrical cell counts over time show the log-normal distribution, whose skewness can be attributed to diffusion of cells in the buffer that is used to meter the blood. We further found that the heterogeneous cell population (i.e. different cell types) shows different diffusion characteristics based on the cell size. Lymphocytes spatially diffuse more as compared to granulocytes and monocytes. The time difference between the cell occurrences follows an exponential distribution and when plotted over time verifies the cell diffusion characteristics. We also characterized the probability of occurrence of more than one cell at the counter within specified time intervals using Poisson counting statistics. For high cell concentration samples, we also derived the required sample dilution based on our particle counting characterization. Buffer characterization by considering the size based particle diffusion and estimating the required dilution are critical parameters for accurate counting results.