Cell viability can be assessed based on the integrity of plasma membrane: the living cells have intact plasma membranes whereas membranes of dead cells are broken. When a cell is exposed to a low voltage field, the electric current cannot go though the intact membrane, which is an electric insulator, if it is viable. Otherwise, as the cellular membrane is broken, electric field can go through the injured cell as there are pores on their membrane. For a normal cell, its size cannot be smaller than its nuclear size, which is the criterion to distinguish between living cells and dead cells.

As a result, when cells in an electrolyte or a particular buffer, they are aligned one by one to a precision measuring pore and exposed to the electric field, each of their information can be captured and the culture condition, including its concentration, viability and volume, can be analyzed. For example, when the living cells get greater volume and pass though the current flow, a greater pulse, in amp-1, can be generated and then amplified. As the cell size is related to the cell volume, a cell size profile in cell population can be produced in terms of pulse height. Since the cells are scanned at such high frequency, a precise result and a high resolution can be produced.

These results from each cell are cumulated and assigned in a calibrated multi-channel analyser with over 500,000 channels. So, for the CASY technology, as the cell flow cytometry, it can present data of each cell as a cell size distribution graph, which has 2 variables, the change in cell volume and that in cell viability. The materials passing though the apparatus can be gated. For the newly invented equipments, they have an automatically lower threshold at 7 um, which can exclude small particles and cell debris in the cell culture. At the same time, there will be an upper threshold to prevent from cell aggregation for counting. However, some of users may set upper threshold to unlimited for cell size. Since the cell size of each cell type is varied, before doing gating, it should ensure the correct cell size is included during the cell size related experiment.

More information can be obtained from:

http://en.wikipedia.org/wiki/CASY_cell_counting_technology