What is the principle of cell viability assays?

A cell viability assay is carried out based on the ratio of viable and dead cells. The principle of these assays is to assess the physical and physiological health of cells in response to extracellular stimuli, or chemical agents. For example, these assays are utilized for evaluating in vitro drug effects in cell-mediated cytotoxicity assays used for measuring cell proliferation. MTT assays are commonly used to measure cellular metabolic activity as an indicator of proliferation, cytotoxicity, and cell viability. Cell viability assays based on membrane integrity assess whether cell membranes are intact or damaged. Propidium iodide is a commonly used dye that is impermeable to viable cells but readily enters cells with damaged membranes. Additionally, cell viability assays based on nucleic acid content involve the quantification of DNA or RNA in cells. Hoechst dyes bind to nucleic acids and allow for the approximation of cell viability by measuring fluorescence activity.