Cell viability is measured as the percentage of living, healthy cells in a population. Cell viability tests are used to determine the general health status of cells, to optimize cell culture or experimental conditions, and to measure cell survival after treatment with tested compounds (e.g. in the drug screening process). Apoptosis tests enable the detection and quantification of cell events associated with the programmed cell death, including caspase activation, exposure of phosphatidylserine (PS) on the cell surface, and DNA fragmentation.