The ATP Cell Viability Assay measures cell viability by detecting cellular ATP levels. This assay utilizes the catalytic reaction of luciferase with D-luciferin, which consumes ATP to produce bioluminescence. The reaction results in a luminescent signal that is directly proportional to the amount of ATP in cells. Because ATP is a marker of healthy, metabolically active cells, it can be used as an indicator of cell viability. When cells die or undergo stress, their ATP levels go down, which will result in a lower luminescent signal. The detection limit of this assay is 1 nM.