What are the most common cell viability assays?

Cell viability assays typically use different types of markers to detect metabolically active cells. Five of the most common cell viability assays include:

1. Tetrazolium Reduction Assays – In these assays, different types of tetrazolium compounds may be used to detect viable cells. Positively charged tetrazolium compounds (MTT) readily penetrate viable cells, which convert MTT into a purple formazan product. 
2. ATP Cell Viability Assays – These assays are based on the premise that only viable cells are capable of synthesizing ATP. 
3. Resazurin Reduction Cell Viability Assays – In these assays, Resazurin, a dark blue cell-permeable indicator dye is used to measure cell viability. 
4. Live-Cell Protease Viability Assays – These assays are based on the premise that live-cell protease activity stops completely after the death of the cell. 
5. Real-time Cell Viability Assays – These assays are based on the premise that only viable cells with active metabolism are capable of reducing a prosubstrate that is added to the cuture medium into a substrate for luciferase.