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.