What is the principle of WST-8 assays?

WST-8 assay is a colorimetric assay used to determine viable cell numbers. It can be used for cell proliferation assays as well as cytotoxicity assays. 

In these assays, WST-8 (2-(2-methoxy-4-nitrophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H tetrazolium, monosodium salt) is used as a chromogenic indicator for cell viability. The assay is based on viable cells reducing the slightly yellow WST-8 to produce an orange-colored formazan dye. The amount of formazan dye produced is directly proportional to the number of viable cells present.