Used in microbiology to detect the presence of viable microorganisms, especially in sterility testing of pharmaceutical and medical products. It helps differentiate live bacteria from dead ones by penetrating cell membranes and reacting with dehydrogenase enzymes in metabolically active cells. When reduced, it forms a red, insoluble compound called formazan, which appears as a visible red color or precipitate. This color change indicates microbial metabolic activity, making it a valuable tool in quality control for sterile manufacturing processes. Also used in semen analysis to assess sperm viability, where live sperm remain white and dead sperm turn red under the microscope.