The NBT (nitroblue tetrazolium) reduction assay is used to -

Nitro blue tetrazolium is used in a diagnostic test, paicularly for chronic granulomatous disease and other diseases of phagocyte function. When there is an NADPH oxidase defect, the phagocyte is unable to make reactive oxygen species or radicals required for bacterial killing. As a result, bacteria may thrive within the phagocyte. The higher the blue score, the better the cell is at producing reactive oxygen species. Ref: Ananthanarayan & Panikers textbook of microbiology 9th edition pg.no. : 175