Nitro blue tetrazolium is a chemical compound composed of two tetrazole moieties.

NBT serves as the oxidant and BCIP is the AP-substrate (and gives also dark blue dye).

One application is in activity stains in gel electrophoresis, such as with the mitochondrial electron transport chain complexes.

When there is an NADPH oxidase defect, the phagocyte is unable to make reactive oxygen species or radicals required for bacterial killing.

The higher the blue score, the better the cell is at producing reactive oxygen species.