Nitroblue tetrazolium (NBT, CAS 298-83-9) is a yellow, water-soluble, heterocyclic organic compound. It is a ditetrazolium salt that functions as a redox indicator and chromogenic substrate. Its chemical structure features two tetrazolium rings linked by a biphenyl group. The primary application of NBT is in biochemical and clinical diagnostics. It is widely used as a substrate for the detection of alkaline phosphatase (AP) and other enzymes in techniques such as ELISA, Western blotting, and immunohistochemistry. Upon enzymatic reduction, the soluble, colorless NBT is converted into an insoluble, deep blue-to-purple formazan precipitate, providing a visual or spectrophotometric signal. It is also a key reagent in the NBT reduction test, a classic assay used to diagnose chronic granulomatous disease (CGD) by assessing the oxidative burst activity of phagocytes. NBT is considered a stable and reliable reagent for colorimetric detection. It is typically used in conjunction with 5-bromo-4-chloro-3-indolyl phosphate (BCIP) to enhance sensitivity. As a light-sensitive compound, it should be stored protected from light.