Nitroblue Tetrazolium (NBT) is a chromogenic substrate that, like other tetrazolium compounds, can be reduced to produce a colored formazan derivative. Conventionally, a semi-quantitative microscopic NBT assay is used to determine the production of superoxide anion (O2(-)) in various phagocytic cells. This microscopic assay is conducted by counting the cells containing blue NBT formazan deposits, which are formed by reduction of the membrane permeable, water-soluble, yellow-colored, nitroblue tetrazolium (Y-NBT) by O2(-). NBT test, the oldest and most recognized diagnostic test for chronic granulomatous disease (CGD), relies on light microscopy to provide a mostly qualitative determination of phagocyte NADPH oxidase activity: production of blue reduced NBT formazan in normal cells but not in those from patients with CGD. NBT can also be used as a chromogenic activity stain for oxidoreductases in gels or solutions. More commonly NBT is often paired with 5-bromo-4-chloro-3-inolyl phosphate (PCIB) for the colorimetric detection of alkaline phosphatase activity. Alkaline phosphate converts PCIB to a product that reduces NBT to its formazan derivative, resulting in a black-purple precipitate.