Fast Blue RR has been used to detect alkaline phosphatase activity.
Fast blue RR is mainly used in histological stainings. It binds to the reaction product of alkaline phosphatase and precipitates it as an insoluble compound. The sites of alkaline phosphatase activity appear black. Fast blue RR is also used for the detection of esterase activity. It couples with naphthol (released from naphthol acetate) and forms an insoluble azo-dye which appears red/blue.