Kinetic colorimetric method
This assay is based on the reducing ability of the ketoamines formed from the non-enzymatic condensation products of glucose and proteins (fructosamines), to reduce tetrazolium salts to a colored dye in alkaline solution. The oxidized tetrazolium salt, acting also as the reaction starter, is reduced by the fructosamine forming a purpure formazan complex.
Fructosamine + NBT+2 NBT+: Fructosamine complex
The rate of color development is proportional to the concentration of fructosamine in the sample.