Enzyme amplified immunoassays.

Enzyme amplification has been applied to improve the sensitivity of many immunoassays which use alkaline phosphatase as the label. The activity of the enzyme is amplified by using NAD, derived by hydrolysis of NADP, to activate catalytically a substrate cycle that generates a coloured product. In routine use, an amplification factor of 100-fold is easily achieved and, with longer incubation times, this can be increased to allow the detection of as few as 3,000 enzyme molecules. In order to avoid the limitations of spectrophotometry, an electrochemical version of the amplifier has recently been developed which performs with comparable sensitivity in a prototype device. The many advantages of electrochemical detection may ultimately herald the arrival of a new generation of simple, sensitive and inexpensive diagnostic systems.