RAPID ELECTROCHEMICAL ASSAY FOR THEOPHYLLINE IN WHOLE-BLOOD BASED ON THE INHIBITION OF BOVINE LIVER ALKALINE-PHOSPHATASE

A disposable electrochemical test strip for determining clinically relevant concentrations of theophylline (0-300 μM) in whole blood is described, based on the generation of p-aminophenol from p-aminophenyl phosphate by the action of bovine liver alkaline phosphatase. Theophylline is an uncompetitive inhibitor of alkaline phosphatase and thus inhibits this process. The test strip consists of a screen-printed, carbon-based electrode system containing the enzyme and substrate in separate layers. Application of a 20-μl blood sample to the strip initiates the enzymic reaction, which will proceed to an extent that is inversely dependent on the amount of theophylline in the sample. After a 2-min incubation, the p-aminophenol generated is quantified by its electrochemical oxidation at + 150 mV (vs. Ag/AgCl) on the underlying carbon electrode. Caffeine and theobromine (0-1 mM), phenylalanine (< mM) and endogenous alkaline phosphatase (<2 U ml -1) do not interfere. © 1990.