Abstract: A portable apparatus for detecting the presence of at least one methylxanthine chemical species such as caffeine or theophylline in a beverage comprises a first portion comprising an effective concentration of phosphodiesterase enzyme, a second portion comprising cyclic AMP, and means for indicating inhibition of degradation of the cyclic AMP by the phosphodiesterase due to the presence of the methylxanthine species. A method for determining the presence of at least one methylxanthine chemical species in a beverage comprises contacting at least one test portion of the beverage with effective concentrations of at least one phosphodiesterase enzyme and cyclic AMP, and further contacting the test portion with means for indicating the inhibition of the degradation of the cyclic AMP by the phosphodiesterase due to the presence of the methylxanthine species.