An electrochemical biosensor, comprising a thick-film screen-printed Ir/C working and counter electrodes, was developed for the detection of the enzymatic activity of adenosine deaminase for potential patient management with liver disease. Present methods of detecting adenosine deaminase involve spectrometric methods that require costly, nonportable equipment that is ill-suited for a point-of-care testing. Studies dealing with the electrochemical detection of adenosine deaminase intermediates such as xanthine and hypoxanthine required the use of +0.5 V, vs a Ag/AgCl reference electrode, and a large sample volume, 1 mL or more. The proposed biosensor used a sample volume of 3 mu L and a potential of +0.27 V vs Ag/AgCl. This biosensor used the enzymes xanthine oxidase (EC 1.1.3.22) and purine nucleoside phosphorylase (EC 2.4.2.1) coimmobilized on the thick-film screen-printed working electrode to detect enzymatically generated H2O2. Constant potential measurements showed that this biosensor had good linear performance over a 0-36 units/L of adenosine deaminase concentration range in phosphate buffer, mouse plasma, and mouse whole blood. The effect of the variation in the quantity of enzyme on the current output and the interference from intermediate-formed compounds were also explored.