A negative urinary glucose oxidase test in the presence of glucbsuria may occur in patients with alcaptonuria or the carcinoid syndrome. It may also be noted after ingestion of aspirin or L-dopa. The responsible agents are potent reducing metabolites such as gentisic acid, homogentisic acid, or 5-hydroxyindole acetic acid. These agents produce the misleading reaction by keeping the indicator dye o-tolidine in its reduced form. In the presence of such reducing agents the tests for glucosuria should be carried out either with Tes-Tape or by modifying the manufacturer's instructions and only partially immersing the Clinistix strip.