False-positive results for ketone with the drug mesna and other free-sulfhydryl compounds.

All free-sulfhydryl compounds tested produced false-positive reactions in the Legal test for ketones. The color developed in the ketone pad of urine dipsticks [N-Multistix SG, Multistix 10 SG (Ames), and Chemstrip 9 (Boehringer-Mannheim)] was misinterpreted for ketone bodies, both by visual and automated reading. In contrast to the reaction with true ketones, a drop of glacial acetic acid added onto the ketone pad of dipsticks discharged the false-positive red color. A red-violet also developed instantly with free -SH compounds in the Acetest tablet assay (Ames), but quickly faded. In general, the presence of acidic groups such as -COOH and -SO3H in the structure appeared to increase the nitroprusside reactivity of free -SH compounds, whereas the presence of a -NH2 group appeared to decrease it. Currently, false-positive ketone reactions ascribable to a free -SH group are most likely to be seen for urine containing mesna. The false-positive test for ketones caused by free -SH compounds can be recognized and ruled out by proper procedures. On the other hand, this chromogenic reaction with free thiols might be used for monitoring urinary excretion of mesna.