A method for determining nitrofurantoin in urine in the presence of phenazopyridine hydrochloride and its metabolites.

The high urinary drug concentrations usually encountered after administering nitrofurantoin in the chemotherapy of urinary tract infections are often measured by the nitromethane-Hyamine method. We show here that, if the urinary tract analgesic, phenazopyridine hydrochloride, and its related metabolite(s) are in the urine, they interfere with this determination of nitrofura ntoin. Nevertheless, the nitrometha ne-Hyamine method may be modified to determine nitrofurantoin quantitatively in urine collected from human subjects who have been treated with the analgesic and nitrofurantom concomitantly.