Drug interference with measurement of metanephrines in urine

The influence of 35 commonly used drugs on measurement of metanephrines in urine was evaluated. Two concentrations of drugs were chosen for study based on usual doses and the percent of dose excreted unchanged in the urine. At "medium" drug concentrations, only phenylephrine falsely elevated metanephrine levels, whereas at a 10-fold higher drug concentration, guanethidine, hydrocortisone, imipramine, isoetharine, levodopa, phenobarbital, and phenylephrine caused positive interference. Propranolol and theophylline caused a negative interference at the two concentrations studied. The significance of these results is discussed.