For patients taking procainamide, you’ll need to monitor serum drug levels and the active metabolite N-acetylprocainamide (NAPA) to prevent toxic reactions. To suppress ventricular arrhythmias, the therapeutic serum concentration of procainamide should range between 4 and 12 mcg/mL. Therapeutic levels of NAPA should average between 9 and 20 mcg/mL.