Table 3.  Effects of drugs and other artifacts on therapeutic drug monitoring results. Drug Artifact Sequela All drugs Serum Separator Tubes Silicon gel can bind drug, decreasing concentrations. Aminoglycosides 1. Glass tubes 2. Beta-lactams 1. Glass binds drug, decreasing concentrations. 2. High concentrations of beta-lactam antibiotics inactive aminoglycoside, decreasing concentrations. Bromide Increased serum chloride Depending on assay, chloride can not be distinguished from bromide, resulting in artifactually high chloride concentrations. Digitalis glycosides Red Stoppers Stopper may bind drug, decreasing concentrations. Phenobarbital, phenytoin, primodone Drugs Concentrations decreased by phenobarbital induced drug metabolism and other drugs which stimulate drug metabolism; concentrations increased by cimetidine, chloramphenicol and other inhibitors of drug metabolism; concentrations increased by clorazepate if concentrations are high. Thyroid hormones Drugs Phenobarbital increased peripheral metabolism and may decrease concentrations. Theophylline Drugs Concentrations increased by drugs which decrease metabolism, including enrofloxacin, imidazole antifungals, cimetidine, others. "Target" peak concentrations for aminoglycosides depend on infecting organism, and specifically the minimum inhibitory concentration (MIC) of the infecting organism.  The target peak concentration should be 4 to 10 times the MIC.  Trough concentration should equal or be below that recommended in order to minimize toxicity.