The serum drug concentration should never be interpreted in isolation, and the patient's clinical condition must be considered. The sample must be taken at the correct time in relation to dosage interval.

Phenytoin

Phenytoin has a low therapeutic index and a narrow target range. Although the average daily dose is 300 mg, the dose needed for a concentration in the target range varies from 100 to 700 mg/day. Because phenytoin has non-linear (zero-order) kinetics, small increases in dose can result in greater increases in blood level.

Aminoglycosides

Gentamicin, tobramycin, netilmicin and amikacin are antibiotics with a low therapeutic index. After starting treatment, measurements should be made before and after the third to fifth dose in those with normal renal function, and earlier in those with abnormal renal function. Levels should be repeated, if the dose requires adjustment, after another 2 doses. If renal function is stable and the dose correct, a further check should be made every 3 days, but more frequently in those patients whose renal function is changing rapidly. It is often necessary to adjust both the dose and the dose interval to ensure that both peak and trough concentrations remain within the target ranges. In spite of careful monitoring, the risk of toxicity increases with the duration of treatment and the concurrent use of loop diuretics.