ThERAPEUTIC INDEX Medications with a high therapeutic index (TI) have a wide safety margin. Therefore, there is no need for routine serum medication‑level monitoring. Medications with a low TI require close monitoring of serum medication levels. Nurses should consider the route of administration when monitoring for peak levels (highest plasma level when elimination = absorption). For example, an oral medication can peak from 1 to 3 hr after administration. If the route is IV, the peak time might occur within 10 min. (Refer to a drug reference or a pharmacist for specific medication peak times.) For trough levels, obtain a blood sample immediately before the next medication dose, regardless of the route of administration. A plateau is a medication’s concentration in plasma during a series of doses.