YNHH Laboratory Manual

The informational value of drug levels is greatly enhanced when levels are obtained at the most appropriate times. Listed below are our recommendations for sampling times for commonly monitored drugs. We have also indicated approximate times to achieve steady state, since levels obtained after steady state is reached have long term predictive value.

The [actual] time the specimen was obtained must be known to interpret the level, whether the specimen was drawn at the optimal sampling time or not. The actual time of draw should be entered on the requisition by the [person obtaining the specimen]. This will ensure that the draw time is entered into the patient's record along with the serum concentration. In addition, it should be indicated whether the draw is a peak or trough, when appropriate, so that this information may also be recorded with the level.

For greatest utility, the best time to draw a trough level is just before the next dose. For convenience, trough levels may be obtained from 0-30 minutes before the dose, provided the actual time of draw is indicated on the requisition. Except where otherwise indicated, peak levels should be drawn 30 minutes after an IV dose and 1-2 hours after an oral dose. In general, time to peak after oral dose is highly variable. Thus, unless otherwise indicated, therapeutic ranges refer to trough levels.

AMINOGLYCOSIDES (Gentamicin, Tobramycin, Amikacin): Draw peak 30 minutes after completion of a 30-minute infusion. Draw a trough just before the next dose. With normal renal function, steady state is achieved within 16 hours.

AMIODARONE Therapeutic range refers to trough levels. Steady state is reached in 2-6 months, or sooner if a loading protocol is followed.

CARBAMAZEPINE Peaks and trough should be within therapeutic range. Because time to peak is highly variable, only trough levels should be routinely monitored. These should be maintained in the lower portion of the therapeutic range. Steady state is reached after 2-6 days.

CHLORAMPHENICOL Both peak and trough levels are needed. Draw peak levels 2-3 hours after an oral or IV dose. Draw trough levels just before the next dose. Steady state is usually achieved in 12-36 hours (Performed by reference laboratory).

CYCLOSPORINE Both peak (collected two hours after dose) and trough levels may be utilized. Appropriate levels vary with indication and concurrent immunosuppressive therapy. Steady state is usually achieved in 1-2 days.

DIGOXIN Draw trough level; or draw random level [no earlier than] 4 hours after an IV dose or 6 hours after an oral dose (It is often convenient to give the dose in the evening and obtain the level in the morning). With normal renal function, steady state is usually achieved within 1 week.

ETHOSUXIMIDE Obtain trough or random level. Steady state is reached after 1-2 weeks (Performed by reference laboratory).

LAMOTRIGINE Therapeutic range is not well defined. Steady state is reached in 5-8 days.

LIDOCAINE Therapeutic range refers to levels obtained at any time after steady state is reached during a continuous IV infusion. Steady state is usually reached in 2-3 hours with a loading dose or 6-12 hours without a loading dose.

LITHIUM Therapeutic range refers to levels drawn 12 hours after an oral dose. With normal renal function, steady state is usually achieved in 3-6 days.

METHADONE Therapeutic range refers to trough levels. Steady state is reached in 3-6 days.

METHOTREXATE Obtain levels at times prescribed in protocol.

PHENOBARBITAL Obtain trough or random level. Steady state is reached after 1-3 weeks.

PHENYTOIN A level should be obtained after a loading dose. Additional trough levels should be obtained 2-3 days after initiating therapy and every 3-7 days thereafter until steady state is reached. At steady state, measure trough or random levels. Time to steady state may vary from 1 to more than 4 weeks.

PRIMIDONE Therapeutic range refers to trough levels at steady state. Steady state levels of primidone are usually achieved in 2-3 days, but steady state levels of the active metabolites PEMA and phenobarbital require 1-4 weeks (Performed by reference laboratory).

PROCAINAMIDE During a continuous infusion, sample 30 minutes after a loading dose, at 2 hours, at 12-24 hours, and at any time after steady state is achieved. During intermittent oral dosing, peak and trough levels of both procainamide and the metabolite, N-acetylprocainamide, should be within the therapeutic ranges. Because time to peak is highly variable, particularly with sustained release formulations, only trough levels should be routinely monitored. They should be maintained in the lower half of the therapeutic range. Additional measurements (nominal peaks) are made if and when signs of toxicity are noted. With normal renal function, steady state is usually achieved within 24 hours.

QUINIDINE Both peak and trough levels should be within the therapeutic range. Because time to peak is highly variable, particularly with sustained release formulations, only trough levels should be routinely monitored. They should be maintained in the lower half of the therapeutic range. Additional measurements (nominal peaks) are made if and when signs of toxicity are noted. With normal hepatic function, steady state is usually reached within 24-48 hours.

SIROLIMUS Therapeutic range refers to trough levels and may vary with indication and concurrent immunosuppressive therapy. Steady state is usually achieved in 1-3 weeks.

TACROLIMUS Therapeutic range refers to trough levels and may vary with indication and concurrent immunosuppressive therapy. Steady state is usually achieved in 3-12 days.

THEOPHYLLINE During a continuous infusion, sample 30 minutes after loading dose, at 4-6 hours, at 12-18 hours, and at any time after steady state is achieved. During intermittent dosing with aminophylline elixir, measure both peak and trough levels. Both should be within the therapeutic range. With sustained release preparations, time to peak is highly variable. Trough levels only need be monitored. They should be maintained in the lower half of the therapeutic range. Additional measurements (nominal peaks) are made if and when signs of toxicity are noted. Steady state is usually achieved within 1-2 days.

TRICYCLIC ANTIDEPRESSANTS (Amitriptyline, Nortriptyline, Imipramine, Desipramine): Therapeutic ranges refer to trough levels at steady state. With amitriptyline or imipramine, the ranges refer to the total concentration of the parent drug and its active metabolite (nortriptyline or desipramine, respectively). Steady state is usually achieved within 1-3 weeks.

VALPROIC ACID: Therapeutic range refers to trough levels at steady state. Steady state is usually achieved in 2-3 days.

VANCOMYCIN Vancomycin peak levels are rarely indicated. The peak therapeutic range refers to peaks drawn 30 minutes after a 60-minute infusion. Because vancomycin is in a distribution phase at this time, concentrations are changing very rapidly. Failure to adhere rigorously to infusion and sampling time results in values that are not readily interpreted. Because the peak is drawn during the distribution phase, it is not useful for pharmacokinetic calculations. All requests for vancomycin levels must be discussed with the Pharmacy (688-2245). Troughs are drawn just before the next dose. With normal renal function, steady state is usually achieved within 24 hours.

Updated: July-2008