Therapeutic Drugs - Optimal Time to Draw Samples

Laboratory monitoring of therapeutic agents is an endeavor that requires a great amount of coordination between laboratory, nursing and pharmacy personnel. To adequately evaluate the appropriate dosage levels of many drugs, the collection and testing of specimens for trough and peak levels is necessary. The trough level is the lowest concentration in the patient's bloodstream, therefore, the specimen should be collected just prior to administration of the drug. The peak level is the highest concentration of a drug in the patient's bloodstream.

Note: Random drug levels may be ordered and are appropriate for monitoring drug usage if drug is being administered by continuous IV infusion.

The following guidelines should be used when collecting therapeutic drug samples:

Aminoglycosides: Gentamicin & Tobramycin IV Trough: Collect just prior to next dose
IV Peak: Infuse over 30 minutes, wait 30 minutes after infusion is complete to collect.
IM Therapy: Draw trough just prior to next dose and peak at one hour after injection
Carbamazepine Collect just prior to next dose
Digoxin One determination is needed within 6 - 24 hours after the previous dose.
Phenytoin Collect just prior to next dose
Phenobarbitol Collect just prior to next dose
Quinidine Collect just prior to next dose
Theophylline IV Infusion: Wait 30 minutes after loading dose and resample at 4 - 8 hour intervals
IV Bolus: Draw 30 minutes after infusion has completed
Oral Dose: Measurement of peak concentrations is recommended, with sample drawn 4 hours following dose for slow release preparations, or 2 hours after dose for regular forms
Vancomycin Trough: Collect just prior to next dose
Peak: Draw 1 ½ hours after IV infusion has completed.
References: Interpretation in Therapeutic Drug Monitoring, Daniel Baer and William Dito, 1981, Phlebotomy Handbook, Garza and McBride, 3rd Ed, p. 173-174.