Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (627 page)

   Drug has narrow therapeutic range.

   To provide or confirm an optimal dosing schedule.

   To confirm cause of organ toxicity (e.g., abnormal liver or kidney function tests).

   Other diseases or conditions exist that affect drug utilization.

   Drug interactions that have altered desired or previously achieved therapeutic concentration are suspected.

   Drug shows large variations in utilization or metabolism between individuals.

   Need medicolegal verification of treatment, cause of death or injury (e.g., suicide, homicide, accident investigation), or to detect use of forbidden drugs (e.g., steroids in athletes, narcotics).

   Differential diagnosis of coma.

   Applications

   The clinician must be aware of the various influences on pharmacokinetics— factors such as half-life, time to peak and to steady state, protein binding, and excretion.

   The route of administration and sampling time after last dose of drug must be known for proper interpretation. For some drugs (e.g., quinidine), different assay methods produce different values, and the clinician must know the normal range for the test method used for the patient.

   In general, peak concentrations alone are useful when testing for toxicity, and trough concentrations alone are useful for demonstrating a satisfactory therapeutic concentration. Trough concentrations are commonly used with such drugs as lithium, theophylline, phenytoin, carbamazepine, quinidine, tricyclic antidepressants, valproic acid, and digoxin. Trough concentrations can usually be drawn at the time the next dose is administered (
this does not apply to digoxin
). Both peak and trough concentrations are used to avoid toxicity but ensure bactericidal efficacy (e.g., gentamicin, tobramycin, vancomycin).

   IV and IM administration should usually be sampled 30 minutes to 1 hour after administration is ended to determine peak concentrations (meant only as a general guide; the laboratory performing the tests should supply its own values).