Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (628 page)

   Blood should be drawn at a time specified by that laboratory (e.g., 1 hour before the next dose is due to be administered). This trough concentration should ideally be greater than the minimum effective serum concentration.

   If a drug is administered by IV infusion, blood should be drawn from the opposite arm.

   The drug should have been administered at a constant rate for at least 4–5 half-lives before blood samples are drawn.

   Unexpected test results may be due to interference by complementary and alternative medicines (e.g., high digoxin levels may result from interference from danshen, Chan Su, or ginseng).

Criteria

   Available methodology must be specific and reliable.

   Blood concentration must correlate with therapeutic and toxic effects.

   Therapeutic window is narrow with danger of toxicity on therapeutic doses.

   Correlation between blood concentration and dose is poor.

   Clinical effect of drug is not easily determined.

   Drugs for Which Drug Monitoring May Be Useful

   Antiepileptic drugs (e.g., phenobarbital, phenytoin)

   Theophylline

   Antimicrobials (aminoglycosides [gentamicin, tobramycin, amikacin], chloramphenicol, vancomycin, flucytosine [5-fluorocytosine])