Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (637 page)

   Drug detected is a process impurity.

   Drug detected is an expected metabolite of a prescribed drug.

   Incorrect prescription filled.

   Drug obtained elsewhere.

   Drug added to specimen.

   Result is a false positive (for IA screens).

   Clinic or laboratory error.

   Knowledge of drug metabolic profiles is important.

   For many drugs, parent drug and metabolite should be present. For example, buprenorphine and norbuprenorphine would be expected to be present in a patient taking buprenorphine. The presence of norbuprenorphine is indicative of metabolism.

   Knowledge of the pharmacokinetic characteristics of drugs is important.

   Drug absorption rates vary based on different formulations and routes of administration.

   Serum/plasma half-lives and urinary elimination half-lives assist in assessing drug detection times and therefore the likelihood that a test on a specimen collected at a specified time after drug administration will be positive.

   Quantitative urine levels may be useful to identify