Commonly Prescribed Medications and Potential False-positive Urine Drug Screens

Nancy C. Brahm; Lynn L. Yeager; Mark D. Fox; Kevin C. Farmer; Tony A. Palmer

Disclosures

Am J Health Syst Pharm. 2010;67(16):1344-1350. 

In This Article

Discussion

The most commonly used UDS tests are immunoassays, as they allow large-scale screenings with rapid detection at minimal expense.[40] The disadvantage of immunoassays, when compared with the use of GC–MS ("the gold standard"), is false-positive results.[7] A false-positive result for individuals with court-ordered or work-related screening can lead to legal interventions, workplace disruptions, or questions of honesty. A more specific confirmatory test should be conducted prior to releasing the initial screening results.[8]

Accounts of false-positive UDS results for drugs of abuse have been reported with a number of rapid-screening immunoassay products. After receiving positive results for illicit or abused substances, additional confirmatory testing was done in some cases. This included more detailed patient interviews and secondary analysis with GC–MS to more precisely identify the offending substances. Awareness of the potential for false-positive results and confirmatory follow-up information are particularly important for the patients who seek care at the Bedlam evening clinic. Patients may be unaware of potential false-positive reactions when they use prescribed medications and of the correct follow-up procedures to properly resolve the situation. Health care delivery may be provided for the working poor by additional resources, such as the Department of Health or other service providers.

UDS sample timing was found to be important in several of the reports.[2,3,15,17,27] In other reports, the presence of metabolites with different structures, pharmacokinetics, or pharmacodynamic properties may have been factors in the results.[10,26] Reagent specificity or sensitivity is another concern.[1,6,41] Patient-specific factors, such as the physiological effects of weight or diet (as they apply to urine acidity or alkalinity) were not included in this review of formulary agents. Based on the reports reviewed, no single reagent was identified with false-positive results. The ranges of the results were developed for the adult, not pediatric, population.[32]

With the increased availability of onsite drug testing and the variety of products associated with reports of false-positive results in the literature, confirmation of results is needed. Failure to follow up to determine if a false positive may have occurred could result in unnecessary adverse consequences for the patient (e.g., incarceration, employment denial, loss of privileges).

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