At my institution, almost all opiate immunoassay–positive samples that are not confirmed as positive by GC-MS have concentrations of drug plus active metabolites that are slightly or moderately below the 100-ng/mL (μg/L) cutoff typically applied by most laboratories. As illustrated by naloxone in this case, however, false-positive results occasionally occur. Other drugs that can cause false-positive results for some opiate immunoassays include dextromethorphan, diphenhydramine, ephedrine/pseudoephedrine, doxylamine, chlorpheniramine, brompheniramine, quinolone antibiotics, and rifampin. Poppy seeds contain morphine and lesser amounts of codeine, and ingestion of sufficient quantities of poppy seed–containing foods may cause a clinical false-positive opiate immunoassay result at the 300-ng/mL cutoff appropriate for clinical applications.
Opiate immunoassays generally have poor detection for oxycodone and oxymorphone (parent drug and oxycodone metabolite). Thus, separate oxycodone/oxymorphone-specific immunoassays are required for their detection.
Most clinical laboratories lack the capability to confirm initial drug-positive immunoassay results by GC-MS or LC-MS/MS. Thus, the clinician must interpret these test results in the context of the particular clinical situation. When such test results are confounding or are not compatible with the clinical picture, as illustrated by this case, confirmatory testing may be warranted. Other reasons for confirmatory testing include the detection of drugs of abuse in children, during pregnancy, for organ transplantation candidates, in pain-management programs, and for hospital employee fitness for duty. In such legally or clinically sensitive cases, child custody or access to care or employment may be denied if nonprescribed opiates are present.
The proper interpretation of results of urine tests for drugs of abuse may not be straightforward. Opiates in particular have several potential confounders. Close communication between clinical and laboratory staff is likely to lead to the most appropriate patient care and follow-up.
Author Contributions: All authors confirmed they have contributed to the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; and (c) final approval of the published article.
Authors' Disclosures of Potential Conflicts of Interest: No authors declared any potential conflicts of interest.
Role of Sponsor: The funding organizations played no role in the design of study, choice of enrolled patients, review and interpretation of data, or preparation or approval of manuscript.
- Received for publication May 11, 2010.
- Accepted for publication May 20, 2010.
- © 2010 The American Association for Clinical Chemistry