In this clinical case study, Straseski et al. detail a patient's presentation that resembled an opiate overdose, which was ultimately found to be a false-positive result produced by the immunoassay. Unfortunately, this finding is not unusual in clinical practice when caring for a patient.
In 2003, Clinical Chemistry published the National Academy of Clinical Biochemistry guidelines for the testing of poisoned patients (1). A high degree of consensus was reached for the recommendations in these guidelines, among which were: (a) The emergency department and clinical laboratory should develop a close relationship to optimize the tests that are ordered and performed; (b) the laboratory should provide a list of the major cross-reacting substances for each drug class against which to check when a positive result is reported; and (c) immunoassays should detect most opioids, not just codeine and morphine.
As is noted in the case study, most point-of-care immunoassays have important sensitivity and specificity limitations. The case study points to a potential, but uncommon, false-positive opiate result caused by the child's naloxone dose. Other drugs that have the potential to produce a positive screening result for opioids include dextromethorphan, diphenhydramine (methadone assays only), poppy seeds, quinine, quinolones, rifampin, and verapamil (methadone assays only). In addition to false-positive results, false-negative results may occur as well. The latter are commonly seen when semisynthetic or synthetic forms of opioids are used. Fentanyl and oxycodone are usually not detected in urine screens. Methadone is another commonly missed drug if the immunoassay is dependent on a structural similarity to morphine.
Urine drug screens are frequently used in healthcare; however, accurate interpretation is vital because decisions on treatment and disposition are often based on these results. Clinicians need to be aware that the preliminary tests performed by immunoassays are presumptive only. A confirmatory test (e.g., GC-MS) is required before decisions can be made on the basis of the drug screen. Often, the results for the confirmatory test are not produced a timely fashion. Because treatment of the poisoned patient often does not require an antidote and because many drugs that require antidotes are not found in the point-of-care drug screen, treatment should be based on the presenting toxidrome and not on the drug screen.
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