Context: Millions of assays are performed each year to monitor for substance abuse in various settings. When common medications cross-react with frequently used testing assays, false-positive results can lead to invalid conclusions.
Objective: To evaluate cross-reactivity of quinolone antimicrobials in common opiate screening assays and to assess the in vivo implications of this phenomenon.
Design, setting, and participants: The reactivity of 13 quinolones (levofloxacin, ofloxacin, pefloxacin, enoxacin, moxifloxacin, gatifloxacin, trovafloxacin, sparfloxacin, lomefloxacin, ciprofloxacin, clinafloxacin, norfloxacin, and nalidixic acid) was tested in 5 commercial opiate screening assays from September 1998 to March 1999. In 6 healthy volunteers, we confirmed the cross-reactivity of levofloxacin or ofloxacin with these opiate screening assays.
Main outcome measure: Opiate assay activity (threshold for positive result, 300 ng/mL of morphine).
Results: Nine of the quinolones caused assay results above the threshold for a positive result in at least 1 of the assays. Four of the assay systems caused false-positive results for at least 1 quinolone. Eleven of the 13 compounds caused some opiate activity by at least 1 assay system. At least 1 compound caused opiate assay activity in all 5 assay systems. Levofloxacin, oflaxacin, and perfloxacin were most likely to lead to a false-positive opiate result. Positive results were obtained in urine from all 6 volunteers.
Conclusion: Greater attention to the cross-reactivity of quinolones with immunoassays for opiates is needed to minimize the potential for invalid test interpretation.