Some antibiotics may cause false positives for opiates:
"A total of 13 quinolone antibiotics were solubilized to represent expected urinary concentrations for testing via 5 different commercial immonoassays (levofloxacin [levaquin], ofloxacin [floxin], gatifloxacin, enoxacin, moxifloxacin, sparfloxacin, pefloxacin, trovafloxacin, lomefloxacin, ciprofloxacin [cipro], cinafloxacin, norfloxacin, and nalidixic acid. The assays included EMIT II reagents, AxSYM flourescence polarization immunoassay, CEDIA, Roche Abuscreen OnLine reagents, and Beckman opiate reagents. In vitro testing against a positive concentration of morphine (300 ng/mL) revealed that 9 of the quinolones (levofloxacin, ofloxacin, perfloxacin, enoxacin, gatifloxacin, lomefloxacin, moxifloxacin, ciprofloxacin, and norfloxacin) caused false positives in at least one of the assay systems. The quinolones that cross reacted most often included levofloxacin (4/5), ofloxacin (4/5), perfloxacin (3/5) and enoxacin (2/5)."
Baden LR Horowitz B, Jacoby H Eliopoulos GM Quinolones and false positive urine screeing for opiates by immunoassay technology JAMA 2001; 286:3115-3119.
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