How often do false-positive phencyclidine urine screens occur with use of common medications?

BACKGROUND: Previous reports describe false-positive urine immunoassay screens for phencyclidine (PCP) associated with use of tramadol, dextromethorphan, or diphenhydramine. The likelihood of these false positives is unknown.
OBJECTIVE: We sought to find the relative frequency of false-positive PCP screens associated with these medications and to look for any other medications with similar associations.
METHODS: In an IRB-approved study, we retrospectively reviewed charts of all ED encounters with positive urine screens for PCP in our hospital from 2007 through 2011, inclusive. Urine samples were tested for drugs of abuse using the Siemens Syva EMIT II Immunoassay. Our laboratory routinely confirmed all positive screens using GC-MS with results classified as either "confirmed" (true positive) or "failed to confirm" (false positive). We recorded all medications mentioned in the chart as current medications or medications given before the urine sample. We used Fisher's exact test to compare frequencies of tramadol, dextromethorphan, diphenhydramine, and other medications between the two groups.
RESULTS: Tramadol, dextromethorphan, alprazolam, clonazepam, and carvedilol were significantly more frequent among the false-positive group, but the latter three were also associated with polysubstance abuse. Diphenhydramine was more frequently recorded among the false-positive group, but this was not statistically significant.
CONCLUSION: False-positive urine screens for PCP are associated with tramadol and dextromethorphan and may also occur with diphenhydramine. Positive PCP screens associated with alprazolam, clonazepam, and carvedilol were also associated with polysubstance abuse.