Figure 4 Variability in sensitivity of marketed PCP and tricyclic Belnacasan ic50 antidepressant screening immunoassays. The plotted circles indicate the concentration of compound that

produces equivalent reaction to 25 ng/mL PCP or 1000 ng/mL desipramine (tricyclic antidepressant … In our own medical system, we had anecdotally observed that PCP abuse by patients presenting to our ED was rare, whereas intentional dextromethorphan and meperidine usage Inhibitors,research,lifescience,medical was more common, suggesting that there may be many false positive PCP screening tests. We therefore examined the causes of PCP positive screens in our medical system (Additional file 1, tab V). Over the course of 24 months (January 2007 through January 2009), we had 10 patient samples with positive PCP screens, nine on the Syva EMIT assay and only one on the Biosite Triage assay system (Additional file 1, tab V). Urine samples from

these 10 patients were also analyzed by GC/MS, using a protocol to definitively identify a wide range of clinically important legal and illicit drugs Inhibitors,research,lifescience,medical [28]. Of these, only one patient showed the definitive presence of PCP Inhibitors,research,lifescience,medical by GC/MS (30 year old female); this positive screen occurred on the Syva EMIT system. The one positive PCP screen on the Biosite Triage assay in our sample occurred in a 48 year old female, with GC/MS analysis showing apparently very high urine concentrations of diphenhydramine, a medication reported to cause positives on the Biosite Triage PCP assay at high

urine concentrations Inhibitors,research,lifescience,medical (Additional file 1, tab P). The eight patient samples (average age 23.6 years, range 2–44 years old; 5 males, 3 females) that had a positive PCP screen on the Syva EMIT assay not accounted for by the presence of PCP all showed apparently high urine concentrations of dextromethorphan by GC/MS that would be consistent with intentional or inadvertent overdose of dextromethorphan. Inhibitors,research,lifescience,medical One of these samples also showed the presence of apparently high urine concentrations of meperidine by GC/MS. Relative to PCP, dextromethorphan (Tanimoto similarity = 0.565) and meperidine (Tanimoto similarity = 0.538) have similarity coefficients higher than some of the PCP metabolites compared to their parent drug (Additional file 1, tab P). The common misuse of dextromethorphan suggests that, in medical settings where PCP use is uncommon, false before positives on some marketed PCP assays (e.g., Syva EMIT) due to dextromethorphan can occur more frequently than true positives. In these cases, PCP screening assays may become more effective as ‘dextromethorphan overdose screens’ than as PCP screens. However, it should be pointed out that at least five of the currently marketed PCP immunoassays are reported to be insensitive to dextromethorphan (Additional file 1, tab P; Figure ​Figure4A).4A). The Biosite Triage system used in some hospitals in our medical system is an example of this.