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Castelli R2, Thornton SL, Akpunonu P, Gerona R. 
“True or False? Analysis of phencyclidine positive urine drug immunoassays with liquid chromatography-time-of-flight mass spectrometry”. 
NAACT Abstract. 2015;? (62).

BACKGROUND: Urine drug immunoassays (UDS) are frequently ordered despite literature demonstrating their poor specificity and lack of clinical utility. The phencyclidine (PCP) screen, in par- ticular, is described to have poor specificity. We sought to analyze PCP positive UDS samples from an academic medical center with liquid chromatography-quadrupole time-of-flight mass spectrom- etry (LC-QTOF/MS) to determine characteristics of confirmed and unconfirmed specimens.

METHODS: At a tertiary care academic medical urine samples with a PCP positive UDS result were identified from January 2014 to January 2015 and then sent for LC-QTOF/MS analysis using a library of 550 drugs. This includes 285 novel psychoactive substances (NPS) of which 10 are PCP analogues. Results were grouped as PCP confirmed, known interfering substance con- firmed, or unknown interfering substance. Our institution’s PCP UDS reporting limit is 25 ng/mL while the LC-QTOF/MS limit of quantification for PCP is 5 ng/mL. For this study known interfer- ing substances were dextromethorphan (DXM), diphenhydramine (DPH), doxylamine, tramadol, venlafaxine, meperidine, ketamine, lamotrigine, and thioridazine. Clinical data (age, sex, location of patient when UDS ordered, and disposition) was gathered by retrospective chart review. Groups were compared for statistical difference using Fisher exact probability test.

RESULTS: 53 samples were collected and tested. Mean age was 37 years (range 22–72) and 39 (74) were male. Thirty-seven (68) of the screens were ordered in the emergency department. Twenty- four (45) were confirmed true PCP positives. Of the false posi- tives, only 6 (21) were from known interfering substance with 3 being attributed to dextromethorphan alone, 2 to tramadol alone and 1 to DXM and DPH combination. In the DXM/DPH combi- nation case, the novel psychoactive substances (NPS) mcPP and pentylone were also detected. They were the only NPS detected in this study. No PCP analogues were detected. In the remaining falsepositive cases (n = 24) no known interfering agent was identified. In this group cotniine was detected in 20 cases, caffeine/theophyl-line/theobromine in 15 cases, buspirone in 2 cases and citalopram in 1 case. There was no statistical significant difference in any clinical data between the 3 groups.

CONCLUSIONS: In this study, only 45 of collected positive PCP USD were confirmed to be true positives. In 80 of false positives cases, no known interfering substance could be identified. Further testing is warranted to identify the cause of these false positives. This study further highlights the poor specificity of the PCP UDS.

Key Words: Laboratory, Phencyclidine, Dextromethorphan
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