Performance Evaluation of an Automated Fentanyl Immunoassay

Background: High-throughput fentanyl immunoassays have recently emerged for clinical use, but early reports have demonstrated relatively high false-positive rates. The purpose of this study was to compare 2 immunoassays, the ARK and ARK II fentanyl immunoassays, and to demonstrate the clinical impact of implementing the ARK II assay. Methods: The ARK and ARK II fentanyl assays were performed on a Roche c 502 chemistry analyzer. Positive and negative percentage agreement was assessed for each assay with 112 residual patient specimens relative to liquid chromatography–tandem mass spectrometry (LC–MS/MS). Cross-reactivity was assessed with the primary metabolite, norfentanyl, and analogs acetylfentanyl, acrylfentanyl, and furanylfentanyl. The proportion of specimens that did not confirm was assessed retrospectively from the laboratory information system. Results: The concordance of the ARK assay was 75% (kappa 0.46, 95%CI 0.28–0.63) and the ARK II was 93% (kappa 0.86, 95%CI 0.76–0.95) with LC–MS/MS. 30 ng/mL of norfentanyl was required for a positive result by ARK and 15 ng/mL by ARK II. Similar cross-reactivity was observed when fentanyl and norfentanyl were both present in the specimen and with fentanyl analogs. After implementing the ARK II assay, the proportion of specimens that did not confirm by LC–MS/MS decreased from 11.7% per month to 2.0% per month. Conclusions: The ARK II fentanyl immunoassay has improved concordance relative to the original ARK fentanyl immunoassay using LC–MS/MS as the comparator method. Improved analyte specificity resulted in a reduced proportion of clinical samples that do not confirm.