Discovering Cross-Reactivity in Urine Drug Screening Immunoassays through Large-Scale Analysis of Electronic Health Records

Abstract

Background: Exposure to drugs of abuse is frequently assessed using urine drug screening (UDS) immunoassays. Although fast and relatively inexpensive, UDS assays often cross-react with unrelated compounds, which can lead to false-positive results and impair patient care. The current process of identifying cross-reactivity relies largely on case reports, making it sporadic and inefficient, and rendering knowledge of cross-reactivity incomplete. Here, we present a systematic approach to discover cross-reactive substances using data from electronic health records (EHRs).

Methods: Using our institution's EHR data, we assembled a data set of 698651 UDS results across 10 assays and linked each UDS result to the corresponding individual's previous medication exposures. We hypothesized that exposure to a cross-reactive ingredient would increase the odds of a false-positive screen. For 2201 assay-ingredient pairs, we quantified potential cross-reactivity as an odds ratio from logistic regression. We then evaluated cross-reactivity experimentally by spiking the ingredient or its metabolite into drug-free urine and testing the spiked samples on each assay.

Results: Our approach recovered multiple known cross-reactivities. After accounting for concurrent exposures to multiple ingredients, we selected 18 compounds (13 parent drugs and 5 metabolites) to evaluate experimentally. We validated 12 of 13 tested assay-ingredient pairs expected to show cross-reactivity by our analysis, discovering previously unknown cross-reactivities affecting assays for amphetamines, buprenorphine, cannabinoids, and methadone.

Conclusions: Our findings can help laboratorians and providers interpret presumptive positive UDS results. Our data-driven approach can serve as a model for high-throughput discovery of substances that interfere with laboratory tests.

Fig. 1.. Establishing validity of the data and approach.

Each plot corresponds to a screening assay, and each point corresponds to an ingredient. A log₂ (odds ratio) of NA indicates that the association was not tested, as <5 individuals had a false-positive (NA on x axis) or true-positive (NA on y axis) UDS result preceded by exposure to the given ingredient. The green triangle in the upper right of the benzodiazepines plot corresponds to clobazam, which is detected by the screen but not by the confirmation. See Materials and Methods for an explanation of why some other assay targets (e.g., several tricyclic antidepressants) had increased odds ratios for false-positive screens.

Fig. 2.. Validating cross-reactivity by spiking compounds into drug-free urine.

Dashed lines show the cutoff for each screening assay. Plots show only a subset of the tested compounds and a subset of the assays. Data from all spiking experiments are in Table 6 of the online Data Supplement.

Fig. 3.. Estimating percentages of false-positive results explained by (A) ingredient and (B) ingredient category.

Plots include only known cross-reactants whose 95% CI lower bound of odds ratio (OR_FP) was ≥2. Exposures are not mutually exclusive, so percentages in (A) could sum to >100. Exposures in (B) followed the hierarchy assay target > known cross-reactant > new cross-reactant, so each exposure was counted once.