Deceptively simple: Can urine samples from NexScreen cup be re-used for confirmatory testing?
Authors: Pandya Vrajesh , McMillin Gwendolyn , Young Brittany 
Published: November 2022

Abstract

Background
The NexScreen urine drug screen cup (SKU: HCDOAV-8145EF1KT, NexScreen LLC, CO, USA) is a Clinical Laboratory Improvement Act (CLIA)-waived test used at our affiliated mental health institute. Specimens testing positive are sent for Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) confirmation. The manufacturer instructs patients to urinate directly into the NexScreen cup. Doing so raised concern about whether this urine can be re-used for confirmatory testing. Modifying the test by collecting urine in another container before pouring it into the NexScreen cup would reclassify the test as high complexity. This would require further regulatory oversight and negate the convenience of a CLIA-waived test. Requesting the patient to urinate into a second container is not always feasible, as patients may be unable or unwilling to provide another sample. A straightforward solution would be to take specimens from the NexScreen container and use them for confirmatory testing. However, the assay components of the NexScreen drug test strips may contaminate specimens and generate false positives with highly sensitive LC-MS/MS assays. In addition, false negatives on confirmatory testing could occur if NexScreen test strips or the plastic cup adsorb drug compounds from patient urine. The manufacturer provides no guidance in this regard.

Objective
The goal of this study was to investigate if re-using urine from NexScreen cups for LC-MS/MS confirmatory testing was appropriate.

Methods and Results
We pooled 25 NexScreen-negative patient specimens and confirmed by LC-MS/MS that no NexScreen-detectable compounds were present in this pool. To evaluate drug conjugate leaching into the specimens, 30 mL blank urine was poured into 10 NexScreen cups, followed by incubation for 1 hour at room temperature, which is the maximum time recommended by the manufacturer for reading NexScreen results. All 10 cups tested negative for all drugs. In addition, none of the negative samples tested positive on LC-MS/MS for any drug compounds targeted, proving that the test strips in the NexScreen cups do not interfere with LC-MS/MS testing. Next, to assess drug adsorption by the NexScreen drug cup, a representative analyte from each of the 13 drug classes was chosen and spiked at concentrations slightly above the published NexScreen positivity cutoffs. Then, 30 mL of spiked urine was placed into 10 NexScreen cups, incubated for 1 hour, and aliquots were sent for LC-MS/MS confirmation. Interestingly, 11 of the 13 analytes achieved at least 95% recovery, while buprenorphine and THC-COOH had 94% and 87% recoveries, respectively, indicating some minor adsorption. However, all analytes, including buprenorphine and THC-COOH, recovered well above the cutoffs for LC-MS/MS assays, suggesting that there is no significant adsorption to alter confirmatory results.

Conclusion
Use of an aliquot from a NexScreen cup for confirmatory testing is appropriate within the manufacturer-recommended readability window of 1-hour post-collection.