Comparison of dual-target and mono-target strips for the detection of fentanyl and xylazine

Abstract Background Lateral flow immunoassay strips are usually formatted with a single test line to detect a single target, but alternative designs that provide two or more test lines are also possible. In this study, we compared the performance of a dual test strip that detects both fentanyl and xylazine with two brands of mono-tasker strips (fentanyl only, xylazine only). Methods Performance metrics tested were the limit of detection for the targeted substances in warm and cold DI water, “hard” tap water, saline solution, and artificial urine; interferences; and cross reactivity with fentanyl analogs. Results Nearly all lots of test strips showed significant increases in the limit of detection (worse performance) in hard water. All the strips show intermediate concentration ranges where the visual intensity of the test line is weak, meaning that the target is sometimes detected and sometimes not detected. One lot of dual strips detected xylazine at 250 ng/mL, which was significantly better than the two brands of monotasker xylazine strips. Conclusion Performance of the dual strips was comparable to that of the monotaskers. However, the strips are more complicated to interpret. Adoption of dual strips may require additional support for users.