Analytical performance and visual evaluation of fentanyl and xylazine test strips

Testing street drugs for the presence of active adulterants such as fentanyl and xylazine can provide the user some confidence as to the safety of their drugs.

We obtained 3 different commercially available xylazine and 3 different commercially available fentanyl test strips and evaluated them for analytical sensitivity using drug standards. The specificity of the fentanyl strips against fentanyl analogues was also assessed. Powdered fentanyl, xylazine, and fentanyl analogue standards were dissolved in water and serially diluted to bracket the manufacturer's stated test strip sensitivity. Each dilution was tested in duplicate until a negative result was obtained. The ability to discern positive from negative results under different lighting conditions was also assessed for one of the strips (two lots of reagents).

All three xylazine test strips detected the drug at concentrations below the manufacturer's stated limits, however one strip was substantially more sensitive (at 50 ng/mL) than the other two (at 250 ng/mL). One of the fentanyl strips had the best sensitivity (at 3.5 ng/mL), well below the stated sensitivity of 20 ng/mL. The other two were less sensitive (at 7 and 250 ng/mL). For specificity against fentanyl analogues, all of the strips tested positive for all of the analogues tested, but at different levels. The visual endpoints for all of the strips were acceptable under normal lighting conditions, but cannot be read when the ambient light is low.

Based on these results, we selected one xylazine (Shanghai Accurature Diagnostics) and one fentanyl strip (W.P.H.M.) based on the best analytical sensitivity. The fentanyl strip chosen had varying degrees of specificity against the other manufacturers.