Assessing the real-world performance of xylazine test strips for community-based drug checking in Los Angeles

BACKGROUND: The veterinary sedative xylazine is increasingly found in illicit fentanyl and has been associated with numerous health harms. Xylazine test strips (XTS) are an emerging technology that can theoretically assist consumers in avoiding xylazine, but they require real-world validation. We leverage community-based drug checking program data to compare real-world XTS performance to 'gold standard' methods. METHODS: Samples were initially assessed by dissolving 1 mg of drug product in 1 mL water and dipping an XTS ("first generation" Wisebatch™) in the sample. Subsequently, confirmatory testing was performed by sending samples to the National Institute of Standards and Technology for qualitative analysis using direct analysis in real-time mass spectrometry (DART-MS). A subset was analyzed quantitatively with liquid chromatography gas spectrometry (LC-MS) to quantify xylazine, fentanyl, and other compounds. RESULTS: A total of n = 1570 drug samples were analyzed between June 2023 and May 2025, and a total of n = 801 XTS were used. N = 715 comparisons between xylazine test strips and mass spectrometry results could be made, including n = 333 among samples that tested positive for fentanyl. Of these, n = 63 samples were confirmed to contain xylazine by mass spectrometry, of which the majority contained low concentrations (average concentration 2.3%; 78% of samples contained less than < 1% xylazine by weight). Of the 63, n = 34 were correctly identified as positive by XTS, yielding sensitivity of 54.0 %. Of n = 270 xylazine negative samples, n = 235 were correctly categorized (specificity = 87.0%). Most false positives occurred with lidocaine present. CONCLUSIONS: In our sample, with a large percentage of low concentration xylazine samples, "first generation" Wisebatch XTS had a relatively low sensitivity, but higher specificity. This highlights the value of confirmatory testing and the complicated and often confusing nature of point-of-care test strips for novel substance detection. Lot testing and validation studies are needed to improve quality control in this area.