The Detection of Xylazine in Tijuana, Mexico: Triangulating Drug Checking and Clinical Urine Testing Data.

Introduction: Xylazine is a veterinary anesthetic increasingly present alongside illicit fentanyl in the United States and Canada, presenting novel health risks. Although xylazine remains less common in the Western US, Mexican border cities serve as key trafficking hubs and may have a higher prevalence of novel substances, but surveillance there has been limited.

Methods: We examined deidentified records from the Prevencasa free clinic in Tijuana, describing urine and paraphernalia testing from patients reporting using illicit opioids within the past 24 hours. Xylazine (Wisebatch and Safelife brands), fentanyl, opiate, methamphetamine, amphetamine, benzodiazepine, and nitazene test strips were used to test urine and paraphernalia samples. Paraphernalia samples were also analyzed with mass spectrometry.

Results: Of n=23 participants providing urine and paraphernalia samples concurrently, 100%, 91.3%, and 69.6% reported using China White/fentanyl, methamphetamine, and tar heroin, respectively. The mean age was 41.7 years, 95.7% were male, 65.2% were unhoused, and 30.4% had skin wounds currently. Xylazine positivity in urine for the 2 strip types used was 82.6% and 65.2%. For paraphernalia testing, the xylazine positivity was 65.2% and 47.8%. Confirmatory testing of paraphernalia samples by mass spectrometry indicated a 52.2% xylazine positivity, as well as fentanyl (73.9%), fluorofentanyl (30.4%), tramadol (30.4%), and lidocaine (30.4%). Mass spectrometry suggested lidocaine triggered n=3 and n=0 false positives among the xylazine test strip types.

Discussion: Xylazine is present on the US-Mexico border, requiring public health intervention. High lidocaine positivity complicates the clinical detection of xylazine via testing strips. Routine urine testing for xylazine in clinical scenarios is likely feasible, yet confirmatory urine studies are needed.