Detection of Xylazine or Medetomidine and Their Metabolites in Bodily Fluids

"Based on the observed half-life of xylazine in plasma (12 hours), a reasonable estimate for window of detection is several days after last use, but this can also be affected by assay limit of detection, dosing and mode of administration, hepatic and/or renal function, and potentially pharmacogenetic differences, and/or drug–drug interactions [34]. Particularly for xylazine, unavailability of commercial standards for major metabolites presents a challenge to their routine inclusion in clinical and/or forensic assays, and it is currently unknown whether their detection might improve the sensitivity or window of detection for exposure. The short half-life of dexmedetomidine suggests an even shorter window of detection for medetomidine, and this is consistent with the detection of medetomidine metabolites in the absence of medetomidine in urine.

"Both xylazine and medetomidine are lipophilic with large volumes of distribution which could potentially result in prolonged elimination kinetics in a setting of chronic dosing. For example, in patients chronically exposed to fentanyl, the major norfentanyl metabolite may be detectable in urine for 3–4 weeks post-cessation, reflecting an elimination phase which is significantly longer than the 2–4 days that norfentanyl is generally detectable in urine after short term medical administration during clinical anesthesia [42,43]."