BackgroundUrine drug monitoring for medications for opioid use disorder (MOUD) such as buprenorphine can help to support treatment adherence. The practice of introducing unconsumed medication directly into urine (known as “spiking” samples) has been increasingly recognized as a potential means to simulate treatment adherence. In the laboratory, examination of the ratios of buprenorphine and its metabolite, norbuprenorphine, has been identified as a mechanism to identify “spiked” samples. Urine levels of naloxone may also be a novel marker in cases where the combination buprenorphine–naloxone product has been administered. This case study, which encompasses one provider’s practice spanning two sites, represents a preliminary report on the utility of using urinary naloxone as an indicator of “spiked” urine toxicology samples. Though only a case study, this represents the largest published evaluation of patients’ naloxone levels to date.Case presentationOver a 3-month period across two practice sites, we identified 1,223 patient samples with recorded naloxone levels, spanning a range of 0 to 12,161 ng/ml. The average naloxone level was 633.65 ng/ml with the majority (54%) of samples < 300 ng/ml. 8.0% of samples demonstrated extreme values of naloxone (> 2000 ng/ml). One practice site, which had increased evidence of specimen tampering at collections, had a greater percent of extreme naloxone levels (> 2000 ng/ml) at 9.3% and higher average naloxone level (686.8 ng/ml), in contrast to a second site (570.9 ng/ml; 6.4% at > 2000 ng/ml) that did not have known reports of specimen tampering.ConclusionsWe postulate that naloxone may serve as an additional flag to identify patient “spiking” of urine samples with use of the combination product of buprenorphine–naloxone.