Blood fentanyl concentrations and clinical presentation among patients with substance use disorders in the emergency department.

Fentanyl remains a leading drug involved in overdose fatalities. However, elevated blood fentanyl concentrations can be observed in the living population who have acquired sufficient tolerance, which can complicate the interpretation of toxicological findings. Limited data is available providing fentanyl concentrations in relation to effect profiles. We present data for 39 recreational drug users presenting to the emergency department for non-overdose reasons, including drug use history, clinical presentation, and toxicological findings. Quantitation of fentanyl and 36 other drugs and metabolites in whole blood samples was performed using a multidrug-targeted liquid-chromatography-tandem mass spectrometry method that is routinely employed for forensic casework. Excluding patients that were administered fentanyl during hospital care, or had fentanyl concentrations below the limit of quantitation, the remaining 24 patients had a mean blood fentanyl concentration of 20.1 ng/mL (median, 9.6 ng/mL). Many cases overlapped with driving under the influence of drugs and postmortem fentanyl concentrations reported in the literature despite the absence of observable drug toxicity. Polydrug use was also common in this population, with methamphetamine being the most co-detected substance. Our findings demonstrate that blood fentanyl concentrations are unreliable predictors for effects, particularly in the drug-using population.