(68) Which Should You Order? Blood vs. Urine Toxicology LC-QTOF-MS Testing

Background & Introduction: Overdose rates remain at record levels, further necessitating interventions appropriate to patient condition and substance exposure. Polysubstance exposure, both intentional and unintentional, complicates clinical presentation, overdose management, and pathological sequelae. In the face of rising rates of polysubstance use and the appearance of novel psychoactive substances (e.g., nitazenes, novel benzodiazepines) in the illicit drug supply, there has been increased need and use of expanded comprehensive toxicology testing. Ascertaining the efficacy and limitations of testing measures in biological matrices is key to effective overdose treatment and local drug supply biosurveillance.

Methods: This study compared blood and urine toxicology testing in the setting of overdose. A consecutive sample of Emergency Department (ED) patients were prospectively recruited at the time of visit for unintentional drug overdose. Participants consented to provide blood and urine samples for study testing during the ED visit. Biological samples underwent comprehensive toxicology testing via liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS) at the local hospital toxicology laboratory. This testing method screened samples against an in-house library of 300+ drugs and metabolites, 200+ fentanyl analogs, and a shared library of 1700 additional pharmaceuticals. The untargeted data collection allows for rapid updates to the testing library upon discovery of novel substances, detection of unknown substances, and storage of testing results for retrospective review. Toxicology testing findings were grouped based on parent substance when possible and cross compared between biological matrices to highlight strengths, weaknesses, and limitations of blood versus urine testing across substances. A substance was considered unique if the substance was solely detected in either blood or urine, and not unique if detected in both matrices for an individual. Metabolites were grouped under parent compounds thus parent in one and metabolite in other would not meet the definition of unique.

Results: A total of 76 eligible participants were enrolled over seven months in 2022 and had both blood and urine LC-QTOF-MS toxicology testing performed. Among all study participants, 1287 total substances (parent and metabolites) were found in urine and 958 substances were found in blood. 176 substances were uniquely detected (indicating these substances were not detected in blood of the same individual) in urine and 99 substances were uniquely detected in blood. The most common unique urine findings were naloxone (30), xylazine (15), levamisole (13), cocaine (8), gabapentin (8), and methamphetamine (8). Common unique urine opioid findings included fentanyl (10), tramadol (8), buprenorphine (6), and morphine (4). The most common unique blood findings were caffeine (18), delta-9-THC (11), alprazolam (3), aripiprazole (3), diazepam (3), hydroxyzine (3), naloxone (3), oxycodone (3), diphenhydramine (2), and lidocaine (2). All 76 patients presented with polysubstance exposure. The median time from triage to urine and blood collection was 4.60 hours and 4.87 hours, respectively. In the study sample, fentanyl and fentanyl with xylazine were commonly detected on overdose: 68 (89%) participants had fentanyl detected and 26 (34%) participants had xylazine with fentanyl detected in either blood or urine.

Conclusion & Discussion: This study provides detailed comparative data of toxicology testing in blood and urine that can serve as a guide for the future development of testing protocols and support decision-making in the clinical setting where testing multiple biological matrices may not be practical or feasible. In the clinical management of overdose, urine testing is generally used over blood testing as it is less invasive and the window of detection is longer. However, if obtained early in the clinical course after overdose, blood samples may provide a better indicator of acute exposure. Characteristics of the specific testing method must be considered as some substances can be better detected in blood versus urine. Notably, in our results, benzodiazepines were detected more often in blood versus urine. Detailed knowledge of the testing method characteristics including strengths and weaknesses across biological matrices is essential to accurate result interpretation, particularly given the high rates of polysubstance exposure in participants with an ED visit for overdose. Though resource intensive, an in-house hospital-based LC-QTOF-MS method allowed for comprehensive analysis of a wide variety of substances in both blood and urine.

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