Abstract
Background
Drug overdose deaths remain the primary cause of unintentional injuries in the United States. We examined the validity of a fentanyl test strip (FTS) in detecting fentanyl and its related analogs in water-based illicit drug solutions.
Methods
Illicit drugs obtained from law enforcement (N=343) were tested using a lateral flow chromatographic immunoassay FTS and liquid chromatography-tandem mass spectrometry (LC-MS/MS) at a clinical chemistry laboratory in Baltimore, Maryland.
Results
The FTS assay detected fentanyl at 200ng/mL in water, detected 13 additional fentanyl analogs, and failed to detect carfentanil and furanyl fentanyl at or below 1000 ng/mL. Overall sensitivity and specificity for detecting fentanyls was 98.5% and 89.2%; and the false negative and positive rate was 1.5% and 10.9%. False negatives (n=2) occurred in fentanyl and a fentanyl precursor. False positives (n=23) occurred in the presence of other illicit drugs and compounds (56.5%) or when fentanyls were present below 40ng/mL (43.5%). False positive/negative rates remained low when testing cocaine and prescription opioid samples.
Conclusions
FTS is a valid drug checking tool, however, rapid immunoassays and other drug checking instruments that can detect a wider range of fentanyl analogs including carfentanil need to be prioritized to minimize accidental exposure to the full spectrum of fentanyls.
Keywords: Harm reduction, Drug testing, Overdose, Opioids, Substance Use
Introduction
Unintentional injuries were the fourth leading cause of death in the U.S. in 2021 behind heart disease, cancer and COVID-19.1 During this period, drug overdose deaths were the primary cause of unintentional injuries and resulted in 107,622 deaths. The majority (66%) of these deaths involve illicitly manufactured fentanyl (IMF) and other potent and lethal synthetic opioids, including carfentantil.1 In an analysis of 10 Eastern and Midwest states conducted in 2017, 14 fentanyl-related compounds (i.e., analogs) were involved in 21% of opioid overdose deaths and 1,236 of these deaths were due to carfentanil, which is 10,000 times more potent than morphine.2 Accidental overdoses are driven by an underlying unsafe drug supply that lacks quality controls and federal oversight. Drug suppliers are meeting high societal demand for controlled substances and counterfeit prescription opioids through illicit channels. Without providing access to drug checking programs or access to laboratory testing services that provide an empirical method of testing drug samples, both consumers and suppliers will continue to face challenges in predicting the contents of their drugs, which in turn heightens the risk of drug morbidity and mortality.3,4
The Centers for Disease Control and Prevention (CDC) and the Substance Abuse and Mental Health Services Administration (SAMHSA) recently authorized the use of federal funds to support fentanyl test strip (FTS) distribution—a drug checking method that has shown preliminary validity and acceptability—given the magnitude of deaths involving illicit fentanyl. FTS are low-cost (~1 USD per strip) lateral flow chromatographic immunoassay tests originally designed for the rapid qualitative detection of fentanyl in urine but that has been used increasingly to test water-based drug solutions prior to drug consumption. This harm reduction tool allows people who use drugs (PWUD) and drug suppliers to chemically analyze drug solutions (i.e., drugs dissolved in water) rather than urine prior to use to rapidly detect the presence of fentanyl and a number of fentanyl analogs in the community. Although testing is usually conducted prior to use, for people who use multiple times a day, testing after a negative reaction can also be informative. Studies have shown that FTS programs are acceptable and feasible from the perspective of PWUD5,6,7 and service providers8 and help to build community trust, raise awareness, and promote risk reduction behaviors. The first North American FTS programs have been implemented within syringe services programs3,9 and supervised injection facilities,10 however, programs have expanded to emergency rooms and drug treatment centers in recent years.
A few validation studies have been conducted on the Rapid Response Fentanyl Single Drug Test Strip (BTNX, Canada; cut off 20ng/mL), which are in circulation throughout North America. Initial studies have shown that the accuracy of this FTS is moderate to high11,12 though affected by the concentration of fentanyl present, with false negatives detected at concentrations at or less than 5% in solution.13 There is also evidence of cross-reactivity of the strips with 13 fentanyl analogs,14 as well as with amphetamines and diphenhydramine at concentrations at or above 1mg/mL.15 Key gaps in our knowledge on the validity of this highly-utilized immunoassay include which fentanyl analogs can be detected and whether there are higher false positive and negative rates when testing specific drugs (e.g., cocaine, counterfeit prescription opioids). Our study examined: (1) the limit of detection (LOD) of FTS for fentanyl and 17 analogs using high-precision Liquid Chromatography with Tandem Mass Spectrometry (LC-MS/MS); and (2) the sensitivity and specificity of FTS in detecting IMF compared to LC-MS/MS in a heterogeneous sample of illicit drugs, including cocaine and prescription opioids; and (3) the reasons why false positives or negatives may be triggered when using this immunoassay.
Methods
Study Design
The study consisted of three experiments. In Experiment 1, we established the lowest detection limit of the FTS for a range of fentanyls. Through Experiment 2, we calculated the overall sensitivity and specificity of the FTS at detecting the presence of fentanyls dissolved in 5mL of sterile water. Experiment 3 consisted of quantitative testing of samples that generated false positive or negative results to determine if the concentration of fentanyl present impacted the results obtained.
Instruments and Setting
All FTS and LC-MS/MS testing was conducted at the Johns Hopkins Advanced Clinical Chemistry Diagnostics Laboratory (ACCDL) in Baltimore, Maryland. The ACCDL is licensed by the State of Maryland and the Drug Enforcement Agency for testing of controlled substances (schedule I through V) and agreed to comply with local regulations to conduct this study. The study team also established a Material Transfer Agreement with the Baltimore City Police Department in order to obtain drug samples and testing data. Experiments were conducted according to guidelines established by the Scientific Working Group for Forensic Toxicology.
We tested the validity of a widely used low-cost lateral flow chromatographic fentanyl immunoassay (Rapid Response FTS, BTNX, Canada) with a cut-off of 20ng/mL when used for testing urine. We adhered to BTNX’s instructions when using the FTS (e.g., dipping the strip vertically into a sterile container, waiting 10 to 15 seconds, laying the strip to dry on a flat sterile surface, and recording the result at 5 minutes; one line = “positive”; two lines including a faint second line = “negative”; other = “indeterminate”). Definitive LC-MS/MS testing was conducted using dilutions of the ethanolic solutions provided by the BPD (Thermo Fisher Vanquish U-HPLC System and Thermo Fisher Vantage Triple Quadrupole Mass Spectrometer from Thermo Electron Northpoint Parkway, Suite 100, West Palm Beach, FL, 33407) with a LOD cutoff of 40ng/mL.
Experiment 1: LOD study using laboratory-grade fentanyl and fentanyl analogs
We conducted serial dilution analysis of fortified solutions of laboratory-grade fentanyl and high-priority fentanyl analogues as reported by the Baltimore City Police Department from 2015 to 2019 as well as the National Forensic Laboratory Information System (NFLIS) from 2015 to 2020, as well as known blank samples using both FTS and LC-MS/MS. The tested analogs are listed in Table 1; they were purchased from Cayman Chemical Company. Some analogs were not available for purchase and were excluded from the study. We tested concentrations at or below 1000 ng/mL. The experiment enabled the establishment of the LOD; the LOD is the lowest concentration of an analyte that consistently yields signal greater than the average signal of the blank. We also tested the repeatability of readings by confirming the results in triplicate.
Table 1:
Detection Limits Established using Laboratory-Grade Fentanyl and Fentanyl Analogs for the BTNX Rapid Response Fentanyl Single Drug Test Strip (20ng/ml)
| JHU (Clinical Chemistry Laboratory) |
BTNX (Manufacturer) |
|
|---|---|---|
| Country | United States | Canada |
| Solvent | Water | Urine |
| Fentanyl | 200 ng/ml | 20 ng/ml |
| Fentanyl Analogs reported by BTNX that were tested | ||
| Acetyl Fentanyl | 100 ng/ml | 150 ng/ml |
| Butyryl Fentanyl | 100 ng/ml | 700 ng/ml |
| Carfentanil | Not detected* | 1,000 ng/ml |
| Furanyl Fentanyl | Not detected* | 500 ng/ml |
| Valeryl Fentanyl | 500 ng/ml | 700 ng/ml |
| Fentanyl Analogs not reported by BTNX that were tested | ||
| 4+ANPP | 200 ng/ml | -- |
| Alfentanyl | Not detected* | -- |
| Alpha+methylacetyl fentanyl | 500 ng/ml | -- |
| B+methylfentanyl | 500 ng/ml | -- |
| FIBF | 200 ng/ml | -- |
| FBF | 100 ng/ml | -- |
| n+methyl norfentanyl | 500 ng/ml | -- |
| norfentanyl | Not detected* | -- |
| ortho+methylacetyl fentanyl | 100 ng/ml | -- |
| para+chloroisobutyryl fentanyl | 1000 ng/ml | -- |
| phenyl fentanyl | 500 ng/ml | -- |
| thiofentanyl | 200 ng/ml | -- |
| Fentanyl Analogs reported by BTNX that were not tested | ||
| 3-Methyl Fentanyl | -- | 500 ng/ml |
| Ocfentanil | -- | 250 ng/ml |
| p-Fluoro Fentanyl | -- | 200 ng/ml |
| Remifentanil | -- | 70,000 ng/ml |
| Sufentanil | -- | 100,000 ng/ml |
at 1000 ng/ml or less
Experiment 2: Qualitative testing using illicit drug samples from law enforcement seizures
Three hundred and forty-three eligible illicit drug samples (i.e., those containing any controlled dangerous substance in powder, crystal according to GC/MS analysis) were delivered by the Baltimore City Police Department to ACCDL. These ethanolic solutions were evaporated the resultant solid was prepared in a solution of 5mL of sterile water and tested by blinded technicians using FTS and LC-MS/MS as specified above to calculate the sensitivity, specificity, false positive rate and false negative rate by drugs tested.
Experiment 3: Quantitative testing using drug samples from law enforcement seizures
Additionally, the ACCDL conducted quantitative testing using Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS) on all samples that emerged as a false positive or negative to explore whether the detection ability of the FTS was impacted by IMF concentration.
Data Analysis
The GC/MS dataset from the Baltimore City Police Department was linked to the resultant FTS and LC-MS/MS data using a pre-assigned alpha-numeric laboratory ID and analyzed by the first author who was not involved in the experiments. The analysis excluded n=5 samples that only contained novel fentanyl analogs that were not designed to be detected by either the FTS or GC/MS: para-Fluorobutyryl fentanyl / 4-FBF / FBF; para-Fluoroisobutyrfentanyl / 4-FIBF / FIBF; Para-chloroisobutyryl fentanyl / CIBF. Some drug categories in Experiment 2 were not able to be assessed due to low sample size.
Results
i. LOD
We found that this FTS assay detected fentanyl at 200 ng/mL in water-based solution (Table 1). Additionally, the assay was able to detect the following 13 fentanyl analogs at or below 1000 ng/mL (among 17 analogs tested): acetyl fentanyl, butyryl fentanyl, valeryl fentanyl, alpha-methylacetyl fentanyl, b-methylfentanyl, para-fluorobutyryl fentanyl, para-fluoroisobutyrfentanyl, n-methyl norfentanyl, ortho-methylacetyl fentanyl, para-chloroisobutyryl fentanyl, phenyl fentanyl, thiofentanyl. The assay failed to detect two fentanyl analogs in water that were reported as detectable by BTNX (i.e., in urine samples): carfentanil and furanyl fentanyl; the assay also failed to detect two additional fentanyl analogs that were included for testing (alfentanyl and norfentanyl).
ii. Sensitivity and Specificity of FTS
We also tested 343 drug samples using FTS and LC-MS/MS (Table 2). Overall sensitivity of FTS for detecting fentanyls relative to LC-MS/MS was 98.5% and the false negative rate was 1.5%. Specificity was 89.2% and the false positive rate was 10.9%. These analyses excluded novel fentanyl analogs (see Methods). A comparison to the GC/MS data provided by the Baltimore City Police Department yielded similar findings.
Table 2:
Sensitivity and Specificity of BTNX Rapid Response Fentanyl Single Drug Test Strip (20ng/ml) for Fentanyl and Fentanyl Analogs using Drug Samples Provided by Baltimore City Law Enforcement
| JHU (Clinical Chemistry Laboratory) |
BPD (Forensic Laboratory) |
|
|---|---|---|
| Country | United States | United States |
| Solvent | Water | Ethanol |
| Instrument | Liquid Chromatography | Gas Chromatography |
| Sample size* (n) | 343 | 343 |
| Contained fentanyl/fentanyl analogs | 131 | 132 |
| Did not contain fentanyl/fentanyl analogs | 212 | 211 |
| Sensitivity (%) | 98.5 | 98.5 |
| False negative rate (%) | 1.5 | 1.5 |
| Specificity (%) | 89.2 | 89.6 |
| False positive rate (%) | 10.9 | 10.4 |
We excluded n=5 samples that only contained novel fentanyl analogs: para-Fluorobutyryl fentanyl / 4-FBF / FBF; para-Fluoroisobutyrfentanyl / 4-FIBF / FIBF*; Para-chloroisobutyryl fentanyl / CIBF
Sensitivity and specificity were also calculated by drug type (Table 3). We were able to obtain sensitivity estimates for detecting fentanyls in samples containing cocaine, prescription opioids, heroin, and 4-ANPP (fentanyl precursor). Specificity was calculated for samples containing cocaine, prescription opioids, methamphetamine, prescription benzodiazepines, synthetic cathinones and cannabinoids. Sensitivity and specificity for these select categories ranged from 90.5%-100.0%.
Table 3:
Sensitivity and Specificity of BTNX Rapid Response Fentanyl Single Drug Test Strip (20ng/ml) for Fentanyl and Fentanyl Analogs using Drug Samples Provided by Baltimore City Law Enforcement - By Drug Category
| Stimulants (Cocaine) | Prescription Opioids (oxycodone, tramadol, morphine) |
||
|---|---|---|---|
| Sample size (n) | 72 | Sample size (n) | 40 |
| Cocaine only | 66 | PO only | 20 |
| Cocaine and fentanyls | 5 | PO with fentanyls | 19 |
| Cocaine with all other CDS | 1 | PO with other CDS | 1 |
| Sensitivity (%) | 100.0 | Sensitivity (%) | 100.0 |
| False negative rate (%) | 0.0 | False negative rate (%) | 0.0 |
| Specificity (%) | 97.0 | Specificity (%) | 90.5 |
| False positive rate (%) | 3.0 | False positive rate (%) | 9.5 |
| Stimulants (Methamphetamine) |
Prescription Benzodiazpines (alprazolam,
clonazepam) |
||
| Sample size (n) | 25 | Sample size (n) | 10 |
| Methamphetamine only | 24 | PB only | 10 |
| Methamphetamine with fentanyls | 1 | ||
| Sensitivity (%) | – | Sensitivity (%) | -- |
| False negative rate (%) | – | False negative rate (%) | -- |
| Specificity (%) | 100.0 | Specificity (%) | 100.0 |
| False positive rate (%) | 0.0 | False positive rate (%) | 0.0 |
| Synthetic Cathinones (eutylone) | Heroin | ||
| Sample size (n) | 17 | Sample size (n) | 47 |
| Synthetic cathinones only | 17 | Heroin only | 2 |
| Heroin and fentanyls | 45 | ||
| Sensitivity (%) | -- | Sensitivity (%) | 100.0 |
| False negative rate (%) | -- | False negative rate (%) | 0.0 |
| Specificity (%) | 100.0 | Specificity (%) | – |
| False positive rate (%) | 0.0 | False positive rate (%) | – |
| Cannabinoids (delta-9-THC, cannabinoid, CBD) | 4-ANPP (fentanyl precursor) | ||
| Sample size (n) | 26 | Sample size (n) | 87 |
| Cannabinoids only | 25 | 4-ANPP with fentanyls | 87 |
| Delta-9-THC with fentanyl | 1 | ||
| Sensitivity (%) | – | Sensitivity (%) | 98.9 |
| False negative rate (%) | – | False negative rate (%) | 1.1 |
| Specificity (%) | 96.0 | Specificity (%) | -- |
| False positive rate (%) | 4.0 | False positive rate (%) | -- |
We excluded para-Fluorobutyryl fentanyl / 4-FBF / FBF; para-Fluoroisobutyrfentanyl / 4-FIBF / FIBF*; Para-chloroisobutyryl fentanyl / CIBF
iii. Quantification Testing to Explore False Positives and False Negatives
Quantification testing showed that many false positives (n=23) occurred when fentanyls were present below the LC-MS/MS cutoff of 40 ng/mL (43.5%; 10/23). False positive tests also occurred in the presence of other CDS (30.4%; 7/23), including cocaine (n=2), oxycodone (n=1), tramadol (n=1), heroin (n=3), delta-9-THC (n=1), MDMA (n=1), or non-CDS compounds (26.1%; 6/23). In the two false negative cases, one sample contained fentanyl only (concentration 227 ng/mL) and one sample contained both fentanyl and 4-ANPP (concentration 1592 ng/mL).
Discussion
Our findings demonstrate that the BTNX Rapid Response FTS detects fentanyl and a select number of analogs in water-based drug solutions. The overall LOD, sensitivity and specificity of this FTS were within the range reported by other laboratory studies.11,12,15 Consistent with previous findings,14 the FTS did not detect carfentanil, the most potent synthetic opioid involved in overdose deaths, as well as several novel fentanyl analogs (e.g., FBF, FIBF). FTS do not detect other dangerous non-fentanyl synthetic opioids (e.g., U-47700, U-48800, U-50488).14 Taken together, these findings are concerning since carfentanil is still in circulation and newer synthetic opioids continue to emerge each year. Given this, multi-drug panels that include a carfentanil assay could be useful to distribute. However, more sophisticated drug hecking programs, such as those that utilize fourier transform infrared spectroscopy or portable gas chromatography-mass spectrometry will be needed to keep up with the evolving drug supply over time.
We were able to detect fentanyls in the presence of cocaine, prescription opioids, and heroin, which has implications for FTS programs targeting individuals who seek these substances. While false positives were more common than false negatives, public health messaging should continue to emphasize that FTS, along with other drug checking services, are not 100% accurate and cannot detect the presence of all potentially dangerous chemicals within a sample. A study limitation –like previous validation studies—is that fentanyl contamination is a relatively rare event; thus investment in larger studies with more fentanyl-contaminated drug samples will be necessary in order to advance the science on drug checking programs to reduce drug-related morbidity and mortality. Other researchers have also highlighted the potential of creating legalized and regulated drugs markets to create safer environments, as seen for marijuana, nicotine and alcohol, which would greatly reduce uncertainty in the drug supply and the need for drug checking programs.16
The Rapid Response FTS also performed well in screening out the absence of fentanyls in cocaine, prescription opioids, methamphetamine, prescription benzodiazepines, synthetic cathinones and cannabinoids though more sensitivity studies will be required among rarer drug combinations. Given the risk of false positives in the presence of methamphetamine, MDMA and diphenhydramine at concentrations at or above 1 mg/mL,15 communicating clear instructions for diluting samples prior to testing is critical. A resource for FTS implementors has been recently developed by Dancesafe, which describes the appropriate uses and caveats of this immunoassay.17
FTS can detect a range of fentanyls in illicit drugs. They may be most useful among those who wish to avoid fentanyl, such as cocaine and counterfeit prescription opioid users. Further studies will help assess the value of FTS-based strategies however, FTS alone will not be sufficient in detecting all circulating fentanyls. While an ELISA-based test for carfentanil exists;18 developing and scaling up low-cost simple immunoassay strips that detect carfentanil and other novel synthetic opioids in the drug supply may achieve greater coverage given that many communities are already educated on FTS utilization; in the long term, moving towards comprehensive drug checking programs with time-updated assays and libraries will provide a better understanding of the drug supply to inform prevention efforts. Legal barriers prohibiting the possession and distribution of drug samples, drug paraphernalia, and drug checking tools will need to be re-examined in order to support successful FTS implementation.
Acknowledgements:
We would like to thank BTNX and the Baltimore City Crime Laboratory for their partnership on this project.
Disclosures and funding:
This study was supported by the National Institute on Drug Abuse (5R03DA049998). Dr. Park is funded by the COBRE on Opioids and Overdose (P20GM125507) from the NIH and serves as a technical consultant for a cooperative grant between Harvard Medical School and the Food and Drug Administration (U01FD00745501). The contents of this manuscript are solely the responsibility of the authors and do not necessarily represent the official views of the funders.
References
- 1.Ahmad FB, Cisewski JA, Miniño A, & Anderson RN (2021). Provisional mortality data—united states, 2020. Morbidity and Mortality Weekly Report, 70(14), 519. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.O’Donnell J, Gladden RM, Mattson CL, & Kariisa M (2018). Notes from the field: overdose deaths with carfentanil and other fentanyl analogs detected—10 states, July 2016–June 2017. Morbidity and Mortality Weekly Report, 67(27), 767. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Maghsoudi N, Tanguay J, Scarfone K, Rammohan I, Ziegler C, Werb D, & Scheim AI (2021). Drug checking Services for People who use Drugs: a systematic review. Addiction. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4.Palamar JJ, Salomone A, & Barratt MJ (2020). Drug checking to detect fentanyl and new psychoactive substances. Current opinion in psychiatry, 33(4), 301. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 5.Peiper NC, Clarke SD, Vincent LB, Ciccarone D, Kral AH, & Zibbell JE (2019). Fentanyl test strips as an opioid overdose prevention strategy: findings from a syringe services program in the Southeastern United States. International Journal of Drug Policy, 63, 122–128. [DOI] [PubMed] [Google Scholar]
- 6.Krieger MS, Goedel WC, Buxton JA, Lysyshyn M, Bernstein E, Sherman SG, Rich JD, Hadland SE, Green TC and Marshall BD, 2018. Use of rapid fentanyl test strips among young adults who use drugs. International Journal of Drug Policy, 61, pp.52–58. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Sherman SG, Morales KB, Park JN, McKenzie M, Marshall BD, & Green TC (2019). Acceptability of implementing community-based drug checking services for people who use drugs in three United States cities: Baltimore, Boston and Providence. International Journal of Drug Policy, 68, 46–53. [DOI] [PubMed] [Google Scholar]
- 8.Glick JL, Christensen T, Park JN, McKenzie M, Green TC, & Sherman SG (2019). Stakeholder perspectives on implementing fentanyl drug checking: Results from a multi-site study. Drug and alcohol dependence, 194, 527–532. [DOI] [PubMed] [Google Scholar]
- 9.Park JN, Frankel S, Morris M, Dieni O, Fahey-Morrison L, Luta M, Hunt D, Long J and Sherman SG, 2021. Evaluation of fentanyl test strip distribution in two Mid-Atlantic syringe services programs. International Journal of Drug Policy, 94, p.103196. [DOI] [PubMed] [Google Scholar]
- 10.Karamouzian M, Dohoo C, Forsting S, McNeil R, Kerr T and Lysyshyn M, 2018. Evaluation of a fentanyl drug checking service for clients of a supervised injection facility, Vancouver, Canada. Harm reduction journal, 15(1), pp.1–8. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 11.Green TC, Park JN, Gilbert M, McKenzie M, Struth E, Lucas R, Clarke W and Sherman SG, 2020. An assessment of the limits of detection, sensitivity and specificity of three devices for public health-based drug checking of fentanyl in street-acquired samples. International Journal of Drug Policy, 77, p.102661. [DOI] [PubMed] [Google Scholar]
- 12.Ti L, Tobias S, Lysyshyn M, Laing R, Nosova E, Choi J, Arredondo J, McCrae K, Tupper K and Wood E, 2020. Detecting fentanyl using point-of-care drug checking technologies: a validation study. Drug and alcohol dependence, 212, p.108006. [DOI] [PubMed] [Google Scholar]
- 13.McCrae K, Tobias S, Grant C, Lysyshyn M, Laing R, Wood E, & Ti L (2020). Assessing the limit of detection of Fourier-transform infrared spectroscopy and immunoassay strips for fentanyl in a real-world setting. Drug and Alcohol Review, 39(1), 98–102. [DOI] [PubMed] [Google Scholar]
- 14.Bergh MSS, Øiestad ÅML, Baumann MH, & Bogen IL (2021). Selectivity and sensitivity of urine fentanyl test strips to detect fentanyl analogues in illicit drugs. International Journal of Drug Policy, 90, 103065. [DOI] [PubMed] [Google Scholar]
- 15.Lockwood TLE, Vervoordt A, & Lieberman M (2021). High concentrations of illicit stimulants and cutting agents cause false positives on fentanyl test strips. Harm reduction journal, 18(1), 1–9. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 16.Ivsins A, Boyd J, Beletsky L, & McNeil R (2020). Tackling the overdose crisis: the role of safe supply. International Journal of Drug Policy, 80, 102769. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 17.DanceSafe. Accessed on 06/21/22. https://dancesafe.org/product/fentanyl-testing-strips-instruction-sheets/
- 18.Wharton RE, Casbohm J, Hoffmaster R, Brewer BN, Finn MG, & Johnson RC (2021). Detection of 30 fentanyl analogs by commercial immunoassay kits. Journal of analytical toxicology, 45(2), 111–116. [DOI] [PubMed] [Google Scholar]