One brand of fentanyl test strips were examined using 84 drug samples containing fentanyl, fentanyl analogs, mixtures of heroin and fentanyl analogs, non-fentanyl synthetic opioids, NPS, traditional drugs of abuse and their adulterants and others prescription drugs used as adulterants.
Fentanyl Test Strips
One brand was used for this study: SIGNIFY ANALYTICS(FYL) Test Strip-Cut Off 20ng/ml
Preparation of Drug Solution for Fentanyl Test Strip Experiments
All drug solutions (listed in Table 3), were previously analyzed by GCMS. All were prepared in a range of six dilutions from 1000ug/ml to 0.01ug/ml in deionized water. (1000ug/ml, 100ug/ml, 10ug/ml, 1ug/ml, 0,1ug/mL, 0,01ug/ml).
[1ug/ml is equivalent to 1000ng/mL]
Fentanyl, fentanyl analogs, synthetic opioids and NPS were obtained from street samples. Several samples with the same compound were tested on the same range of solution as the amount of them are not known due to lack of analytical standards.
The cut-off listed at Table 1 has been chosen according to the low cut-off results of the different samples tested (These results are available at Table 3.) Analytical standards of traditional drugs of abuse and their adulterants and other prescription drugs were used for testing the strips.
Fentanyl Test Strip Experiments
The test strips were dipped into the drug solutions for 10-15 seconds according to instructions provided by other manufacturers and sources of information.
Test strips were placed in a non-absorbent flat surface and the result was interpreted at 5 minutes.
Interpretation of test results
According to the manufacturers instructions, the test should be considered negative when two colored bands appear (C and T band).
In the present study, it’s described selectivity and sensitivity (Table 1 and Table 2) of the fentanyl test strips, as well as cross-reactivity and false positives.
As expected, all tests were positive (Fig.1) for fentanyl for all fentanyl and fentanyl analogues. Therefore these Fentanyl Strips are selective for Fentanyl and most Fentanyl analogs.
➢ Fentanyl Analogs
Specificity is the ability to assess unequivocally the analyte in the presence of components which may be expected to be present. These strips have a low specificity for fentanyl, as they are also reacting positively to other fentanyl analogs.
All these positives can contribute positively under a harm reduction perspective when detecting fentanyl and fentanyl analogs as adulterants.
➢ Other Substances
The strip bands generated negative results (Fig.1) when non fentanyl synthetic opioids were tested, excepting U-47700 (false positive), as well as NPS, excepting FUB-AMB (synthetic cannabinoid with false positive), traditional drugs of abuse and their adulterants and others prescription drugs used as adulterants.
Even U-47700 is a synthetic opioid not related to fentanyl chemical structure, this positive might be beneficial in case of its presence as adulterants.
In the case of nitazene-type opioids tested (Fluonitazene, Butonitazene. Metonitazene, Etonitazepine), results were negative. These substances are potent as other fentanyls.
➢ False Positives
The FTS were assessed to determine prevalence of false positives when tested with methamphetamine and MDMA as scientific publications have stated this (Ref. 2 and 3) where common stimulants and cutting agents can create positive results.
MDMA was given a negative result, a faint band appeared, although methamphetamine gave positive to FTS at 20mg/m, but negative at 10ml/mL.
The sensitivity for each compound and mixture of compounds present in the tested samples is detailed in table 1.
These strips (20 ng/mL) have more sensitivity for fentanyl and all fentanyl analogs than other strips (200 ng/mL) with a lower cut-off. For most fentanyl analogs, the sensitivity of the 20 ng strips has improved by lowering the cut-off.
A limitation for the test strips was the low sensitivity for detecting ultra-potent fentanyl analogue carfentanil (cut-off 200 ug/mL).
In summary the fentanyl test strips tested displayed a broad selectivity, detecting all fentanyl analogues present on the recreational drug market. None of the fentanyl strips were affected by the presence of heroin, illicit or other pharmaceutical drugs.
Very high concentrations of various non-fentanyl compounds can give false positives. To avoid this situation and as a practical guidance for testing samples with FTS, we would recommend diluting the sufficient amount of the sample as a grain of rice into 10mL of water. Over diluting is not a problem, the FTS is sensitive enough for fentanyl detection.
These strips have more sensitivity for detection of a wider range of fentanyl analogs.
The immunoassay test strips are a reliable detection method for fentanyl at low concentrations and complex matrices.
TABLE 1: Summary Positive Compounds and Cut-off Level
TABLE 2: Summary Negative Compounds
TABLE 3: Working Table
- Bergh MS-S, Øiestad ÅML, Baumann MH et al. (2021). Selectivity and sensitivity of urine fentanyl test strips to detect fentanyl analogues in illicit drugs. International Journal of Drug Policy 90. 10.1016/j.drugpo.2020.103065.
- Park JN, Sherman SG, Sigmund V, Breaud A, Martin K, Clarke WA. Validation of a lateral flow chromatographic immunoassay for the detection of fentanyl in drug samples. Drug Alcohol Depend. 2022 Nov 1;240:109610. doi: 10.1016/j.drugalcdep.2022.109610. Epub 2022 Aug 31. PMID: 36115223; PMCID: PMC9588631.
- Lockwood, TL.E., Vervoordt, A. & Lieberman, M. High concentrations of illicit stimulants and cutting agents cause false positives on fentanyl test strips. Harm Reduct J 18, 30 (2021). https://doi.org/10.1186/s12954-021-00478-4
- Evaluation of fentanyl test strips. Scanner project (European Union’s Justice Programme- Drugs Policy initiatives)