"In a harm reduction setting, a FTS might be used to test the drug residue in a cooker or baggie for fentanyl before use of the drug. Our results show that the concentrations of diphenhydramine, methamphetamine, and MDMA commonly found in street drugs are at levels that could generate false positives on the FTS. Many cookers and small baggies hold about 0.75–1 mL of water. If we assume there is 5 mg of methamphetamine in the container that is diluted with 1 mL of water, the concentration of methamphetamine will be 5 mg/mL and would trigger a false positive on the FTS. If the residue were dissolved with 10 mL of water, the methamphetamine concentration would be 0.5 mg/mL and would render a true negative on the FTS. If the drug residue instead consisted of 95% methamphetamine and 5% fentanyl, the 10 mL dilution would ensure that the methamphetamine concentration would not interfere with the FTS while the true positive result would come from the fentanyl present in the sample. As practical guidance for harm reduction groups, a dilution with at least 50 mL of water will provide a good margin of error for accurate detection of fentanyl in cooker or powder residues while avoiding false positives from other drugs. Over dilution is not a likely problem; the FTS is sensitive enough that if there was just 0.5 mg of fentanyl residue in a cooker and it is dissolved in a 10-L bucket of water (50 µg/L or 50 ng/mL), the FTS will still detect the fentanyl present."
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). doi.org/10.1186/s12954-021-00478-4