"In order to help prevent overdoses, lateral flow immunoassay test strips originally designed for monitoring traces of fentanyl and its analogs in urine are being explored as a drug checking technology in harm reduction contexts [17,18,19,20]. One commonly used fentanyl test strip or “FTS” (BTNX Inc., Markham, ON, Canada) is a lateral flow chromatographic immunoassay for the qualitative detection of fentanyl in urine at the cutoff concentration of 20 ng/mL. A positive result on this test strip gives one line, a negative result gives two lines, and an invalid test gives either no line or no control line . The “off label” use of the FTS in a harm reduction context involves preparation of a solution of the drug to be checked. For example, the residue in a cooker or baggie may be dissolved in a little water and then tested with the FTS. BTNX Inc. provides information about specificity of their test strip response, but for fentanyl 20 ng/mL FTS, the only drugs tested were fentanyl (detected at 20 ng/mL in urine) and norfentanyl (detected at 375 ng/mL in urine). In addition, a suite of pharmaceuticals were found to be non-interfering at levels of 100 ug/mL in a urine matrix [21, 22]. We have found that common stimulants and cutting agents that are often present in illicit drugs can create false positives. The problem arises from the cross-reactivity of the antibody for these other substances . Although the affinity of the antibody for these substances is much lower than for fentanyl, if they are present at sufficiently high concentrations, they can cause a false positive result [24, 25]. As we consider the 4th wave of the pandemic, it can be expected that drug users will need to test stimulants to see if they contain fentanyl."
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