Fentanyl test strips are an important tool for detecting opioids that can cause fatal overdoses.
By testing over 200 fentanyl analogs with two brands of fentanyl test strips (FTS), Kathleen Hayes found that the test strips can detect many, but not all, fentanyl analogs. The types of analogs that a given FTS can detect are related to the hapten used to elicit the strip's antibodies. This work is now published in Harm Reduction Journal.
Carfentanil is a fentanyl analog 50 times stronger than fentanyl. In the photo below, one brand of test strips (BTNX) shows one red line which means it detects carfentanil, and the other brand shows two red lines, which means it does not.