Fentanyl test strips: how they work, how to use them, and their blind spots

Fentanyl test strips (FTS) are an immunoassay — the same lateral-flow technology as a pregnancy test — repurposed for harm reduction. They are the single highest-value test in the current drug supply, and they have specific limits you must know. How to use them - Dissolve a small sample in water. The dilution matters: the usual guidance is about 1/4 teaspoon (or a small defined amount) of residue in a defined volume of water — follow the specific product's instructions, because the strip is calibrated to a concentration. - Dip the strip (do not submerge past the line), wait the stated time, read. - Counter-intuitive like all these strips: ONE line = positive (fentanyl detected), TWO lines = negative. A faint second line still counts as negative-for-fentanyl, but read it in good light. The blind spots - High-dose threshold effect ("hook effect"): at very high fentanyl concentrations some strips can paradoxically read falsely negative; test a diluted sample. - Analogue coverage is incomplete: many strips detect fentanyl and several analogues, but NOT all novel analogues, and typically NOT nitazenes (a separate class now in the supply). A negative strip does not mean "no potent opioid". - It tests the portion you dissolved. Fentanyl is not evenly mixed; a negative on one part of a batch does not clear the rest. The "chocolate chip cookie" problem. The real use Even with those limits, a positive is decisive information. Combine with: never use alone, have naloxone present, start low. General testing and safe-storage discipline: https://wiki.soapbox.community/wiki/Testing_Labelling_and_Safe_Storage_of_Dosed_Preparations Sources: CDC and FDA on FTS; dancesafe.org usage guidance.