Call us at: 888-217-2719. Fentanyl residue testing utilizes sophisticated lab techniques to detect even the tiniest amounts of the drug. Samples are analyzed using mass spectrometry methods to identify fentanyl’s chemical signature. This provides definitive identification and rules out false positives. The limits of detection can reach as low as 1 nanogram per sample. State-of-the-art equipment and experienced technicians are critical to getting accurate results from minute fentanyl residues.