Start With the Assay Target

Opiates commonly refer to morphine and codeine, while semisynthetic and synthetic opioids are chemically distinct. A routine opiate immunoassay is commonly built around morphine-like structures. It should not be assumed to detect fentanyl, methadone, or buprenorphine. Oxycodone also commonly needs its own assay target.

An assay result is bounded by its antibody design, cutoff, and cross-reactivity package insert. Cross-reactivity is not a universal property of a drug class. A fentanyl assay may have distinct response characteristics for fentanyl and norfentanyl, and another manufacturer may use a different cutoff. Read the active method and reagent lot documentation rather than importing expectations from a different platform. In this case, a negative fentanyl screen means no assay signal met the 2 ng/mL cutoff for the tested fentanyl target in that specimen; this does not exclude exposure outside the target, cutoff, specimen timing, or detection window.

Federal workplace cutoffs and panel criteria are for covered federal programs. Clinical, hospital, and most workplace testing outside that framework use locally selected panels and cutoffs. Before interpreting not detected, verify that the ordered assay actually targets the drug or metabolite in question.