Immunoassay screening is the first of the two-stage laboratory testing process. Antibodies engineered to bind particular drug classes produce a measurable signal when the target is present above the screening cutoff level. Specimens below the cutoff are reported negative and go no further; specimens at or above it are presumptive positives that must be confirmed with a specific method such as GC-MS before anything is reported. Because immunoassays react to drug classes, they can cross-react with structurally similar legal compounds — historically, some decongestants and even poppy seeds triggered presumptive positives — which is exactly why confirmation testing and MRO review exist. Instant point-of-collection test devices use the same immunoassay chemistry in a portable format.