Cross-reactivity

Cross-reactivity is the tendency of an immunoassay to respond to a compound other than its intended target because the two share a similar molecular structure.

Antibodies bind by shape, and compounds that resemble the target closely enough can occupy the same binding site and contribute to the signal. Manufacturers of test kits publish the degree to which known substances cross-react, and laboratories take this into account when interpreting screens. It is a property of the specific assay, so it varies from one kit to another.

Published work has reported that some cannabinoids, including CBN, can interfere with certain THC immunoassays, while CBD generally shows little cross-reactivity in the assays that have been studied. The practical consequence is that a screening result cannot on its own identify which compound triggered it, which is why confirmatory testing exists. Confirmatory methods identify compounds by mass and do not share this limitation. Whether any given product will affect any given screen cannot be predicted from its label.