Immunoassay
EIAAn immunoassay is a rapid screening test that uses antibodies to detect whether a class of molecules is present in a sample above a set concentration.
Antibodies are chosen to bind a target such as the THC metabolite THC-COOH, and a color, light or electrical change signals binding. The method is fast, inexpensive and suited to screening many samples, which is why most workplace urine drug tests begin with it. It reports a result relative to a cutoff rather than a precise amount.
Because antibodies recognize molecular shape, related compounds can bind the same antibody, a property known as cross-reactivity. Standard panels are aimed at THC metabolites, not at CBD, but other cannabinoids can in principle interfere with some assays. A screen that reads at or above the cutoff is called non-negative and is sent for confirmatory testing by mass spectrometry, which identifies exactly what is present. The screening result alone does not establish which compound triggered it, and no product can promise a particular screening outcome.