Competitive and non-competitive immunoassays are two ways to measure substances in the body using antibodies. In a competitive immunoassay, the substance in the sample competes with a labeled version of itself to bind to an antibody so the more substance present, the weaker the signal you get. In contrast, a non-competitive (or sandwich) immunoassay uses two antibodies to “trap” the substance, and the signal increases as the amount of substance increases. Simply put, competitive tests show less signal with more substance, while non-competitive tests show more signal with more substance.