How does competitive ELISA work?

In a competitive ELISA, the antigen in a sample competes with a reference antigen for binding to a specific antibody. As the amount of sample antigen increases, less antibody binds to the reference antigen, producing a lower signal.

Competitive ELISA is often used for small antigens or when only one suitable antibody is available. Unlike most ELISA formats, its signal is typically inversely proportional to the amount of target in the sample.