The technique was developed in 1976 Yorde and his coworkers (1). In the competition ELISA for antibody detection, the test sample (containing free antibody) and enzyme-labelled antibody conjugate are simultaneously added to the antigen coated wells. They compete with each other for the antigen. If free antibody is present in the test sample it binds to the antigen and subsequent addition of substrate, after washing, does not elicit colour development. Hence, colour development in this assay is inversely proportional to the quantity of specific antibody in the test sample. In simple terms when the amount of the antigen or the antibody analyzed in the serum is low, high absorbance is obtained, whereas low absorbance is observed with greater quantities. Sera with preservatives like sodium azide are unsuitable for competition ELISA format, since the preservative may inactivate the enzyme. It is also known as a blocking/inhibition ELISA and perhaps the most complex of all the ELISA techniques. Different ELISA formats like direct, indirect, sandwitch all can be adapted to a competitive format. The competitive/inhibition ELISA is predominantly used to measure the concentration of an antigen or antibody in a sample by detecting interference in an expected signal output. Competitive ELISA is useful for the measurement of low molecular weight targets. Some competitive ELISA kits use labeled antigen instead of a labeled antibody. The labeled antigen and the sample antigen (unlabeled) compete for binding to the primary antibody. The lower the amount of antigen in the sample, the stronger the signal due to more labeled antigen in the well.

In a competitive/blocking ELISA, immunoglobulins from positive serum samples compete to inhibit the mAb from binding to its specific binding site on antigens and subsequently prevent color development whereas non-reactive sera produce a strong colored reaction. Competitive/blocking ELISAs have been confirmed to be significantly sensitivite and specific for detecting antibody to Bluetongsue Virus (2), West Nile virus (3), and Francisella tularensis (4), and even a higher sensitivity than i-ELISA for detection to Mycoplasma meleagridis (5) and PRRSV (6).