This assay employs the quantitative sandwich enzyme immunoassay technique. Antibody specific for AT III has been pre-coated onto a microplate. Standards and samples are pipetted into the wells and any AT III present is bound by the immobilized antibody. After removing any unbound substances, a Horseradish Peroxidase (HRP)-conjugated antibody specific for AT III is added to the wells. Following a wash to remove any unbound reagent, a substrate solution is added to the wells and color develops in