This assay employs a two-site sandwich ELISA to quantitate sELAF in samples. An antibody specific for sELAF has been pre-coated onto a microplate. Standards and samples are pipetted into the wells and any sELAF present is bound by the immobilized antibody. After removing any unbound substances, a biotin-conjugated antibody specific for sELAF is added to the wells. After washing, Streptavidin conjugated Horseradish Peroxidase (HRP) is added to the wells. Following a wash to remove any unbound avidin-enzyme reagent, a substrate solution is added to the wells and color develops in proportion to the amount of sELAF bound in the initial step. The color development is stopped and the intensity of the color is measured.