Since the colorimetric signals result solely from binding of HRP-Streptavidin to biotinylated peptides, signal intensities measured in the assay are directly proportional to the amount of biotinylated peptide bound to the capture antibody and are inversely proportional to the amount of unlabeled target protein/peptide in standards or samples. A calibration curve can be established then using signal intensities resulting from competition by known concentrations of unlabeled peptide standards, and the concentration of target protein/peptide in each sample can be calculated based upon comparison of colorimetric signal intensities to those of the known standards..