Competitive binding assays having improved linearity

A method for providing an improved homogeneous competitive binding assay for quantifying an analyte in a sample, wherein the steps of the assay method comprise the following:

• a) preparing a reaction mixture comprising the sample and a plurality of reagents;
• wherein the plurality of reagents includes:
• a labeled competitive binding compound,
• an antibody specific for the analyte, and an aqueous solvent;
• and
• b) measuring the amount of labeled competitive binding compound bound to the antibody in the reaction mixture as a measure of the amount of the analyte in the sample;
• wherein the improvement comprises including in the plurality of reagents a predetermined amount of unlabeled competitive binding compound, wherein, upon addition to the reaction mixture, the predetermined amount still permits the analyte in the sample to bind the antibody, and improves the linearity of the relationship between the amount of analyte present in the sample and the amount of analyte measured in step b);
• and wherein the steps of the method for providing the improved assay comprise the following:
• i) conducting the assay with samples containing varying known amounts of the analyte in the sample in the absence of any additional unlabeled competitive binding compound in the reaction mixture;
• ii) calculating the linearity of the relationship of the assay in step i) between the known amount of the analyte in the sample and the amount of analyte measured in step b) of the assay;
• iii) conducting the assay with samples containing varying known amounts of the analyte in the sample in the presence of a fixed amount of unlabeled competitive binding compound in the reaction mixture;
• iv) calculating the linearity of the relationship of the assay in step iii) between the known amount of the analyte in the sample and the amount of analyte measured in step b) of the assay;
• v) selecting the fixed amount of unlabeled competitive binding compound in step iii) as the predetermined amount for inclusion in the plurality of reagents in the improved assay if the linearity calculated in step iv) is improved in comparison with the linearity calculated in step ii).