Method and element for measuring analytes in biological fluids using immobilized binder-analyte labeled complex

A method for measuring analytes in biological fluids which comprises:

• (1) covalently immobilizing a specific antibody binder to a given analyte on a solid phase support;
• (2) saturating the binding sites on said specific antibody binder with a labeled analyte to the extent steric hinderance permits to form an immobilized specific antibody binder-analyte labeled complex;
• (3) saturating remaining unoccupied binding sites on the immobilized specific antibody binder-analyte labeled complex with unlabeled analyte prior to contacting said complex with a biological specimen;
• (4) contacting a sample of biological fluid to be analyzed for the presence of the given analyte with said immobilized complex, said sample of biological fluid as contacted with said immobilized complex being in an untreated form as obtained from the donor;
• and
• (5) and allowing an analyte, if present in said sample, to compete with the labeled analyte bound to the immobilized binder for binding sites on said binder thus displacing a given amount of labeled analyte which is directly proportional to the amount of analyte present in the sample,
• wherein the affinity of the analyte to the analyte's specific binder is at least about 10.sup.7 l/mol and is higher than the affinity of the labeled analyte to the same binder and wherein the analyte has a molecular weight greater than 20 kD.