The subject invention pertains to methods and materials for accurately assessing the presence or absence of analytes of interest in samples, particularly in physiological samples. The subject invention involves utilizing a ligand binding domain (LBD) of a receptor to selectively capture the analyte target specific for that LBD. In one embodiment, the receptor is a protein or polypeptide. The ligand binding domain is allowed to react with a sample and the presence or amount of ligand (i.e., target analyte) bound by the LBD is determined. Suitable analytes include soluble analytes such as hormones, enzymes, lipoproteins, bacterial or viral antigens, immunoglobulines, lymphokines, cytokines, drugs, soluble cancer antigens, and the like. The methods of the present invention can be performed in both liquid-phase and solid-phase.