Abstract: A method of assaying for an analyte which occurs at least partially bound as a complex with its soluble receptor or binding protein, the method comprising the steps of: i) mixing a biological fluid serum sample containing the analyte to be determined with a detergent for dissociating said complex, ii) mixing the sample from step i) with reagents, including a specific binding partner of the analyte for binding to the analyte, for performing a specific binding assay for the analyte, iii) and mixing the sample from step i) with a sequestrant for the detergent, whereby the binding of step ii) is performed in the presence of the sequestrant.