Methods and reagents are disclosed for determining a total amount of an analyte in an unknown sample suspected of containing the analyte in the presence of endogeneous interfering substances. The methods involve measuring an amount [Y] of the portion of the analyte in the unknown sample that is bound by endogeneous binding substances employing an assay for the analyte. An amount [Z] of analyte in the unknown sample that is not bound by endogeneous binding substances is determined by the formula: [Z]=a[Y]+b, wherein “a” and “b” are predetermined by conducting the assay on samples containing known amounts of the analyte but substantially free from endogeneous interfering substances. Adding [Y] and [Z] yields the total amount [X] of the analyte in the unknown sample.