Protein precipitation reagent

A precipitation reagent for use in analytical systems for the determination of hydrophobic analytes in a biological test sample, particularly analytical systems employing specific binding proteins for such analytes. The precipitation reagent precipitates interfering proteins, hemoglobin, and other interfering substances from a biological test sample while, at the same, maintaining hydrophobic analytes in solution and minimizing the denaturation of specific binding proteins, such as, for example, antibodies, which may be present in an immunoassay system. The precipitation reagent comprises a zinc salt, a glycol, and a straight or branced alcohol from about 1 to 4 carbon atoms, and may optionally contain an acid. A preferred precipitation reagent comprises zinc sulfate, methanol and ethylene glycol, and is particularly useful in a fluorescent polarization immunoassay for the determination of hydrophobic analytes, especially cyclosporine.