An improved extractive reagent composition and method for extracting an immunosuppressant drug, such as sirolimus, tacrolimus or cyclosporine, from blood samples while yielding a test sample extract that has low vapor pressure and is compatible with immunoassay components. The inventive reagent composition comprises dimethyl sulfoxide (DMSO), at least one divalent metal salt and water. The sample extracts resulting from use of each of these combinations have low vapor pressure and are compatible with immunochemistry assays.