Methods are disclosed of designing antibodies for a sandwich assay for a small molecule having a molecular weight of about 500 to about 2,000. The method comprises preparing a first antibody that binds to the small molecule, and preparing a second antibody that binds to the small molecule at a portion of the small molecule other than a portion to which the first antibody binds. The second antibody is prepared from an immunogen that comprises a predetermined portion of the small molecule. The antibodies may be employed in sandwich assays for the small molecule.