To verify a high performance liquid chromatography assay, the development of an alternative, opposite polarity liquid chromatography system is recommended. Results are compared and the more accurate or, if both methods give equivalent results, the simpler or more convenient procedure is chosen. Also useful, but often less rigorous, are comparisons against other selective methods, such as gas or thin-layer chromatography, comparison with theory (mass balance), or use of a second detector, wavelength, eluting solvent, or response measurement (area vs. height).