A solid, water-soluble substantially anhydrous, storage-stable reagent formulation for use in conducting a clinical diagnostic test on a biological specimen is provided. The reagent formulation comprises a mixture containing a reagent capable of participating in a test reaction to effect a measurable change in a test system; and a nitrogen bearing polyoxyalkylene nonionic surfactant. The surfactant has a structure corresponding to that obtained when ethylene diamine is reacted sequentially with propylene oxide and ethylene oxide in the presence of a catalyst. The polyoxypropylene chains of said surfactant have an average molecular weight of between about 750 and about 6750. Methods of preparing the reagent formulations and methods of using them to conduct clinical diagnostic tests are also provided.