Solid phase reagents having immobilized thereon novel single-chain Fv antibodies specific to SS-A/Ro autoantigens have been developed for use in immunoassays to detect the presence or amount of autoantibodies to such autoantigens in a test sample. The solid phase reagents can also be used to purify autoantigen. Diagnostic test kits which contain these single-chain Fv antibodies specific to SS-A/Ro autoantigen useful in immunoassays are also described.