Any measurement of the properties of one or more autoantibodies. An antibody is an immunoglobulin molecule possessing a specific amino acid sequence that binds to a specific antigen, a substance introduced into an organism which initiates an immune response including the production of the very antibodies which bind to it in an effort to destroy it. An autoantibody is an antibody that attacks the cells, tissues, native proteins, or other endogenous molecules of the organism in which it was formed.