Abstract: A method for the diagnosis of IgA nephropathy using a specific binding reaction comprising the steps:a) preparing a substrate capable of binding fibronectin or IgAb) contacting the substrate resulting from step a) with a sample of body fluid drawn from a patient subject to diagnosis to bind any fibronectin-IgA-complex present in said sample to the substrate, andc) determining the presence of complex bound to the substrate using the reaction between the exposed part of such bound complex and a corresponding antibody thereto; anda diagnostic kit for use in such diagnosis.