Osteoarthritis (OA) is the most common degenerative disease of joints world-wide. About two-third of people over 65 years are affected, whereas the etiology is largely unclear. Currently, there is no causative therapy avaible, which is due to the complex pathomechanisms of arthrosis and also due to the lack of diagnostic biomarkers that allow monitoring of therapeutic success of drug candidates. Researchers of the University of Cologne have found that autoantibodies against the cartilage protein thrombospondin 4 (TSP-4) occur in the course of arthrosis. Rheumatoid arthritis is characterized by the presence of autoantibodies, but autoantibodies against TSP-4 have jet not been described for arthrosis. In addition to TSP-4, the researchers have found autoantibodies against the cartilage oligomeric matrix protein (COMP) and the C-type lectin domain family 3 member A (CLEC3A) in the course of OA, which may be used in a combined test together with TSP-4 in order to strengthen significance of diagnosis (Table 1). The present invention is the first serum biomarker test that helps to support the clinical diagnosis of OA and furthermore may provide a tool that allows the monitoring of therapeutic success of drug candidates for the treatment of arthrosis.