Methods, kits, and active ingredients for diagnosing or treating arthritis or a degenerative disease of the skeletal system, or for selection of subjects for therapy. The methods for diagnosing arthritis involve the detection of an autoantibody, which is associated with arthritis, or excluding the presence of an autoantibody against collagen II. The methods for diagnosing a degenerative disease of the skeletal system, involve the detection of an autoantibody against thrombospondin-4 or COMP. The kits contain a detection agent for an autoantibody and can be used for diagnosing arthritis or a degenerative disease of the skeletal system. The active ingredient can be used for treating or preventing autoimmune-associated arthritis.