The serum level of a highly sulfated epitope present on long keratan sulfate chains provides a direct measure of the rate of catabolism of cartilage proteoglycans. Levels of the keratan sulfate epitope are elevated in patients with generalized osteoarthritis (OA), indicating these individuals have elevated rates of cartilage proteoglycan catabolism. In the Pond-Nuki model of canine OA, the serum level of the keratan sulfate epitope rises rapidly after the transection of the anterior cruciate ligament, long before OA lesions can be detected, and remains high for at least 13 weeks.