Objective. To study 3 body fluids for changes in the levels of 5 biomarkers of cartilage metabolism during the early phases of experimental osteoarthritis (OA).Methods. Twenty skeletally mature mixed-breed canines underwent unilateral surgical transection of the anterior cruciate ligament. Samples of joint fluid, serum, and urine were obtained preoperatively and just before necropsy (3 weeks or 12 weeks postoperatively). Biomarkers included 2 markers of cartilage matrix synthesis/turnover (aggrecan 846 epitope and C-propeptide of type II collagen) and 3 markers of cartilage degradation (keratan sulfate proteoglycan epitope, the collagenase-generated cleavage epitope of type II collagen [Col2-3/4C long mono , or CIIC], and crosslinked peptides from the C-telopeptide domain of type II collagen [Col2CTx]). Significant changes in the levels of these biomarkers were determined by paired analyses.Results. Joint pathology was more severe in the 12-week group compared with the 3-week group. In joint fluid, due to limited volume, only Col2-3/4C long mono and Col2CTx were measured. Significant elevations in the levels of both of these markers were observed in experimental joints in both the 3-week group and the 12-week group. In serum, the level of aggrecan 846 epitope was elevated at both 3 weeks and 12 weeks, the level of Col2-3/4C long mono was elevated at 12 weeks, and the level of Col2CTx was elevated at both 3 weeks and 12 weeks. In urine, the level of Col2-3/4C long mono was elevated at 12 weeks after surgery.Conclusion. Levels of biomarkers of intact aggrecan proteoglycan (aggrecan 846 epitope) and type II collagen degradation (Col2-3/4C long mono and Col2CTx) were elevated early after unilateral stifle joint injury, suggesting that these markers are sensitive and specific for early cartilage changes associated with isolated joint injury in this established model of experimental OA.The early stages of osteoarthritis (OA) are difficult to diagnose. Joint structure and function are typically altered substantially before symptoms cause patients to seek medical care, that is, the osteoarthritic process begins long before OA presents as a clinical disease. The insidious onset and "silent" progression of primary OA not only obscure an early diagnosis, but also delay treatment that may help prevent further cartilage destruction and joint failure. Hence, diagnostic tests that can detect and monitor molecular events early in the pathogenesis of OA would be potentially very useful.For nearly a century, synovial fluid analysis has been important in the differential diagnosis of arthritis. Although synovial fluid analysis is useful in distinguishing between inflammatory, septic, and crystal-induced arthritis, its value in OA is limited. Similarly, routine blood and urine analyses can identify certain inflammatory arthritides and metabolic defects that cause joint pain and pathology but, as yet, reveal little about the OA process. No specific molecular markers of the OA process are currently available for clinical use, and the...