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Summary: Objectives: To characterize the utility of nitrotyrosine (NT) as a biomarker for arthritis and joint injury. Design: Synovial fluid, plasma, and urine from patients diagnosed with osteoarthritis (OA), rheumatoid arthritis (RA), anterior cruciate ligament (ACL) injury, meniscus injury and pseudogout, and knee-healthy volunteers were analyzed for concentrations of NT, nitrate and nitrite (NO x ), matrix metalloproteinase (MMP)-3, MMP-1, MMP-9, more than 40 chemokines and cytokines. Results: In OA, plasma and synovial fluid NT were increased versus healthy volunteers. Synovial fluid to plasma NT ratios were elevated in OA patients. Synovial fluid from patients with ACL and meniscus injury and pseudogout had increased levels of NT (P < 0.001). In these samples, NT levels significantly correlated with ARGS-aggrecan neoepitope generated by aggrecanase cleavage of aggrecan (P ≤ 0.001), cross-linked C-telopeptides of type II collagen (P < 0.001), MMP-1 (P = 0.008), and MMP-3 (P ≤ 0.001). In RA, plasma NT decreased following 6 months of anti-tumor necrosis factor (TNF) treatment. For every 1.1% change in log10 NT, there was a 1.0% change in the log10 disease activity scores (DAS28-3 CRP). Both predicted and observed DAS28-3 CRP showed a robust linear relationship with NT. RA plasma NT positively correlated with CRP, MMP-3 and interferon γ-induced protein 10. Conclusions: NT may serve as a useful biomarker for arthritis and joint injury. In RA, NT is highly correlated with several biomarkers and clinical correlates of disease activity and responds to anti-TNF therapy. [Copyright &y& Elsevier]