Synovial fluid oxidative stress index as a biomarker for arthritis severity in thoroughbred racehorses.

Arthritis is a major cause of lameness in racehorses. Even when lameness improves, it may recur after training resumes, making it difficult to determine the disease state and when to end treatment. This retrospective observational study examined whether oxidative stress in synovial fluid could assess arthritis status in racehorses. Synovial fluid samples obtained during routine clinical practice from horses with arthritis and control samples obtained from horses without clinical or radiographic evidence of carpal joint disease were retrospectively analyzed. A total of 72 Thoroughbreds in Japan (3.4 ± 0.9 years) were included; 52 had arthritis and 20 served as controls. Arthritic horses were classified as acute (AC), subacute (SA), or chronic (CH) based on clinical stage, and as with or without radiographic lesions. Diacron-reactive oxygen metabolites (d-ROMs) and biological antioxidant potential (BAP) were measured to calculate the oxidative stress index (OSI = d-ROMs/BAP × 100). OSI was significantly lower in SA, CH, and control groups than in AC horses, and lower in horses without than with radiographic lesions. These findings suggest that OSI in synovial fluid may serve as a quantitative biomarker for arthritis severity and could aid clinical decision-making regarding treatment duration and return to training, particularly in cases without radiographic lesions.