OBJECTIVE: To investigate the reliability of urea as the internal reference of ankle irrigating fluid in correction of synovial fluid and related factor concentration dilution times, and to further determine the factor expression level in synovial fluid. PATIENTS AND METHODS: A total of 91 patients, including 20 cases of non-posterior median foot lesions and 71 cases of unilateral ankle osteoarthritis, were enrolled in this study. AOFAS score, Takakura staging and visual analogue pain scale were given; the venous blood was collected and the ankle fluid and irrigating fluid were obtained from the affected side via puncture. The urea in serum, synovial fluid and irrigating fluid was quantified using an AU5800 biochemical instrument (Beckman Coulter). Statistical analysis was performed for the correlation between urea in serum and synovial fluid; the levels of TNF-alpha, IL-1 beta, IL-6, IGF-1 and HELIX-II in irrigating fluid quantified by enzyme linked immunosorbent assay (ELISA) were further corrected. RESULTS: There was a good linear relationship between urea contents in synovial fluid and serum (R2=0.89), and the regression slope was 0.927. The levels and ratio of urea in serum and synovial fluid were independent of Takakura staging or pain degree. The expressions of IL-1 beta (p=0.000), IGF-1 (p=0.000) and HELIX-II (p=0.010) were significantly increased in synovial fluid in patients with ankle osteoarthritis. There was no significant difference in expression of IL-1 beta (p=0.514), and TNF-alpha was not detected in synovial fluid. CONCLUSIONS: This study confirmed the dynamic stability of urea in the ankle fluid, and it is not affected by the progression of arthritis, age and other factors, which is the standard internal reference of correcting ankle irrigating fluid dilution times. The occurrence mechanism of ankle osteoarthritis may be different from that of other osteoarthritis, and IL-6 and IGF-1 are associated with the progression of the disease. Increased HELIX-II is an independent factor of predicting cartilage injury.