Background: The urate levels and the correlations of urate levels between the serum and synovial fluid (SF) of many arthritic diseases have not been well described.

Objective: Compare urate levels in the serum and SF of gouty arthritis, calcium pyrophosphate dihydrate deposition disease (CPPD), rheumatoid arthritis (RA), septic arthritis, ankylosing spondylitis (AS), and osteoarthritis (OA) patients.

Material and Method: Paired samples of serum and SF from 95 patients comprised of 33 patients with gout, 22 with CPPD, 18 with RA, nine with septic arthritis, three with AS, and 10 with OA were collected simultaneously for urate measurement by photometric test.

Results: Ninety-five patients, including 53 males, with mean (SD) age of 64.1(15.3) years were recruited. In gout, serum and SF urate levels were significantly higher than those of CPPD, RA, septic arthritis, AS, and OA (p<0.01). In all the study population, the serum/SF ratios of urate levels of gout were not different across all groups. However, after excluding 24 patients with creatinine >1.5 mg/dl, the serum/SF ratios of urate were significantly lower in gout compared with the others (p = 0.02). There were strongly positive correlations between serum and SF urate levels in gout similar to CPPD, RA, septic arthritis, AS, and OA (r = 0.81-0.91, p<0.01).

Conclusion: Despite the highest level of serum and SF urate across all groups, the serum/SF urate ratio in gout patients was the lowest, which suggests that SF urate levels are uniquely higher than their serum. In addition, the levels of serum urate of the entire groups strongly reflect their SF levels.

Keywords: Synovial fluid, Urate, Uric acid