Clinical factors and treatment outcomes associated with failure in the detection of urate crystal in patients with acute gouty arthritis

Background/Aims: To investigate the rate of detection of monosodium urate (MSU) crystals in the synovial fluid (SF) of patients with acute gouty arthritis and factors associated with false-negative results. Methods: A total of 179 patients with acute gouty arthritis who had undergone SF crystal examination were identified from the data warehouse of two university hospitals. Clinical and laboratory data were obtained from the medical records. Results: The overall rate of detection of MSU crystals was 78.8%. In univariate analyses, the only significant differences between the variables of crystal-negative and crystal-positive patients were a lower C-reactive protein level (p = 0.040) and fewer patients undergoing emergent surgery in the crystal-positive group (p = $4.5{\times}10^{-6}$). In logistic regression analyses, MSU crystal-negative results were significantly associated with the interval from arthritis onset to crystal examination (p = 0.042), and this was the most significant risk factor for arthroscopic surgery (p = $2.1{\times}10^{-4}$). Seventeen patients who underwent arthroscopic surgery had a significantly longer hospital stay (p = 0.007) and a significant delay in gout treatment (p = $8.74{\times}10^{-5}$). The distribution of crystal-negative patients differed significantly between the SF samples that were evaluated by both the laboratory medicine and the rheumatology departments (p = $1.2{\times}10^{-14}$), and the $\kappa$ value was 0.108. Conclusions: Although several clinical features were associated with detection failure, SF MSU crystal identification was critically dependent on the observer. Considering the impact on the treatment outcomes, implementation of a quality control program is essential.