Synovial Cell Count Poorly Predicts Septic Arthritis in the Presence of Crystalline Arthropathy

Introduction: A synovial cell count greater than 50,000/mm³ is the threshold most commonly used to diagnose septic arthritis. This lab value may be nonspecific in the setting of crystalline arthropathy. The purpose of this study was to evaluate the accuracy of diagnosing septic arthritis using a synovial cell count cut-off of 50,000/mm³ in the setting of crystalline arthropathy. Methods: This was a retrospective review of joint aspirations performed between July 1^st, 2013 and June 30^th, 2016. Synovial fluid samples were evaluated for cell count, crystals, Gram stain, and culture. The sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of the synovial markers were calculated. Results: During the study period, 738 joint aspirations were sent for testing, of which 358 aspirations in 348 patients met inclusion criteria. There were 49 (13.7%) cases of culture-positive septic arthritis, and 47 patients underwent surgical irrigation and debridement. Gout and pseudogout crystals were present in 163 aspirates (45.5%). Three joints (0.8% overall rate) had concomitant crystalline arthropathy and septic arthritis, each of which had a synovial WBC ≥85,000/mm³. Increasing the WBC count cutoff to 85,000/mm³ demonstrated a specificity of 100%, but a PPV of 12.0%. Conclusions: A cut-off of 85,000/mm³ may be more appropriate to diagnose concomitant septic arthritis and crystalline arthropathy. We recommend medical management and observation in patients with crystal-positive joint aspirations unless the synovial cell count is elevated above 85,000/mm³. Prospective studies using this treatment guideline are needed to evaluate its validity and accuracy.