Does the presence of crystal arthritis rule out septic arthritis?

The objective of this study was to determine the incidence of septic arthritis in the presence of joint crystals. A retrospective study was conducted at a university tertiary care referral center. The study population included all patients with synovial fluid crystals in the joint aspirate sent to the laboratory during the 7-year study period. Septic arthritis was defined as a positive synovial culture. Of the 265 joint aspirates containing crystals, 183 (69.0%) contained gout crystals, 81 (30.6%) contained pseudogout crystals, and 1 (0.4%) contained both. Four (1.5%) of the aspirates had positive cultures. The mean synovial WBC of the 4 samples with concomitant crystals and septic arthritis was 113,000 (95% confidence interval [CI] 72,700–153,200), which was significantly higher than the entire population at 23,200 (95% CI 19,400–27,000; p < 0.01). Of note, all 4 patients with concomitant disease had significant co-morbidities and synovial WBC counts greater than 50,000. Septic arthritis and acute crystal-induced arthritis can occur simultaneously; there were 4 cases (1.5%) of concomitant disease in our study population. The presence of crystals cannot exclude septic arthritis with certainty.