To determine the sensitivity and specificity of clinical and laboratory signs for the diagnosis of septic arthritis (SA).

This prospective study included all adult patients with suspected SA seen in the emergency department or rheumatology department at the University Hospital, Clermont-Ferrand, France, over a period of 18 months.

In total, 105 patients with suspected SA were included, 38 (36%) presenting with SA (29 [28%] with bacteriologically documented SA). In the univariate analysis, chills (p=0.015), gradual onset (p=0.04), local redness (p=0.01), as well as an entry site for infection (p=0.01) were most often identified in SA. A history of crystal-induced arthritis (p=0.004) was more frequent in non-SA cases. An erythrocyte sedimentation rate (ESR)>50 mm (p=0.005), a C-reactive protein (CRP) level >100 mg/L (p=0.019), and radiological signs suggestive of SA (p=0.001) were more frequent in the SA cases. Synovial fluid appearance: purulent (p<0.001) and clear (p=0.007), synovial white blood cell (WBC) count >50,000/μL (p < 0.001), differentiated between SA and non-SA.

In multivariate analysis, only chills (odds ration [OR]=4.7, 95% confidence interval [CI] 1.3–17.1), a history of crystal-induced arthritis (OR=0.09, 95% CI 0.01–0.9), purulent appearance of the joint fluid (OR=8.4, 95% CI 2.4–28.5), synovial WBC count >50,000/mm3 (OR=6.8, 95% CI 1.3–36), and radiological findings (OR=7.1, 95% CI 13–37.9) remained significant.

No clinical sign or laboratory test (excluding bacteriological test), taken alone, is conclusive for the differentiation between SA and non-SA, but the association of several signs, notably chills, history of crystal-induced arthritis, radiological findings, and the appearance and cellularity of joint fluid may be suggestive.