"As adjuncts in the diagnosis of septic arthritis, WBC and ESR are poor tests, whereas the jWBC count is a fairly good test. However, none of these tests are perfect, and the clinician should be conservative in approaching the patient with a hot joint."
- WBC is poor at ruling in or ruling out septic arthritis - LR(+) 1.7 (95% CI 1.2 to 2.3) and an LR(-) 0.46 (95% CI 0.19 to 1.1).
- ESR is poor at ruling in or ruling out septic arthritis - LR(+) 0.84 (95% CI 0.60 to 1.2) and an LR(-) 2.4 (95% CI 0.07 to 0.89).
- The upper limit cut-off for jWBC count in the diagnosis of septic arthritis has never been clearly established.
- Medical folklore and emergency medicine texts have suggested cut-offs of 2000, 10,000 or 50,000.
- The diagnostic cut-off that maximized the combination of sensitivity and specificity was a jWBC count of 17,500 cells/mm, with a sensitivity of 83% and a specificity of 67%, an LR(+) value of 2.5 (95% CI 1.8 to 3.6) and an LR(-) value of 0.25 (95%CI 0.07 to 0.89).
- Consequently, a jWBC cut-off of 17,500 cells/mm may allow the clinician to rule out a septic joint (LR(-) 0.25, 95% CI 0.07 to 0.89), which is the principal concern of clinicians.
Li SF, Cassidy C, Chang C, Gharib S, Torres J. Diagnostic utility of laboratory tests in septic arthritis. Emerg Med J 2007;24:75-7.
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