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The recommended approach for patients with suspected septic arthritis is arthrocentesis with fluid analysis sent for: Gram stain, culture, WBC count with differential, and crystal analysis before starting antibiotics.
From a classic meta-analysis of 14 studies involving greater than 6,000 patients the only 3 findings that occur in more than 50% of patients with septic arthritis were
Joint pain (sensitivity, 85%; 95% confidence interval [CI], 78%-90%),
Joint swelling (sensitivity, 78%; 95% CI, 71%-85%),
Fever (sensitivity, 57%; 95% CI, 52%-62%).
Vs.
Sweats (sensitivity, 27%; 95% CI, 20%-34%)
and rigors (sensitivity, 19%; 95% CI, 15%-24%) were less common findings in septic arthritis.
The probability of septic arthritis increases progressively with higher synovial WBC counts:
- <25,000 cells: LR 0.32 (95% CI, 0.23-0.43)
- > 25,000 cells: LR 2.9 (95% CI, 2.5-3.4)
- >50,000 cells: LR 7.7 (95% CI, 5.7-11.0)
- >100,000 cells: LR 28.0 (95% CI, 12.0-66.0)
PMN percentage ?90% suggests septic arthritis with LR 3.4 (95% CI, 2.8-4.2)
VS
PMN <90% lowers the likelihood (LR 0.34, 95% CI 0.25-0.47).
References
Margaretten ME, Kohlwes J, Moore D, Bent S. Does This Adult Patient Have Septic Arthritis? JAMA. 2007;297(13):1478-88.