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.