ESR, CRP and WBC are not sensitive enough to rule out septic joint.

25% of septic joints have WBC under 40k → association with MRSA.

Early septic joint can have low synovial WBCs while inflammatory arthritis (RA, reactive arthritis, etc.) can have markedly elevated synovial WBCs.

Synovial glucose less than half serum glucose is indicative of septic joint.

In Prosthetic joints, WBC >1.7k and PMN > 64% is sufficient to make the diagnosis.

33% of pediatric patients with septic joint are afebrile and 50% with co-existing osteomyelitis.

2% of patients with crystalopathy (gout, pseudogout) have concomitant septic joint.

Coagulopathy and overlying soft tissue infections are relative contraindications to arthrocentesis and should not sway you from pursuing the definitive diagnosis if clinically concerned.

References:

Hariharan, H, et al. Sensitivity of Erythrocyte Sedimentation Rate and C-reactive Protein for the Exclusion of Septic Arthritis in Emergency Department Patients. J of Emerg Med. 2010; 40(4):428–431.

Frazee BW, Fee C, Lambert L. How common is MRSA in adult septic arthritis? Ann Emerg Med. 2009;54(5):695-700.                Baker DG, Schumacher HR. Acute monoarthritis. N Engl J Med. 1993;329(14):1013-1020.