Interpreting Lab Results

Patients with synovial fluid crystals may still have concomitant septic arthritis.

Progressively higher synovial WBC count increases the likelihood ratio (LR) of septic arthritis:

• Synovial WBC count > 50 X10^9/L has a positive LR of 7.7 for septic arthritis

• Synovial WBC count >100 X10^9/L has a positive LR of 28.0 for septic arthritis

• PMN cell count of >90% has a positive LR of 3.4 for septic arthritis