The presence of crystals alone in synovial fluid in patients with acute monoarthritis cannot exclude infection, because septic arthritis and crystal arthritis often coexist. High synovial fluid white blood cell (WBC) counts and an elevated C-reactive protein (CRP) level are common to these conditions and cannot distinguish between them reliably.
Papanicolas and colleagues retrospectively examined 1612 synovial aspirates. From them, 104 patients with clinically significant septic arthritis were identified. They were compared with 295 patients who had isolated crystal arthritis.
There was concomitant septic arthritis in 5.2% of joint aspirates with crystals. A high synovial WBC count and elevated CRP level (higher than 100 mg/L) were predictive of concomitant septic arthritis, with sensitivities of 86.4% and 86.4%, specificities of 48.3% and 54.6%, and negative predictive values of 98.5% and 98.7%, respectively. Gram-positive organisms predominated, constituting 86% of organisms cultured; the most common organism cultured from septic joints was Staphylococcus aureus. Blood cultures were performed in 70.2% of patients with confirmed septic arthritis and were positive with the same organism as cultured in synovial fluid in 42.5%.
The authors noted that blood cultures should be collected and empirical antibiotic therapy considered in most cases of monoarthritis, even if crystals are detected.