• Lito Electra Papanicolas, Paul Hakendorf, and David Llewellyn Gordon.
• Microbiology and Infectious Diseases, Flinders Medical Centre, Bedford Park, South Australia 5042. litop76@gmail.com
• J Rheumatol. 2012 Jan 1;39(1):157-60.

ObjectiveIn acute monoarthritis, the presence of crystals in synovial fluid may lead to a diagnosis of crystal arthritis (CA) before septic arthritis (SA) can be excluded by culture. We aimed to identify the frequency of coexistence of CA with SA and to compare these with regard to synovial fluid microscopy, C-reactive protein (CRP), and blood culture.MethodsWe examined 1612 synovial aspirates from 2004 to 2009 retrospectively. Of these, 104 patients with clinically significant SA were identified. These were compared to 295 patients with isolated CA.ResultsFive percent of joints with CA had concomitant infection. A high synovial white blood cell (WBC) count and elevated CRP (> 100 mg/l) were predictive of concomitant SA with a sensitivity of 86.4%, specificity of 48.3% and 54.6%, and negative predictive values of 98.5% and 98.7%, respectively. In patients with SA who had a blood culture, 42.5% were positive with a matching organism. SA of the shoulder had a 90% rate of bacteremia.ConclusionCrystals alone in synovial fluid from acute monoarthritis cannot exclude SA, as CA and SA frequently coexist. High WBC counts and elevated CRP are common to both SA and CA. Blood cultures should be collected and septic arthritis considered, even when crystals are present, particularly if the shoulder is affected. The exception is when Gram stain is negative and the CRP is < 100 mg/l and joint WBC count is < 10,000/μl. In these circumstances it is very unlikely that there will be concomitant SA.

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