We determine the proportion of methicillin-resistant Staphylococcus aureus (MRSA) in adult septic arthritis patients presenting to the emergency department (ED).

This was a cross-sectional retrospective review in 2 urban academic EDs in northern California, one tertiary care and one public. Subjects included patients who underwent arthrocentesis in the ED from April 2006 through July 2007. We queried the microbiology laboratory databases for synovial fluid cultures sent from the ED. We reviewed synovial fluid culture results and corresponding synovial fluid analyses and then classified positive culture results as true septic arthritis or likely contaminant. For septic arthritis cases, we reviewed medical records and abstracted presenting features. We report our findings with descriptive statistics.

One hundred nine synovial fluid cultures were sent from the EDs. Twenty-three results (21%; 95% confidence interval [CI] 14% to 30%) were positive, of which 9 were likely contaminants; 1 was from a soft tissue abscess and 1 was from bursitis. Of 12 septic arthritis cases, 6 cultures (50%; 95% CI 21% to 78%) grew MRSA, 4 (33%; 95% CI 7% to 60%) methicillin-susceptible S aureus, and 1 each (8%; 95% CI 0% to 24%) Streptococcus pneumoniae, Enterococcus faecalis, and Pseudomonas aeruginosa. Of the 6 MRSA cases, 4 were in male patients; median age of patients was 47.5 years, 3 patients had previously diseased joints, 2 patients injected drugs, 2 patients were febrile, 3 patients had previously diseased joints, median synovial fluid leukocyte count was 15,184 cells/microL (range 3,400 to 34,075 cells/microL), and 5 patients received appropriate ED antibiotics.

In this 2-ED population from a single geographic region, MRSA was the most common cause of community-onset adult septic arthritis. Synovial fluid cell counts were unexpectedly low in MRSA septic arthritis cases.