MP031: Synovial fluid analysis in the diagnosis of septic arthritis: comparing local data to the literature

Abstract

Introduction: A hot, painful, swollen joint is a common presentation to the emergency department. Of the potential etiologies, septic arthritis (SA) is the most devastating. Prompt diagnosis and treatment are essential to improve outcomes. Both culture proven and clinically suspected SA are thought to have the same prognosis, with similar morbidity and mortality estimates. No clinical exam or serum lab finding has the sensitivity or specificity to diagnose or exclude SA. Instead, diagnosis relies mainly on joint aspiration and synovial fluid analysis. A synovial white blood cell count (sWBC) greater than 50,000 cells/microliter is suggestive of SA and organisms seen on gram stain or growing in culture effectively makes the diagnosis. However, culture and gram stain are positive in only 67% and 50% of cases respectively. The objective of this study was to analyze the accuracy of synovial fluid analysis in our local practice environment. Methods: All those encounters with diagnoses related to SA at four adult emergency departments in Calgary between 2013-2014 were reviewed. Hospital records were analyzed for synovial analysis, antibiotic usage and surgical procedures. Results: Of 286 encounters, 87 were determined to satisfy the definition for SA in that culture was positive, gram stain was positive or clinical findings lead to treatment with antibiotics and/or surgical intervention. Gram stain was positive in 22% of cases with cultures positive in 51% of patients. sWBC were less than 50000 in 55% of cases and less than 25000 in 24% of cases. Of 88 gram stains performed, 28% were negative but had positive culture. All positive gram stains were associated with positive cultures. Conclusion: Culture, gram stain and sWBC of patients diagnosed with SA in Calgary show differences compared with the published literature. In Calgary, the majority of SA diagnoses were made clinically. The sWBC is central to making the diagnosis. Interestingly, 55% of patients diagnosed with SA had a count less than 50,000. It remains unclear what features of history, physical exam, imaging and lab analysis lead to the diagnosis of SA in these cases. Future studies will focus on these outliers to see if a more appropriate diagnostic algorithm would be useful in Calgary. Collaboration between infectious disease specialists, orthopedics, and emergency departments guided by local data is needed to ensure accurate and timely diagnosis.