Original ContributionsHow sensitive is the synovial fluid white blood cell count in diagnosing septic arthritis?
Introduction
Atraumatic joint pain is a common complaint encountered in the emergency department (ED). Because septic arthritis is 1 of the most serious causes of atraumatic monoarticular joint pain, the emergency physician must make this diagnosis, usually by arthrocentesis and synovial fluid analysis, in a timely manner.
The American Rheumatologic Association provides the following guidelines for synovial fluid cell count interpretation: noninflammatory > 200 to 2000 synovial fluid white blood cell count (WBC)/mm3; inflammatory = 2000 to 50 000 WBC/mm3; infectious = more than 50 000 WBC/mm3[1]. In the past 2 decades, several authors have reported instances of culture-proven septic arthritis with synovial WBC less than 50 000/mm3[2], [3], [4], [5]. The question is how accurate is the standard synovial fluid WBC cutoff of less than 50 000 WBC/mm3 to rule out septic arthritis?
We evaluated the sensitivity of the synovial WBC cutoff of 50 000/mm3 to diagnose septic arthritis at our institution.
Section snippets
Study design
This retrospective case study was conducted to determine the sensitivity of the synovial WBC cutoff of 50 000 WBC/mm3 to diagnose septic arthritis. The institutional review board at our hospital approved this study.
Study setting and population
The study population includes all patients with infectious arthritis confirmed by synovial fluid culture with growth of a pathologic organism at an academic, urban tertiary care center with 50 000 ED patient visits per year. The study period lasted from January 1996 to December 2002.
Study protocol
Results
The database identified 49 patients with culture-positive synovial fluid aspirates in the 6-year study. Patient demographics are summarized in Table 1. The average age was 63 years with roughly equal number of men and women.
The most common joint involved is the knee (65%), followed by the hip (12%) (Table 2). The most common organism involved is Staphylococcus aureus (55%) followed by beta streptococcus (22%) (Table 3).
Nineteen (39%) of 49 patients (95% CI, 25%-52%) had a synovial WBC of less
Discussion
Septic arthritis is a rare but serious cause of monoarticular joint pain that results from bacterial invasion of the synovium [6]. This invasion can result from direct inoculation, hemotogenous spread, or spread of a local infection. According to Kaandorp et al [7], the incidence of septic arthritis has remained stable at approximately 5.7 of 100 000 patients despite the advent of newer antibiotics and aggressive surgical treatment.
In the cohort study of Kaandorp et al [7], which comprised more
Limitations
Our study has several limitations. First, our study is a retrospective case series and, therefore, by design there is no way to calculate the specificity of the synovial WBC count. However, the design does allow for examination of the sensitivity of current guidelines regarding the diagnosis of septic arthritis. A second limitation is the narrow definition of septic arthritis; we included only cases of culture-proven septic arthritis. This likely eliminated in patients who are already on
Conclusion
Our study shows that the evaluation of synovial fluid using a 50 000 WBC/mm3 cutoff for septic arthritis lacks accuracy. A sensitivity of 61% (95% CI, 48%-75%) is far too low on which to base reliable clinical decisions. It is apparent that a lower cutoff (even at the expense of specificity) may be necessary to properly rule out septic arthritis, especially when significant predisposing medical conditions exist. A large prospective study is needed to better define the predictors of septic
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2021, American Journal of Emergency MedicineCitation Excerpt :Diagnosis of septic arthritis remains clinically challenging as an acutely painful or swollen joint may also be indicative of trauma, crystal arthropathy, rheumatoid arthritis, osteoarthritis, or various systemic diseases that have similar clinical presentations [8,13,14]. Despite its influence on clinical management, joint fluid analysis is not always sufficient to differentiate non-infectious from infectious arthropathy [8,14-17]. Synovial fluid culture is the current gold standard for diagnosis of adult septic arthritis.
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2020, Clinica Chimica ActaCitation Excerpt :In our study, adopting the 50,000 cells/μl cutoff to rule out infections would have missed 3 out of the 7 culture positive septic arthritis diagnoses. Hence, lower counts could be associated with culture positive arthritis and should not be solely used to rule out septic arthritis [26,27]. A %PMN cutoff of > 80% has also been reported to be of superior sensitivity for diagnosing septic arthritis than the TC-BF cutoff of 50,000 cells/μl [26].