Clinical Decision Guidelines
Background: The sensitivity of the standard threshold value for a synovial WBC count in septic arthritis of 50000 WBC/mm3 has been called into question. This study set out to determine the median synovial WBC value of patients presenting to quaternary academic teaching hospital and to establish if the differential cell percentage of synovial neutrophils, lymphocytes, monocytes and neutrophils can assist clinicians in making a diagnosis of septic arthritis.
Methods: This study was a single-center, retrospective, chart review at a quaternary academic teaching hospital. Charts were pulled from a two-year period from 2016 to 2017 for joint aspirations performed in the ED. Patients had to be 18 years or older. 179 charts met these criteria and of those 102 had culture results. A positive culture served as a gold standard. The mean WBC count was recorded and the sensitivity and specificity of a value of 50000 WBC/mm3 was analyzed. The WBC differential was recorded.
Results: Of the 102 charts reviewed, 92 had data for the synovial WBC. An overall median value of 4699 WBC/mm3 (N=91, 539-26593, Q1-Q3) was found. This produced a median value for negative synovial cultures of 2236 WBC/mm3 (N=73, 511-14,700) and a median value for positive synovial cultures of 34,663 WBC/mm3 (N=18, 2833-100,000 Q1-Q3). This had a P-value of 0.0054. A cutoff value of 50000 WBC/mm3 lead to a sensitivity of 31% and a specificity of 100%.
Of the 102 charts, 90 had synovial WBC differentials recorded. The median synovial neutrophil percentage was 83.0 % (59.0-91.0, Q1-Q3) overall, 79.0 (52.0-89.0, Q1-Q3) for negative cultures, and 92.0 % (88.0-96.0, Q1-Q3) for positive cultures. The median synovial lymphocyte percentage was 5.0% (2.0-15.0, Q1-Q3, p < 0.0395) overall, 7.0% (2.0-17.0, Q1-Q3) for negative cultures and 3.0% (1.0-6.0, Q1-Q3) for positive cultures. The median synovial monocyte percentage was 8.0% (4.0-18.0, Q1-Q3, p < 0.0001) overall, 12.0% (4.0-22.0, Q1-Q3) for negative cultures and 3.0% (1.0-6.0, Q1-Q3) for positive cultures.
Conclusion: These results demonstrated a median value for a synovial WBC significantly less than the currently suggested threshold and a low sensitivity for the threshold of 50000 WBC/mm3. Furthermore, an elevated synovial neutrophil percentage, decreased synovial lymphocyte percentage and a decreased synovial monocyte percentage could be indicative of septic arthritis.