What is the role of leukopenia in the assessment of septic arthritis?

Background It is not well-understood how leukopenia affects the synovial white blood cell (WBC) and percent neutrophils (%PMNs) in the setting of septic arthritis . We sought to determine 1. Do synovial WBC and %PMNs differ between patients with culture positive septic arthritis with or without leukopenia ? And 2. Are traditional thresholds of synovial fluid studies for accurately diagnosing septic arthritis still applicable in the leukopenic patient population? Methods A retrospective cohort study was performed at a single institution of 79 non-leukopenic and 11 leukopenic patients diagnosed with culture-positive septic arthritis. Demographic data, serum laboratory values, synovial laboratory values, and culture results were recorded. Significant differences in synovial laboratory values were evaluated using the Wilcoxon-Mann-Whitney test. Results are reported as median, interquartile range, and p values. Results There was a significant difference in synovial WBC in leukopenic patients compared to non-leukopenic patients with culture positive septic arthritis ( p = 0.01). No significant difference was found in the synovial %PMNs between two cohorts ( p = 0.33). Conclusion Leukopenic patients with culture positive septic arthritis have significantly lower synovial WBCs compared to non-leukopenic patients. Traditional thresholds for synovial WBC are not reliable for excluding diagnosis of septic arthritis in leukopenic patients.