BOTTOM LINE: Modify WBC threshold for diagnosing septic arthritis is patients who received recent antibiotics (24h to 2 weeks).
Additional Information
The ISDA 2024 guidelines use a WBC synovial fluid count of greater than 50,000 cells/mm to suggest septic arthritis.
A study of 81 patients with culture proven septic arthritis found the average leukocyte count was 40,408 ± 29,433 cells/µL in those who received antibiotics prior to arthrocentesis vs 93,824 ± 73,875 cells/µL in those who received no antibiotics for two weeks prior.
This is a greater than 50% reduction in mean WBC count!
A retrospective study of 383 patients found that patients who received IV or oral abx within 2 weeks before arthrocentesis had an optimal synovial WBC cutoff of > 16,000 (sensitivity 82%, specificity 76%), compared to >33,000 cells/µL (sensitivity 96%, specificity 95%) in the control group who had not received antibiotics within 2 weeks.
Conclusion:
When a patient has received antibiotics before arthrocentesis, a diagnostic value of >16,000 synovial leukocytes may be considered to guide treatment of septic arthritis. Additionally, in this one study, a diagnostic value of >33,000 synovial WBCs yields the highest accuracy for diagnosis of septic arthritis in patients who have not been given antibiotics before arthrocentesis.
References
Massey PA, et al. Optimal Synovial Fluid Leukocyte Count Cutoff for Diagnosing Native Joint Septic Arthritis After Antibiotics: A Receiver Operating Characteristic Analysis of Accuracy. The Journal of the American Academy of Orthopaedic Surgeons. 2021.
