Synovial White Cell Count in the Diagnosis of Septic Arthritis: Are Current Diagnostic Practices Appropriate?

Introduction & aims: Septic arthritis is an emergency, potentially causing irreversible joint destruction and disability. Synovial WCC and polymorphonuclear cell percentage are the best predictors of septic arthritis likelihood. Yet, synovial white cell and differential count are not routinely assessed. We aim to investigate the incidence of failure to perform these tests, and to develop correct synovial fluid analysis practices. Method: This is a retrospective analysis of native joints having undergone arthrocentesis for suspicion of septic arthritis at Box Hill Hospital (BHH) during September 2011 and September 2013 inclusive. Recruitment was from the Eastern Health Decision Support Service (DSS), a database compiled from all systems within Eastern Health, of which BHH is a member. The study was limited to large joints, including hip, knee and shoulder. All prosthetic joints were excluded from the patient population. All patient histories were examined for suspicion of septic arthritis and subsequent arthrocentesis. Pathology records were accessed to determine incidence of cell count and differential. Results: One hundred and thirty-six cases of joint aspirations were identified within the time frame, of which sixty-seven fitted our criteria for evaluation. All but two cases were delivered using the DSS, which was limited to data compiled only until June 2013. The two remaining cases were identified with a manual search of the radiology and pathology databases from June to September 2013. 22 of the 67 joint aspirates studied did not have a cell count carried out. Four of these 22 cases had a diagnosis of septic arthritis. In five aspirates, there was a failure to confirm a definite diagnosis and they were thus conservatively treated as a septic joint. The remaining acute joints in which no cell count was done were gout (7 cases), pseudogout (5 cases) and rheumatoid arthritis (1 case). Cell counts were not routinely detected for a variety of reasons. Eleven aspirates were deemed too viscous, and in eight cases the sample had clotted prior to pathologist assessment. Two cases had insufficient volume, and one sample was too bloodstained to calculate a cell count and differential; likely due to traumatic aspiration. Conclusions: 33% of acute monoarthritis’ evaluated over the study period failed to have a synovial fluid WCC and differential. This may be due to inadequate samples, or lack of