How reliable are the synovial cell count and blood parameters in the diagnosis of septic arthritis?

This study aims to investigate the reliability of the joint fluid cell count and blood parameters compared to the culture results in the diagnosis of septic arthritis (SA). A total of 192 patients (112 males, 80 females, mean age: 60.3±19.2 years; range, 18 to 98 years) who presented with SA between January 2018 and July 2022 were evaluated retrospectively. The recorded joint fluid cell count, complete blood count (CBC), white blood cell (WBC) count, serum erythrocyte sedimentation rate (ESR) and serum C-reactive protein (CRP) and culture results were analyzed comparatively according to SA diagnosis. The most commonly involved joint was the knee joint (82.3%), which was affected in 158 patients. Thirty-six (18.8%) of the patients who underwent joint aspiration had positive culture result. The cultures were positive in 10 (35.7%) of 28 patients with synovial WBC value greater than 50,000/mm3, while 26 (15.9%) of 164 patients with a synovial WBC value less than 50,000/mm3 had positive culture results (p=0.013). Patients with SA may present variable blood and synovial parameters. Making decision based on the commonly used synovial WBC count cut-off value of 50,000/mm3 may lead to misdiagnosis. To avoid misdiagnosis or delay in treatment, it is of utmost importance not to exclude the diagnosis acutely, and suspicion of SA should remain even with unlikely values.