Synovial fluid sedimentation in the immobile patient: a commentary on modern septic arthritis and the addition of a new variable confounding diagnosis

Gregory Cunningham,¹ Brendan Ricciardo<sup>2

1</sup>Royal Perth Hospital, Perth, Western Australia, Australia; ²Bunbury Regional Hospital, Bunbury, Western Australia, Australia

Abstract: Septic arthritis is a serious cause of morbidity and mortality. Despite recent advances, monoarticular and polyarticular septic arthritis (SA) have a mortality rate of approximately 11% and 30%, respectively. SA has a 40% risk of permanent loss of joint function. Diagnosis of SA is difficult, given that no rapidly available individual test proves 100% sensitive or 100% specific. There are no previous reports on the phenomenon of synovial fluid sedimentation in an immobile patient, although the occurrence has been identified in vitro. This commentary also presents an extended report of a patient who had been immobile and supine for 24 hours before her right knee was aspirated and treated for septic arthritis. Due to her immobilization, the synovial fluid had settled. The color and opacity of the sequential aliquots from one arthrocentesis was noted to change from light straw-colored, to thick opaque purulent material. Laboratory reports showed increasing white cell counts (WCCs), from 2.6 × 10⁹ to 78 × 10⁹ between the sequential samples. This demonstrates a newly identified phenomenon of sedimentation. This might have led to a diagnostic difficulty, had the knee not been fully aspirated. Aspiration serves as a diagnostic tool, because it collects a sample, but it also serves as a treatment measure, because it removes purulent material. Complete aspiration of the joint should be performed for full therapeutic benefit and to avoid the potential diagnostic confusion of a falsely low WCC due to this newly identified phenomenon of synovial fluid sedimentation in the immobile patient.

Keywords: septic arthritis, inflammatory arthritis, joint, sedimentation, orthopedic