Synovial Fluid Cell Counts Remain Stable for 72 Hours Regardless of Storage Temperature: Implications for PJI Diagnosis

Background: Synovial fluid analysis plays a crucial role in the diagnosis of periprosthetic joint infection (PJI). However, the stability of leukocyte counts and the percentage of polymorphonuclear neutrophils (PMN%) under different storage conditions remains uncertain, and many institutions lack immediate access to on-site laboratories. We investigated whether storage temperature (room temperature (RT) vs 4 °C) influences synovial fluid white blood cell (WBC) count and PMN%, and if these parameters are stable for up to 72 hours after aspiration. Methods: We prospectively analysed 106 synovial fluid samples obtained during revision arthroplasty for suspected PJI. Assuming that the population was homogenous according to the inclusion criteria, patient’s samples were randomly allocated either to be stored at RT or at 4°C, in order to obtain two different set of samples. In both set of samples WBC count and PMN% were measured at baseline, 6, 12, 24, 48, and 72 hours using an automated haematology analyser after pre-treatment with hyaluronidase. Changes over time in the same patient synovial fluid and between different storage temperature groups were assessed with independent T test. Results: Both WBC count and PMN% remained stable for up to 72 hours in samples stored at either RT or 4 °C. Mean WBC counts were slightly higher in refrigerated samples, but differences were minimal and not statistically significant. No variation led to reclassification of samples across the ICM 2018 diagnostic thresholds for PJI. Conclusion: Synovial fluid WBC and PMN% remain stable for up to 72 hours regardless of storage temperature. These findings challenge the assumption that immediate analysis is required and support greater flexibility in clinical workflows, particularly in institutions without immediate on-site laboratory availability.