Short- and long-term stability of synovial fluid calprotectin.

Introduction: Information about analyte stability is of crucial importance. The aims of this study were to determine the short- and long-term stability of synovial fluid calprotectin at various temperature conditions (4-8 °C for 7 days, - 20 °C and - 80 °C for 6 weeks).

Materials and methods: Eleven samples from patients were included in this study. The samples were promptly transported at room temperature (RT) to the laboratory immediately after arthrocentesis. Upon arrival, the samples were transferred into plastic tubes without additives and pretreated with hyaluronidase solution. After centrifugation at 1500xg for 10 minutes at RT, the baseline calprotectin concentrations were determined. Seven aliquots were stored in LoBind tubes (Eppendorf) at 4-8 °C and the calprotectin was measured every day. Six additional aliquots were stored at temperatures - 20 °C and - 80 °C and the concentration of calprotectin was measured weekly. Analysis was done using Buhlmann fCAL turbo reagent on analyzer Siemens Atellica Solution (Siemens Healthcare, Erlangen, Germany). Data were analyzed by Microsoft Excel and MedCalc statistical software. The percentage difference (PD%) was calculated. The maximum permissible difference (MPD) was 9.1% for PD%.

Results: The PD% with the corresponding 95% confidence intervals were inside the predefined MPD. The instability equations and correlation coefficient for storage temperatures were PD% = 0.1644 x time (day), r = 0.06, P = 0.614 for 4-8°C, PD% = 0.5190 x time (week), r = - 0.22, P = 0.080 for - 20°C, and PD% = 0.1316 x time (week), r = 0.08, P = 0.545 for - 80°C.

Conclusions: The calprotectin in the synovial fluid is stable when stored long-term for 6 weeks at - 20 °C or at - 80 °C or short-term (7 days) at 4-8 °C.