Faecal calprotectin determination: impact of preanalytical sample treatment and stool consistency on within- and between-method variability

Introduction: We assessed the differences in faecal calprotectin (FC) concentrations measured by two assays depending on the stool consistency and extraction method. Materials and methods: Stool samples were extracted using the EliA Stool Extraction Kit, Caler (R) Cap extraction device and respective weighing methods, while FC concentrations were measured using the EliA (TM) Calprotectin and Buhlmann fCAL (R) Turbo method and checked for within- and between-method variability with regard to extraction method and stool consistency category. Extraction yield was evaluated for impact of different sample incubation time (10 min and 1 h) in extraction buffer for both methods and for impact of different initial sample dilutions (1:50, 1:100, 1:500) for fCAL (R) Turbo method. Results: Results determined from Calex (R) Cap extracts were higher compared to weighing method extracts (mean bias 33.3%; P < 0.001), while no significant difference was found between results obtained with EIiA Stool Extraction Kit and weighing method (mean bias 0.1%; P = 0.484), in both cases irrespective of stool consistency. Buhlmann fCAL (R) Turbo results were higher than EliA (TM) Calprotectin results (mean bias 32.3%, P = 0.025 weighing method; and mean bias 53.9%, P < 0.001 extraction devices), the difference is dependent on stool consistency and FC concentration. Significantly higher FC extraction yield was obtained with longer sample incubation time for both methods (P = 0.019 EliA (TM) Calprotectin; P < 0.001 fCAL (R) Turbo) and with increasing initial sample dilution for fCAL (R) Turbo method (P < 0.001). Conclusion: Preanalytical stool sample handling proved to be a crucial factor contributing to within- and between-FC assay variability. Standardization is urgently needed in order to assure comparable and reliable FC results.