Can 2 Different Fecal Calprotectin Assays be Used Interchangeably in IBD Treatment?

Background: Fecal calprotectin (FC) is a biomarker for inflammation in inflammatory bowel disease (IBD). Interpretation of results can be complicated because of the use of different assays to determine FC. Goals: To assess the agreement between 2 different assays for determining FC in patients with IBD. Methods: Samples from adults and children with IBD were tested with 2 assays: (1) EliA 2 Calprotectin and (2) EK-Cal. Samples were uniformly tested on the same day. Interassay variability was displayed in a Bland-Altman plot. The difference in categorization of the FC result (1: 0 to 250 mg/kg, 2: 250 to 500 mg/kg, 3: >500 mg/kg) was assessed with the linear weighted κ for adults and children separately. Results: A total of 171 patients [mean age: 33 (range: 7 to 81); 92 (54%) female; 117 (68%) Crohn's disease; 53 (31%) ulcerative colitis] were included. Median (interquartile ranges) FC levels were 281 mg/kg (70 to 971) (EK-Cal) and 159 mg/kg (31 to 778) (EliA 2), and the mean delta FC was 89 mg/kg. In the adult population, there was substantial agreement between the 2 assays (κ: 0.72; SE: 0.06; 95% confidence interval, 0.60-0.83) and for pediatric patients, the agreement was almost perfect (κ: 0.83; SE: 0.06; 95% confidence interval: 0.70-0.95). Five of 171 patients (all aged ≥17 y and all with colonic disease) had a difference of 2 categories (1 vs. 3) between assays. Interassay variability was the highest in category 3. Conclusions: The agreement between the EliA 2 and EK-Cal assay in this cohort of IBD patients is substantial to almost perfect. Interassay variability is higher in the highest FC category.