Fecal calprotectin level is nonlinearly associated with GI pathogen detection in patients with and without inflammatory bowel disease

Fecal calprotectin (FCP) is used to monitor inflammatory bowel disease (IBD) activity and can also be elevated in gastrointestinal infections. Our study's objective was to quantify the relationship between FCP levels and lab-confirmed infections in people with and without IBD. We performed a cross-sectional study at a tertiary-care center of all encounters during which FCP and gastrointestinal pathogen polymerase-chain reaction (GI PCR) panel testings were conducted. Using non-parametric tests and quantile regression, we compared the FCP levels by IBD status and pathogen detection. There were 3,347 encounters with FCP and GI PCR testings from 2,780 unique individuals between 1 August 2016 and 17 February 2022. Overall, 54.4% had IBD (n = 1,819). Pathogens were detected in 744 encounters (22.2%), and the detection rate did not differ by IBD status. Median FCP without IBD was significantly elevated when a pathogen was detected (64 vs 41 mg/kg, P = 0.0003, normal ≤50.0 mg/kg), but FCP with IBD was not significantly elevated when a pathogen was detected (299 vs 255 mg/kg, P = 0.207). In quantile regression adjusted for age and IBD, pathogen detection was only significantly associated with higher FCP in the lower two quartiles, though IBD remained significantly associated with higher FCP at all levels (P > 0.001). Pathogen detection by GI PCR is associated with elevated FCP, though this relationship is nonlinear and varies by IBD status. Our findings indicate that FCP may be an adjunct to, but not a substitute for, stool pathogen testing.