Prevalence and Lack of Cross-Sectional Association Between Positive Lactulose Breath Test Findings and Fecal Calprotectin Positivity in Patients with Functional Diarrhea

Background/Objectives: Functional diarrhea (FDr) is a common gastrointestinal disorder, and alterations in the gut microbiota have been proposed as one of its potential pathophysiological mechanisms. Hydrogen/methane breath tests are commonly used to identify findings suggestive of small intestinal bacterial overgrowth (SIBO) and intestinal methanogen overgrowth (IMO), whereas fecal calprotectin (FC) serves as a marker of intestinal inflammation. However, the prevalence of these findings and their relationship in patients with FDr remain unclear. This study aimed to determine the prevalence of positive breath test findings (suggestive of SIBO/IMO) and detectable/elevated FC in patients with a diagnosis of FDr and to evaluate their cross-sectional relationship and phenotypic overlap. Methods: We retrospectively reviewed patients who underwent both a lactulose breath test and FC measurement between March 2015 and April 2021 and fulfilled the Rome IV criteria for FDr. Positive breath test findings suggestive of SIBO/IMO were defined as a hydrogen increase of ≥20 ppm within 90 min or a methane increase of ≥10 ppm at any time following ingestion of 10 g of lactulose. FC was evaluated at two thresholds: the analytical limit of detection (LOD ≥ 11.5 mg/kg), used as an exploratory threshold to distinguish detectable from non-detectable FC, and the conventional clinical cutoff (>50 mg/kg), used to identify clinically elevated FC levels. Results: Among 524 patients with FDr aged ≥ 20 years, 176 (33.6%) had positive breath test findings suggestive of SIBO/IMO, 177 (33.8%) had detectable FC (≥11.5 mg/kg), and 71 (13.5%) met the conventional cutoff (>50 mg/kg). Concurrent breath test positivity and detectable FC (≥11.5 mg/kg) were present in 52 patients (9.9%), while only 19 patients (3.6%) exhibited concurrent breath test positivity and clinically elevated FC (>50 mg/kg). Conclusions: Approximately one-third of patients with Rome IV-defined FDr demonstrated positive breath test findings suggestive of SIBO/IMO, and a similar proportion had detectable FC concentrations (≥11.5 mg/kg), whereas clinically elevated FC (>50 mg/kg) was present in 13.5% (71/524). Positive breath test findings and FC elevation showed no significant cross-sectional association at either threshold. These findings provide descriptive prevalence estimates and do not establish mechanistic relationships between breath test positivity and FC elevation in patients with Rome IV-defined FDr.