Abstract Background Faecal calprotectin (FC) reflects intestinal inflammation in inflammatory bowel disease (IBD), but its relationship with small bowel activity remains incompletely understood. Intestinal ultrasound (IUS) provides a non-invasive assessment of transmural inflammation. This study aimed to evaluate the correlation between FC and IUS-based measures of small bowel inflammation in Crohn’s disease (CD). Methods We performed a retrospective analysis of patients with confirmed ileal CD who underwent IUS at the Unit for Gastrointestinal Ultrasound, Karolinska University Hospital, between October 2023 and December 2024. Patients with additional colonic disease were included only if colonic segments showed no inflammatory activity on intestinal ultrasound, defined by an International Bowel Ultrasound Simplified Activity Score (IBUS-SAS) below 23.8. Demographic data, disease distribution, and IUS parameters - bowel wall thickness (BWT), colour Doppler signal (CDS), bowel wall stratification (BWS), mesenteric fat echogenicity (i-fat), and the IBUS-SAS - were recorded. FC results were documented when available within ±14 days of the IUS assessment. Correlations were assessed using Spearman’s or Kendall’s coefficients, and logistic regression models evaluated the association between FC thresholds and IUS activity. Results Altogether, 67 patients met the inclusion criteria (median age 37.5 years; 34 males). FC correlated moderately with the IBUS-SAS (ρ = 0.48, p 0.01), BWT (ρ = 0.49, p 0.01), and CDS (ρ = 0.43, p 0.01), but not with the Harvey–Bradshaw Index (HBI) (τ = 0.05, p = 0.55). The correlation between HBI and IBUS-SAS was also weak (τ = 0.10, p = 0.27). In logistic regression analyses, the IBUS-SAS ileal score was a significant predictor of elevated FC. For FC ≥ 100 µg/g, each one-point increase in IBUS-SAS was associated with 7.8% higher odds of elevated FC (β0 = –1.42, p = 0.003; β₁ = 0.075, p 0.001). The model showed good discrimination (AUC = 0.76) with an optimal IBUS-SAS cut-off of 12, providing sensitivity 71% and specificity 72%. For FC ≥ 250 µg/g, each one-point increase in IBUS-SAS corresponded to 5.5% higher odds of elevated FC (β0 = –1.97, p 0.001; β₁ = 0.053, p = 0.002). This model achieved an AUC = 0.74 with an optimal IBUS-SAS cut-off of 29, yielding specificity 88.9% and sensitivity 54.5%. Conclusion FC correlated moderately with intestinal ultrasound findings, indicating that combining biomarker testing with imaging enhances non-invasive assessment of small bowel inflammation in CD. Conflict of interest: Rellou, Sofia: No conflict of interest Kordalis, Leonidas: No conflict of interest Petrousis, Grigorios: No conflict of interest Bilican, Gülden: I have no conflicts of interest to declare. Hedin, Charlotte Rose: Grant: C. R. H. Hedin has received specific project grants from Takeda and Tillotts. Personal Fees: C. R. H. Hedin served as a speaker and/or advisory board member for AstraZeneca, Abbvie, Dr Falk Pharma and the Falk Foundation, Galapagos, Janssen, Lilly, Pfizer, Ferring, Takeda, Tillotts Pharma, and received grant support from Tillotts and Takeda. These fees are invoiced by her employer. Bresso, Francesca: No conflict of interest Haas, Stephan L.: SLH served as a speaker and/or advisory board member for Celltrion, Ferring, Janssen, Lilly, Mediahuset, Pfizer, Santax Medico, Takeda, and Tillotts Pharma.
Rellou et al. (Thu,) studied this question.