Calprotectin Measurement in Rectal Mucus Collected with the OriCol™ Sampling Device: A Method-Comparison Study Against Faecal Calprotectin Reference Tests

Background: Faecal calprotectin is recommended by the UK National Institute of Health and Care Excellence (NICE) HealthTech guidance HTG320 (2013) to distinguish inflammatory bowel disease (IBD) from functional bowel disorders, including irritable bowel syndrome (IBS), when cancer is not suspected, and to monitor IBD activity. Stool collection can limit adherence. Home-based and smartphone-enabled assays partly address this gap, but clinic-acquired sampling could provide results soon after consultation. We compared calprotectin in OriCol™ Sampling Device-collected rectal mucus with matched faecal calprotectin. Methods: Sixty-six adults were prospectively recruited from three NHS gastroenterology clinics. Each provided a clinic-acquired rectal mucus sample and matched stool sample. Calprotectin was measured using two CE-marked ELISAs (IDK® Calprotectin and fCAL®), with laboratory staff blinded to clinical category. The primary endpoint was categorical agreement at the 50 µg/g faecal threshold; secondary analyses included Bland–Altman analysis, Cohen’s κ and patient acceptability. Results: Calprotectin was detected across both assays’ working ranges and correlated positively between matrices. Forty-eight participants were eligible for threshold analysis. Regression-derived rectal mucus equivalents produced positive percent agreement of 79% (95% CI 61–91) for IDK® and 68% (51–82) for fCAL®, and negative percent agreement of 53% (27–79) and 40% (12–74), respectively. Chance-corrected agreement was fair for IDK® (κ 0.32, 95% CI 0.02–0.62) and slight for fCAL® (κ 0.07, 95% CI −0.27–0.41), and Bland–Altman 95% limits of agreement between matrices were wide (spanning roughly two orders of magnitude). Among IBD patients, paired results were above threshold in 93% and 86%, respectively. Of 60 respondents, 96.7% would repeat sampling, 55.6% of those expressing a preference (30 of 54) chose OriCol™, and no OriCol™ Sampling Device-related adverse events occurred. Conclusions: OriCol™ enabled acceptable clinic-based rectal mucus calprotectin measurement with positive correlation to faecal results. Above-threshold agreement was encouraging, but the low negative agreement, the fair-to-slight κ values and the wide between-matrix limits of agreement indicate that rectal mucus calprotectin is not equivalent to faecal calprotectin. Below-threshold agreement, in particular, requires confirmation in larger independent cohorts with pre-specified thresholds before clinical implementation based upon current diagnostic platform threshold.