Factors Associated with Clinically Inadequate Bowel Preparation Among Patients with Diabetes Mellitus Undergoing Colonoscopy: A Single-Center Observational Study

Background: Adequate bowel preparation is essential for high-quality colonoscopy, yet patients with diabetes mellitus remain particularly vulnerable to suboptimal cleansing. This study evaluated factors associated with clinically inadequate cleansing among patients with diabetes mellitus undergoing colonoscopy. Methods: This ambispective observational study included 194 adult patients with diabetes mellitus who underwent total colonoscopy at University Hospital Center Split between 1 January 2019 and 30 October 2025 and were prepared using an institutional split-dose PEG/macrogol-based bowel preparation approach. Bowel preparation quality was assessed using the Boston Bowel Preparation Scale (BBPS). Clinically inadequate bowel preparation was defined as total BBPS ≤ 5 or at least one segmental BBPS score ≤ 1. Multivariable logistic regression was used to evaluate associated factors. Results: Median age was 69 years, 131 participants (67.5%) were male, and 179 (92.3%) had type 2 diabetes mellitus. Clinically inadequate bowel preparation was observed in 68/194 participants (35.1%; 95% CI: 28.3–41.8%). In the main complete-case model (N = 147; events = 52), longer diabetes duration remained associated with clinically inadequate preparation after multivariable adjustment. Each 5-year increase in diabetes duration was associated with higher odds after adjustment for age, sex, outpatient status, chronic constipation, and use of tricyclic antidepressants or opioids (adjusted OR 1.27, 95% CI: 1.05–1.54; p = 0.015). Sensitivity analyses supported this association. Conclusions: Clinically inadequate bowel preparation was frequent despite the institutional split-dose PEG/macrogol-based bowel preparation approach. Longer diabetes duration may represent a candidate clinical variable for future risk-stratification studies, but it should not be used as a stand-alone criterion for intensified bowel preparation without prospective validation.