Background Inadequate bowel preparation (BP) is associated with lower adenoma detection rates (ADR). Although it is logically assumed that improving BP would result in higher ADR, recent studies have challenged this assumption, showing that ADR decreased at the highest levels of BP. However, these studies were limited by retrospectively collected data and inadequate adjustment of confounders related to ADR, including colonoscopy withdrawal time. This cross-sectional study aimed to evaluate the effects of BP quality using the Boston Bowel Preparation Scale (BBPS) and withdrawal time on ADR in colonoscopies with adequate preparation.

Methods The study was based on prospectively collected data from patients who underwent screening or surveillance colonoscopy from March to December 2019 and had adequate BP (BBPS >6). All patients received a low-volume polyethylene glycol solution with ascorbate. Withdrawal time was defined as the time from cecal inspection to exit from the anal verge, excluding the time for biopsy and cold snare polypectomy.

Results A total of 1,007 patients with adequate BP were included. The overall ADR was 47%. The BBPS-9 rate and ADR did not differ between endoscopists. ADR (52% vs. 41%) was higher in the good BP group (BBPS 6, 7, 8) than in the highest BP group (BBPS 9). The median withdrawal time was shorter in the highest BP group compared to the good BP group (7 minutes vs. 8 minutes). There was no significant difference in indications for colonoscopy between the good and highest BP groups. Patients with polyps and adenomas were older, more likely to be male, and had longer colonoscopy withdrawal times than those without polyps and adenomas. However, in multivariate analysis, longer withdrawal time, but not BBPS, was significantly associated with ADR (OR 1.12; 95% CI 1.07–1.18).

Conclusions ADR decreased at the highest BP levels. However, longer withdrawal time, not the highest BBPS, was significantly associated with ADR in the adequate BP group. The decrease in ADR in cases with the highest BP may be the result of shorter withdrawal times. Endoscopists should be careful not to miss polyps due to overconfidence and shorter observation times when the BP is perfect.