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Background: Experts have stated that adenoma detection rates (ADR) of individual endoscopists should be measured to assess colonoscopy quality. Aim: The purpose of this study was to quantify the reliability of the ADR as a quality marker. Methods: We simulated a population of endoscopists and patients using published data on adenoma prevalence and adenoma miss rates. For each endoscopist, the ADR was calculated. The proportion of ADR variance attributable to endoscopist and the area under the ROC (AUROC) curve for low-performing endoscopists (lowest quartile or decile) were also calculated. Results: In the base-case analysis (200 patients per endoscopist, miss rate 22 %, adenoma prevalence 24 %), only 13 % of ADR variance was attributable to endoscopist performance (AUROC up to 0.73). An ADR cutoff of <16.5 % identified approximately half of endoscopists in the lowest performance decile (test sensitivity = 53 %), but most (79 %) of the endoscopists identified by this cutoff were NOT low performers (i.e., false positives). In sensitivity analysis, increasing the number of patients per endoscopist, reducing the variance of adenoma prevalence between endoscopists (i.e., performing case-mix adjustment), and increasing the variance in performance between endoscopists all improved ADR test characteristics (AUROC up to 0.88). However, regardless of assumptions, a substantial proportion of endoscopists would be misclassified if a simple ADR cutoff were utilized. Conclusions: The ADR has limited reliability as a quality marker under real-world assumptions. Simple cutoffs are likely to either be insufficiently sensitive or have high false positive rates. Future studies should identify alternative means for assessing endoscopist performance. [ABSTRACT FROM AUTHOR]