Visual estimation of polyp size is imprecise yet still predominantly determines surveillance intervals, resection techniques, and risk stratification. We therefore aimed to quantify its systematic bias against objective measurement standards.  

This prospective cohort study (NCT06822816) at the Montreal University Hospital Center included 574 polyps from 289 consecutive patients (mean age 66.3 years, 47.1% female). Thirteen endoscopists performed visual size estimations of polyps. Each polyp was also measured using one or more objective methods, and a single objective size was retained as the reference standard according to the order: (1) on-site microscopy (DinoLite models AF4515ZTL and AM8917MZTL) of fresh polypectomy specimens before formalin fixation (169 polyps, 29.4%), (2) virtual scale endoscopy (VSE, Scale-Eye, Fujifilm, Tokyo, Japan) (351 polyps, 61.1%), or virtual tape measurement (AccuMeasure, VTM Technologies, Haifa, Israel) (54 polyps, 9.4%). The primary outcome was measurement bias, defined as the mean difference between visual and objective measurements. Secondary outcomes included the impact of morphology, lesion size, and histology. 

Visual estimation demonstrated a significant systematic overestimation with a mean bias of 0.36 mm (95% CI: 0.20–0.52, P<0.001). Bias varied by reference method: 0.82 mm versus on-site microscopy (95% CI: 0.33–1.31, P=0.003), 0.10 mm versus virtual scale endoscopy (95% CI: 0.01–0.19, P=0.115), and 0.60 mm versus virtual tape measurement (95% CI: 0.08–1.12, P=0.074). Overestimation was most pronounced for diminutive polyps (0.37 mm, P<0.001), for polypoid morphology (0.41 mm, P<0.001), and for neoplastic histology (0.47 mm, P=0.002).  

Visual estimation systematically overestimates colorectal polyp size. Although overestimation appears modest when all objective methods are pooled, it may be attenuated by combining methods of varying accuracy. Indeed, when compared specifically with the most accurate method—on-site microscopy—the systematic visual overestimation is larger. This bias also varied by morphology, among other lesion characteristics, and was more pronounced for polypoid lesions. This consistent bias may influence clinical decisions, including risk stratification, surveillance intervals, and appropriate resection techniques, particularly for polyps near critical size thresholds. Routine implementation of objective measurement tools should be considered.