Impact of formalin fixation on colonic polyp size and post-polypectomy surveillance thresholds: evidence from a prospective paired analysis

Background Post-polypectomy surveillance is anchored to polyp size, with a 10-mm cut-off separating a return to screening from 3-year follow-up and a 5-mm cut-off defining the diminutive category. For most polyps the only numerical size record available to the clinician is the pathologist’s post-fixation measurement, yet whether formalin fixation shifts lesions across these thresholds is disputed. Methods Colorectal polyps ≥ 5 mm removed by snare polypectomy during routine colonoscopy and retrieved intact were measured fresh by the endoscopist and again after 8–24 h in 10% neutral buffered formalin by a blinded pathologist, using identical millimetric rulers. Comparative analyses were restricted to neoplastic lesions; unaided in-vivo visual assessments were examined descriptively. Results Sixty-three polyps from 53 patients were analysed; 55 (87.3%) were neoplastic. Mean size fell from 10.09 to 8.98 mm (reduction 1.11 mm, 95% CI 0.86–1.36; 13.7% per lesion; p < 0.001); 67.3% shrank and none enlarged. Shrinkage was greater in lesions < 10 mm than ≥ 10 mm (18.3% vs. 8.3%; p = 0.005) and in sessile than pedunculated lesions (17.3% vs. 7.5%; p = 0.008). Four of the eight lesions measuring 10–11 mm when fresh fell below 10 mm, and the number classified as diminutive rose from 7 to 15. In-vivo assessment showed wider dispersion and twice the threshold discordance. Conclusions Formalin fixation produces a small but directional size reduction that disproportionately affects small and sessile polyps and can shift lesions across both decision thresholds. Recording a fresh ex-vivo measurement at polypectomy is a simple, no-cost step that standardises the reference point.