Variable interpretation of polyp size by using open forceps by experienced colonoscopists

Background: Endoscopic measurement of colorectal polyps by using open forceps is commonly used in clinical trials but is subject to several sources of error.

Objective: Assess error rates in polyp size determination with open forceps at the step of measurement after the forceps are aligned on the polyp.

Design: This was a prospective assessment of 49 gastroenterologists who received training on 10 photographs of polyps with aligned forceps and then measured 10 additional test polyps from photographs. One of the test photographs was excluded because of incorrect forceps alignment.

Setting: Data analyzed at an academic medical center.

Intervention: Photographs displayed in a webinar.

Main outcome measurements: Description of rates of accurate measurements, including the fraction correct within 50% and 25% margins of error.

Results: A total of 37% of all measurements were correct to the exact millimeter, 34% were larger, and 29% smaller compared with the reference standard. A total of 47 of 49 doctors measured all 9 polyps within 50% of the reference standard, and 21 measured all 9 correctly within a 25% error margin.

Limitations: Other potential sources of error in open forceps measurement were not evaluated.

Conclusion: Open forceps polyp size determination is subject to error at the step of using the fully aligned forceps as a scale for measurement. A margin of error of 50% up or down is needed to prevent this step in size determination from causing errors in polyp matching in clinical trials comparing diagnosis-only imaging to colonoscopy.