Muscularis propria detected at colonic biopsy is not a critical value

•Muscularis propria rarely occupies specimens from colonic biopsies.•Histologic features differentiate muscularis propria from muscularis mucosa.•Muscularis propria detected at colonic biopsy fails to correlate with perforation. Perforation is a rare but potentially serious complication of cold forceps colonic biopsies. The presence of muscularis propria might portend increased risk of perforation and warrant urgent communication with endoscopists. However, identifying muscularis propria at time of biopsy is difficult for many reasons, including histologic overlap with muscularis mucosa. Incidental muscularis propria obtained in this manner has yet to be studied. Our objectives were to characterize the morphology of muscularis propria obtained incidentally via cold forceps colonic biopsies, establish practical criteria for diagnosing muscularis propria, and determine whether diagnosing muscularis propria correlates with perforation clinically. We retrospectively reviewed 3 specimens from nontargeted cold forceps colonic biopsies for which pieces of muscularis propria were presumed to be visualized based on larger size or lower nuclear density compared to the corresponding muscularis mucosa. These specimens were then compared to normal transmural control tissue from colectomy to confirm whether nuclear density or other features could distinguish muscularis propria from muscularis mucosa. Muscularis propria in the control tissue had lower nuclear density, smoother cytoplasmic texture, and more cytoplasmic pallor compared to the muscularis mucosa in the control tissue. This constellation of features was seen in all 3 specimens obtained via biopsy and therefore confirmed the presence of muscularis propria, though all patients lacked perforation clinically. Large size, low nuclear density, smooth cytoplasm, and pale cytoplasm identify objects as muscularis propria at time of colonic biopsy. The presence of muscularis propria fails to correlate with perforation clinically and does not warrant urgent communication.