Colonic Mast Cell Density and Diarrhoea: Evidence Against a Threshold Effect in Biopsies From 3508 Patients

BACKGROUND: Elevated mast cell (MC) counts in colonic biopsies are frequently observed in patients with chronic diarrhoea or unspecific abdominal symptoms, potentially leading to a diagnosis of mast cell activation syndrome. However, the clinical relevance of MC quantification in this setting remains controversial. AIM: This study aimed to determine the proportion of patients with elevated MC counts who presented with diarrhoea compared to those with normal counts. As a secondary endpoint, we assessed how often MC quantification was performed and what the underlying clinical indications were. METHODS: We analysed a large US national pathology database with 3.3 million colonoscopies performed between January 2009 and December 2023. Screening colonoscopies for colon cancer detection were excluded. For patients with multiple colonoscopies, only the first procedure was included. This resulted in a total of 2,059,147 colonoscopies. RESULTS: Colonic mast cells were evaluated in 3508 reports (0.17%). In 1167 cases the report contained either the expressions indicating increased MC, or a MC count > 20 MC/high power field (hpf). Diarrhoea was present in 58.3%, 54.1%, 54.7%, 50.0% and 33.3% of patients with MC count of < 20/hpf, 20-39/hpf, 40-59/hpf, 60-98/hpf and ≥ 99/hpf, respectively. CONCLUSION: These data do not show a relationship between diarrhoea and elevated MC in the colonic mucosa. Therefore, we suggest that the incorporation of MC quantification into the histologic evaluation of colonoscopic biopsies obtained for common GI indications may not be warranted.