High frequency of microbial overgrowth among symptomatic patients with autoimmune atrophic gastritis

Autoimmune atrophic gastritis causes achlorhydria, which may predispose to gastrointestinal microbial overgrowth (MO). We sought to evaluate the association between autoimmune atrophic gastritis and MO.

From a database of patients at our institution who had undergone MO testing (via intestinal aspirate culture or glucose or lactulose hydrogen/methane breath testing), we identified patients with autoimmune atrophic gastritis via ICD codes. We then confirmed that these patients had a histologically confirmed diagnosis. Finally we queried the electronic health record to identify patients with non-atrophic chronic gastritis who had undergone MO testing.

We identified 52 patients with autoimmune atrophic gastritis who underwent aspirate culture; 48 (92.3%) met criteria for small intestinal bacterial overgrowth (SIBO) at a threshold of ≥10³ CFU/mL, and 39 (75%) at ≥10⁵ CFU/mL. Among patients who had multiple aspirates, 83.3% had recurrent SIBO. Among 24 patients who underwent breath testing, 11 (45.8%) tested positive, primarily reflecting intestinal methanogen overgrowth. A notable discordance was observed among seven patients who underwent both testing modalities, with five patients having a positive culture but negative breath test. Finally, comparing MO results from patients with autoimmune atrophic gastritis to those with non-atrophic chronic gastritis identified significantly higher test positivity in autoimmune atrophic gastritis (75% vs. 34.3% at ≥10⁵ CFU/mL, P<0.00001; 92.3% vs. 65%, P=0.000043), but not for breath tests (45.8% vs. 30.3%, P=0.106).

MO is common among symptomatic patients with autoimmune atrophic gastritis, and intestinal aspirate culture identifies more positive cases than breath testing in this population. Findings are subject to selection bias and the inherent constraints of both MO testing modalities, and require confirmation with prospective studies.