Hydrogen breath test in patients with severe constipation: the interference of the mixing of intestinal content.

Background: The diagnostic accuracy of the hydrogen (H2 ) breath test might be reduced by the release of preformed H2 , trapped in hard stools. Test solution ingestion might induce the mixing of colonic content and a false positive result. We studied severely constipated patients, at diagnosis and after the normalization of bowel function, to clarify whether this mechanism affects test results.

Methods: Twenty functional constipated patients, 10 consecutive patients with functional diarrhea and 10 healthy volunteers underwent (i) a H2 breath test after lactulose, to exclude differences among the groups in fermenting capacity; (ii) breath H2 excretion monitoring after non-absorbable, non-fermentable PEG-electrolyte solution, to exclude the role of the delivery to the colon of preexisting fermentable substrates or of the release of preformed H2 entrapped in the feces; (iii) H2 measurement during a 7-h fasting period, to exclude the role of spontaneous variations of breath gas excretion; and (iv) breath H2 excretion monitoring after PEG, after normalization of bowel function.

Results: All the subjects excreted similar amounts of H2 after lactulose. After PEG, only severely constipated patients showed significant breath H2 excretion, theoretically able to induce a false positivity of the lactose breath test in 70% of patients and a false positivity of glucose breath tests in 50% of patients. Breath H2 excretion after PEG disappeared if fecal consistency improved after therapy.

Conclusions: Severely constipated patients may harbor preformed gas in hard stools which can be released when mixing of the intestinal content is induced. This mechanism may interfere with breath test results.