Predicting a Response to Antibiotics in Patients with the Irritable Bowel Syndrome

Antibiotics for presumed small intestinal bacterial overgrowth have been shown to improve irritable bowel syndrome symptoms in at least 40% of subjects. A lactulose breath test for small intestinal bacterial overgrowth has been used to select patients who will respond. However, its predictive value, using the classic definition of a positive lactulose breath test, has been disappointing.

We conducted a retrospective evaluation to study characteristics of the lactulose breath test that may be predictive of a response to antibiotics in patients with the irritable bowel syndrome.

A clinical practice database was interrogated for consecutive patients who had a lactulose breath test for irritable bowel syndrome symptoms and a subsequent antibiotic course. Hydrogen + methane levels with carbon dioxide correction were plotted against time. Various profiles of the breath test curves were catalogued and compared with respect to their predictive value for symptom response to antibiotics.

Lactulose breath test graphs of 561 patients of all irritable bowel syndrome subtypes were grouped into categories based on their hydrogen + methane levels with respect to time. Of subjects whose hydrogen + methane rise was <20 ppm throughout the test (group 1; N = 95), 94.7% improved after antibiotics (95% CI 90.1-99.3). Of those with a rise <20 ppm within the first 90 min but a rise >50 ppm thereafter (group 3; N = 53), 47.2% improved (95% CI 33.7-60.6). The difference between groups 1 and 3 was statistically significant P < 0.001.

A lactulose breath test appears to be useful in predicting response to antibiotics in patients with the irritable bowel syndrome. A hydrogen + methane rise <20 ppm throughout the duration of the test is most predictive. This observation contradicts the classic definition of a positive lactulose breath test.