Background and Aims: Validity of the lactulose breath test (LBT) to diagnose small intestinal bacterial overgrowth (SIBO) has been questioned. Therefore, a study was planned to compare LBT with glucose breath test (GBT) to diagnose SIBO in irritable bowel syndrome (IBS) patients and controls. Methods: 175 diarrhea-predominant IBS patients and 150 apparently healthy controls were enrolled. IBS was diagnosed according to Rome II criteria. Breath samples were collected every 10 min up to 180 min. Breath H2 and CH4 were measured using an SC MicroLyzer. SIBO was positive with a sustained increase in breath H2 or CH4 or both ≥10 ppm over a baseline value within <90 min in case of LBT and within <120 min in GBT. Results: SIBO was positive in 60/175 (34.3%) patients by lactulose and in 11/175 (6.2%) patients by GBT. In controls, LBT was positive for SIBO in 45/150 (30%) patients and in 1/150 (0.66%) patients by GBT. Positive LBT for SIBO was not significantly different in patients and controls; while using GBT, SIBO was significantly higher (p < 0.01) in patients as compared to controls. By using GBT as gold standard for SIBO, sensitivity, specificity, positive predictive value and negative predictive value of LBT in IBS patients was 63.6, 67.7, 11.7 and 96.6% respectively. Conclusion: LBT is not a good test to discriminate SIBO in IBS patients from controls.

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