SUMMARY The hydrogen breath test was performed by ingestion of 20 g lactulose and analysis of end-expiratory air. Eighteen patients undergoing colonoscopy, 17 receiving antibiotics, 12 prepared for colon surgery, and 15 controls were examined. The test was repeated under control conditions in the treated patients. Eleven of 55 subjects failed to produce significant amounts of hydrogen under control conditions. This 20% proportion of non-hydrogen producers is much higher than that reported by other investigators. The hydrogen production was very markedly depressed after preparation for colonoscopy and antibiotic therapy. The effect of neomycin and enemata as used in preparation for colon surgery was less marked. Hydrogen production by the colonic flora is thus subject to individual variations and may be affected by various therapeutic regimens. All these may cause false negative results when using the hydrogen breath test to evaluate carbohydrate absorption. The test should therefore not be performed for a considerable time after therapeutic manipulation of the colonic flora.The introduction of the hydrogen breath test (Bond and Levitt, 1972) 23 November 1977 predictable amounts of hydrogen in the process. The purpose of the present study was to evaluate the effect of alterations in the colonic flora on the hydrogen breath test. Methods SUBJECTSSixty-two subjects were studied from the outpatient and inpatient population of our hospital. Twenty-seven were females and 35 males, aged 13-79 years, with a mean of 52-6 ± 15-3 years.

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