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Introduction The lactulose breath test can be used to diagnose small intestinal bacterial overgrowth, but its interpretation remains controversial due to rapid colonic fermentation, which may lead to false positives. Irrespective of this, lactulose provides a measure of bacterial fermentation in the intestines via endogenous hydrogen gas production.Methods A survey-based study was conducted in 731 subjects undergoing lactulose breath testing at a single UK centre ( NCT05122520). Participants completed a 32-item questionnaire on medical history and gastrointestinal symptoms. Following a low-fermentable diet and overnight fast, baseline breath samples were collected before ingestion of 10 g lactulose in 200 ml water. Breath samples were then collected every 15 minutes for 135 minutes using Exetainer tubes (Labco, UK) and analysed by gas chromatography (Agilent, CA). Intestinal fermentation was defined by peak hydrogen production after 60 minutes using 10 ppm incremental cut-offs and analysed against survey responses using X2 tests.Results Peak breath hydrogen levels of ≥60 ppm were associated with diarrhoea-predominant bowel habit and abdominal pain with or without bloating ( table 1). Strongest associations were observed at thresholds of ≥80 ppm for daily frequency (p < 0.001). Peak breath hydrogen values of ≥70 ppm showed significant association with atopic dermatitis (p = 0.009). Participants with a peak of ≥60 ppm hydrogen were more likely to report that symptoms improved after taking antibiotics (p = 0.015) whilst those with a peak of ≥90 ppm hydrogen were more likely to report brain fog (p = 0.005) and symptoms starting after food poisoning (p = 0.045).Conclusion We propose a clinical threshold of ≥80 ppm hydrogen after 60 minutes to define excessive intestinal fermentation on lactulose breath test. These findings suggest intestinal dysbiosis may play a role in symptoms of diarrhoea-predominant IBS and eczema, warranting further mechanistic and interventional studies.Abstract P429 Table 1Breath hydrogen peak (ppm) on lactulose breath test and association with symptoms of irritable bowel syndromePredominant bowel habitH2 ppmAbdominal painBloatingBothAnyWeeklyDailyAnyWeeklyDailyAnyWeeklyDailyDiarrhoea50.001.936.183.862.561.639.850.685.11860.012.153.013.530.788.176.304.350.01070.065.104.013.430.542.108.335.289.02580.149.005<.001.091.125.024.030.010<.00190.099.016.007.063.094.019.026.024.006Mixed50.332.670.495.261.444.964.244.640.97660.980.887.457.737.609.459.881.986.84370.907.789.511.610.299.324.950.552.95180.977.892.838.663.466.687.995.654.92190.185.217.168.483.617.720.271.465.295Constipation50.001.004.075.261.444.964<.001.009.03560.012.039.157.737.609.459.010.048.05670.065.087.324.610.299.324.081.133.08980.149.252.206.663.466.687.172.289.05290.099.192.196.381.686.527.114.190.081Significant associations (p < 0.05) indicated in bold.Darker green = greater positive association. Darker red = greater negative association.