Lactose breath test in adults: Are symptoms predictive of lactose malabsorption?

Background/objective Lactose malabsorption (LM) is common, yet not all malabsorbers develop lactose intolerance (LI). We aimed to assess whether symptoms recorded during the hydrogen breath test (HBT) predict LM and whether methane (CH4) measurement adds diagnostic information. Methods In this retrospective study, 199 adults without organic gastrointestinal disease underwent a standardized lactose HBT between January 2022 and February 2025. Pre-test and intra-test gastrointestinal symptoms were graded on a 0–10 Numerical Rating Scale (NRS) and used to classify LI as mild, moderate, or severe. LM was defined by a ≥20 ppm rise in H2 from baseline; CH4 positivity by a ≥10 ppm rise. Univariable and multivariable analyses explored the association between symptoms and test outcomes. Results LM was present in 56,8% of patients. Overall, 84.9% reported at least one symptom during the test. Flatulence during testing was the symptom most strongly associated with LM (p < 0,01) and correlated with peak H2 values, whereas pre-test symptoms showed no association with LM. Patients who experienced at least one severe symptom (NRS ≥6) were significantly more likely to be malabsorbers (39,8% vs 18,6%; p < 0,01), while lower symptom grades were frequent in both LM+ and LM− subjects. CH4 positivity was observed in 19,1% of patients; in 3,5% of the cohort, LM was detected only by CH4 despite a negative HBT. Higher baseline CH4 anticipated a greater post–challenge increase. Conclusions In adults with unexplained functional gastrointestinal symptoms, standardized symptom recording during lactose breath testing provides clinically useful context, with flatulence emerging as the most informative symptom for LM. Grading intolerance according to symptom severity effectively helps to identify lactose malabsorbers. Adding methane measurements may further improve diagnostic accuracy. Overall, these findings support an individualized diagnostic pathway that blends gas profiling with standardized symptom scoring—guiding targeted management while limiting unnecessarily restrictive diets.