A method for diagnosing a Helicobacter pylori infection in a patient treated with proton-pump-inhibitors (PPIs) comprising the steps of administering to the patient an acid, collecting a first breath sample, administering to the patient 13C-labeled urea, waiting for a time of 10 to 60 minutes, thereafter collecting a second breath sample from the patient, measuring the content of 13C in the CO2 of the first and second sample and determination of a 13C/12C ratio by mass spectroscopy in the respective samples characterized in that a difference Δδ of the 13C/12C ratio of the first breath sample and 13C/12C ratio of the second breath sample is calculated and the value of the difference in the range of 2 to 2,9% indicates the presence of a H. pylori infection in the patient.
