Abstract: The method is based on the carbon-13 labelled urea breath test, which comprises a) administering to the patient an aqueous solution of citric acid at pH comprised between 2 and 2.5; b) collecting a sample of the patient's breath with the object of determining the baseline content of carbon-13; c) administering to the patient a suitable amount of carbon-13 labelled urea; d) collecting a sample of the patient's breath with the object of determining the content of carbon-13 after the administration of the labelled urea; and e) analyzing the breath samples collected to determine the carbon-13 content before and after the administration of urea labelled with carbon-13. The kit has a container for the citric acid, another for the carbon-13 labelled urea, exetainers destined to containing the patient's breath, and means to blow the breath into the exetainers.