breath test, any of various tests in which a person’s breath is analyzed for presence of something abnormal. Subgroups called the 13C breath tests and 14C breath tests involve administration of organic compounds labeled with carbon 13 (heavy carbon) or carbon 14 (radioactive carbon) and measuring the subsequent levels of labeled carbon dioxide in the patient’s breath; the labeled compound may be found to be metabolized normally, too fast, or too slow in the GI tract.
