Breath tests have been used in research laboratories for over 25 years. Originally, the tests were based on the use of 14C, rather than on the nonradioactive isotope, 13C. When 13C became widely available at a reasonable cost, research groups in the United States and Europe developed methodologies to measure 13C abundance in samples of CO2. The tests used a variety of substrates and measured pancreatic function, fat absorption, bacterial overgrowth and P450 mixed-function oxidase.