Breath tests (BTs) are used in gastroenterological practice to study (
patho)physiologicaI and metabolic processes in an indirect way. In the
se tests the appearance in breath of a metabolite of a specific test s
ubstance is studied. The assumption underlying each BT is that one ste
p-the process of interest-in the absorption and metabolism of the trac
er is rate-limiting. Both hydrogen gas excretion and carbon dioxide ap
pearance in breath can be studied. When a carbon-labelled test substan
ce is used, the stable isotope C-13 is preferred to the radioactive is
otope C-14. Measurements of C-13 in expired air are performed by mass
spectrometry. Because of the indirect nature of BTs, involving a seque
nce of reactions and metabolic pools, they usually supply semiquantita
tive data. The tests are nevertheless useful because they often replac
e invasive techniques with a simple procedure that is safe because the
re is no radioactivity involved. BTs have been used to measure gastric
emptying, the presence of Helicobacter pylori in the stomach, small-b
ower bacterial overgrowth, exocrine pancreatic function as well as liv
er metabolic capacity; other potential applications of BTs are being s
tudied.