The urea breath test (UBT) uses the non-radioactive isotope carbon-13 (13C). It relies on the ability of Helicobacter pylori bacteria to produce the enzyme urease, which breaks down urea. The patient drinks a solution of urea labeled with carbon-13, and the concentration of this isotope in their exhaled breath is measured before and after drinking. An increase in the concentration confirms the presence of an active infection.