Customary ¹³CO₂ breath tests—and also ¹⁵N urine tests—always start with an oral administration of a test substrate. The test person swallows a stable isotope labelled diagnostic agent. This technique has been used to study several pathophysiological changes in gastrointestinal organs. However, to study pathophysiological changes of the bronchial and lung epithelium, the inhalative administration of a stable isotope labelled agent appeared more suitable to us. [1-¹³C]Hexadecanol and [1-¹³C]glucose were chosen. Inhaled [1-¹³C]hexadecanol did not yield ¹³CO₂ in the exhaled air, but [1-¹³C]glucose did. To study the practicability of the [1-¹³C]glucose method and the reproducibility of the results, 18 inhalation tests were performed with healthy subjects. In 6 self-tests, the optimum inhalative dose of [¹³C]glucose was determined to be 205 mg. Using the APS aerosol provocation system with the nebulizer ‘Medic Aid’ (Erich Jaeger Würzburg), a 25% aqueous solution was inhaled. Then, breath samples were collected at 15 min. intervals and analysed for ¹³CO₂. 75–120min after the end of inhalation a well-reproducible maximum δ¹³C value of 6‰ over baseline (DOB) was detected for 12 healthy probands.

Speculating that the pulmonary resorption of the [¹³C]glucose is the rate-limiting step of elimination, decompensations in the epithelium ought to be reflected in changed [1-¹³C]glucose resorption rates and changed ¹³CO₂ output.

Therefore, we speculate that the inhalation of suitable ¹³C-labelled substrates will pave the way for a new group of ¹³CO₂ breath tests aiding investigations of specific pathophysiological changes in the pulmonary tract, such as inflammations of certain sections and decompensations of cell functions.