Lower respiratory tract infections (LRTIs) remain a major global health burden, with diagnosis often challenged by clinical heterogeneity, overlapping symptoms, difficulty in obtaining quality biological specimens and limitations of conventional microbiological testing. Exhaled breath contains thousands of volatile organic compounds (VOCs), arising from both host metabolism and microbial activity, and provides a non-invasive window into the metabolic landscape of infection. Emerging evidence from in vitro studies, animal models, and clinical investigations supports the existence of pathogen-specific VOC signatures. However, most published studies have relied on offline methods, such as gas chromatography-mass spectrometry (GC-MS), and robust proof-of-concept for real-time, bedside analysis in diverse, real-world populations is lacking. Proton transfer reaction-mass spectrometry (PTR-MS) offers rapid, quantitative, and non-invasive assessment of breath VOCs in both spontaneously breathing and mechanically ventilated patients.

Inclusion Criteria:

• Patient aged > 1 month, either spontaneously breathing or intubated and mechanically ventilated
• Having suspected lower respiratory tract infection (dyspnea or tachypnea or cough or sputum production or fever or decreased breath sounds on auscultation or any other suspicious diagnostic test (imaging, blood analysis)) or previously documented chronic airway colonization
• Willing to provide informed consent and affiliated with a health insurance system

Exclusion Criteria:

• Patients or representative unable to provide informed consent
• Persons under guardianship or curatorship or persons deprived of liberty