The ILDnose study a multinational, multicenter, prospective, longitudinal study in outpatients with pulmonary fibrosis. The aim is to assess the accuracy of eNose technology as diagnostic tool for diagnosis and differentiation between the most prevalent fibrotic interstitial lung diseases. The value of eNose as biomarker for disease progression and response to treatment is also assessed. Besides, validity of several questionnaires for pulmonary fibrosis is investigated.

Patients will be included in the study after signing written informed consent. eNose measurements will take place before or after a routine outpatient clinic visit at the same location as the regular visit, ensuring minimal inconvenience for patients. First, patients will be asked to rinse their mouth thoroughly with water three times. Subsequently, exhaled breath analysis will be performed in duplicate with a 1-minute interval. An eNose measurement consists of five tidal breaths, followed by an inspiratory capacity maneuver to total lung capacity, a five second breath hold, and subsequently a slow expiration (flow <0.4L/s) to residual volume. The measurements are non-invasive and will cost approximately 5-10 minutes in total, including explanation and informed consent procedure. There are no risks associated with this study and the burden for patients is minimal.

After the measurement, patients will complete a short survey about questions relevant for the data analysis (food intake in the last two hours, smoking history, medication use, comorbidities, and symptoms of respiratory infection). In addition, patients will complete the L-PF questionnaire and the Global Rating of Change scale (GRoC). The L-PF questionnaire consists of 21 questions on a 5-point Likert scale about the impact of pulmonary fibrosis on quality of life, and takes about 3 minutes to complete. The GRoC consists of one question on a scale from -7 to 7: were there any changes in your quality of life since your last visit? Symptoms (cough and dyspnea) will be scored on a 10 cm VAS scale from -5 to 5.

Next to eNose measurements, demographic data and physiological parameters of patients will be collected from the medical records at baseline, month 6, and month 12. Parameters such as age, gender, diagnosis, time since diagnosis, comorbidities, medication, pulmonary function (forced vital capacity (FVC) and diffusion capacity of the lung for carbon monoxide (DLCO)), laboratory parameters (i.e. auto-immune antibodies), HRCT pattern, BAL results and if applicable also genetic mutations, will be recorded and stored in an electronic case report form. These parameters will be collected as part of routine daily care, patients will not undergo any additional tests for study purposes. HRCT scans will be re-analysed centrally by an experienced ILD thoracic radiologist. Mortality and lung function parameters will also be collected at 24 months, if this information is available.

Inclusion Criteria:

• Patients with a diagnosis of fibrotic ILD, as discussed in a multidisciplinary team meeting (50% incident patients and 50% prevalent patients). Patients are classified as 'incident' if they received a diagnosed in a multidisciplinary team meeting within the past six months. Patients will be required to have fibrosis on a HRCT scan <1 year before enrollment in the study defined as reticular abnormality with traction bronchiectasis, with or without honeycombing, as determined by a radiologist. No minimum extent of fibrosis will be required.

Exclusion Criteria:

• Alcohol consumption ≤ 12 hours before the measurement
• Physically not able to perform eNose measurement

Patients with ILD