Patients will receive the EQ-5D-5L measuring their health-related quality of life. The EQ-5D-5L is a standardised self-report questionnaire developed by the EuroQol group measuring the perceived health status of patients. It consists of the EQ descriptive system comprising five dimensions and the EQ visual analogue scale (VAS).

Each dimension of the EQ descriptive system (i.e. mobility, self-care, usual activities, pain/discomfort, anxiety/depression) has 5 levels of response options (i.e. 1= no problems, 2= slight problems, 3=moderate problems, 4= severe problems and 5= extreme problems) and the combined answers of the dimensions result in a five-digit number describing the patient's health.

Patients are also asked to fill in the EQ VAS where they have to report their current perceived health status on a vertical analogue scale. This scale ranges from 'best imaginable health state, i.e. 100' to 'worst imaginable health state, i.e. 0'.

Health literacy will be assessed in this study because poor health literacy might be an important risk factor for therapy non-adherence and it was shown to be associated with a poorer capacity to comprehend and to follow medical advice as well as having poorer health outcomes. The self-administered single-item variant of the subjective health literacy screener (SHLS) will be given to the patients at baseline. In a validation study, the question 'how confident are you in filling out medical files by yourself' scored significantly better than two other investigated questions to detect patients with inadequate health literacy. This question will be scored on a 5-point likert scale (i.e. none, a little, some, most, all of the time), where a score between 0 and 2 (i.e. none, a little or some of the time) will be seen as inadequate health literacy.

The patient health questionnaire (PHQ-9) will be used to assess symptoms of depression, as it is a reliable and validated self-administered nine-item questionnaire based on the nine diagnostic criteria for DSM-IV depressive disorders. The score ranges between 0 and 27, since each item can be scored from 0 (not at all) to 3 (nearly every day). Cut-off points of 5, 10, 15, and 20 represent the lower limits of mild, moderate, moderately severe and severe depression, respectively.

Barriers will be assessed with the adapted version of the 'immunosuppressive medication adherence barrier scale' (IMAB) developed by the transplant360 Task force. The questionnaire consists of 27 items that are scored on a 5-point likert scale (i.e. never, rarely, sometimes, often, always) resulting in a median score for all patients. Several barriers will be explored such as health beliefs, forgetfulness, disruption of the routine and complexity of the regimen.

Patients will receive the K-BILD questionnaire measuring their health-related quality of life. The K-BILD is a 15-item self-completed questionnaire measuring the health-related quality of life (HRQL) of patients with interstitial lung disease. It consists of three domains: breathlessness and activities, psychological and chest symptoms. Each question has a seven-point response scale resulting in a total score ranging between 0 and 100 with a higher score reflecting a higher HRQL.

The St- George's Respiratory questionnaire (SGRQ) is a 50-item questionnaire consisting of two parts. The first part evaluates the symptoms (i.e. cough, sputum production, wheezing and breathlessness) and the second part evaluates the activities (i.e. activities causing breathlessness or limited by breathlessness) and the impacts (e.g. on employment, daily life, etc.). Each question is scored as positive (i.e. =1) or negative (i.e. =0) and the total score is expressed as a percentage where 0% represents the best possible health status and 100% the worst possible health status. A total score can be obtained as well as a score for each of the three components.

DLco data will be retrieved from the medical file of the patient.

Patients will report their medication adherence by means of the validated Basel Adherence Assessment Scale for Immunosuppressive Medications (BAASIS). The questionnaire includes five questions about taking, dosing and timing of medication intake, as well as drug holidays, and treatment discontinuation. All deviating answers (i.e. questions answered by 'yes') will be defined as non-adherence.

Patients need to have the psychological capacity to adhere to their treatment meaning that they need to have adequate knowledge about their disease (e.g. severity of the disease) and treatment (e.g. side effects, dosing regimen). A specific questionnaire developed for this study will be given to patients to measure their knowledge about IPF and Pirfenidone. It consists of 6 true/false/I don't know questions on the treatment of IPF. Correct answers will receive a point resulting in a score ranging between 0 and 6 with higher scores reflecting a better knowledge.

To measure the physical activity in this study, the two-item Brief Physical Activity Assessment Tool based on the physical activity guidelines, will be used. The total score ranges between 0 and 8 with a score of 4 or more reflecting an adequate physical activity level.