Pulmonary diseases are common among patients with Systemic Lupus Erythematosus (SLE) and associated with a significantly increased morbidity, mortality, and lower self-reported health related quality of life.

The investigators aim to diagnose and categorise SLE patients in regards of pulmonary disease and introduce alternative diagnostic tools.

The investigators will perform a cross-sectional study of a population-based study population of SLE patients. The participants will undergo review of medical record, a clinical assessment, body plethysmography, six-minute walk test, high-resolution computed tomography of the thorax (HRCT), thoracic and diaphragmatic ultrasound, blood sampling with analysis of autoantibodies, and three questionnaires. After the investigations, pulmonary diseases among the participants will be diagnosed at a multidisciplinary discussion by a specialised pulmonologist with contribution from a rheumatologist and a radiologist.

The investigators believe that the study will increase the understanding of pulmonary diseases among SLE patients, which could improve overall disease management. The investigators will introduce new alternative diagnostic tools, that could ease diagnosing pulmonary disease among SLE.

Background Pulmonary diseases are common among patients with Systemic Lupus Erythematosus (SLE) and cover a variety of manifestations including pleuritis and interstitial lung disease. Pulmonary diseases among SLE patients are associated with an increased morbidity, mortality, and lower self-reported health related quality of life.

Objective The primary objective of the study is to diagnose and categorise a population-based study population of SLE patients in regards of pulmonary diseases. The secondary objective of the study is to introduce alternative diagnostic tools to diagnose pulmonary disease among SLE patients, including thoracic ultrasound and circulating biomarkers.

Methods and Analysis The study will be reported according to the STROBE guidelines. The investigators will perform a cross-sectional study of a population-based study population of SLE patients. The participants will undergo review of medical record, a clinical assessment, body plethysmography, six-minute walk test, HRCT, thoracic and diaphragmatic ultrasound, blood sampling with analysis of autoantibodies, and three questionnaires translated to Danish - Saint George's Respiratory Questionnaire, Systemic Lupus Activity Questionnaire, and Lupus Impact Tracker. After the investigations, pulmonary diseases among the participants will be diagnosed at a multidisciplinary discussion by a specialised pulmonologist with contribution from a rheumatologist and a radiologist. The results will be blinded to the investigators except for Henrik Zachar Langkilde who will have access to results (except for the MDD diagnose) while the investigations are performed, and before the unblinding a statistical analyse plan will be reported. At the second of October 22 participants have had a visit.

Discussion Pulmonary diseases among SLE patients are common and of importance to the patients, but an area not studied as extensively as other disease manifestations. The investigators believe that this study will increase the understanding of pulmonary disease among SLE patients, which could improve overall disease management for the patients. The investigators hope that the alternative diagnostic tools introduced will ease to diagnose pulmonary disease among SLE patients. The study is approved by the Committees on Health Research Ethics and an umbrella organisation under the Danish Data Protection agency. The project, the protocol and the study design are discussed with two SLE patient partners, and the patient partners suggestions have been incorperated in the project.

Inclusion Criteria:

• Followed at a Department of Rheumatology in Region Southern Denmark.
• Living in Region Southern Denmark.
• Diagnosed with Systemic Lupus Erythematosus.
• Fulfil the 2019 European League Against Rheumatism / American College of Rheumatology classification criteria.
• Speak and understand spoken and written Danish

Exclusion Criteria:

• Participants not having their native lungs
• Not participating in visit day, without a valid reason.

All patients with Systemic Lupus Erythematosus, in Region Southern Denmark (approximately 300 individuals)