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2.2. Sarcoidosis case population Patients aged 18 years or older diagnosed with sarcoidosis from January 1, 2001 to December 31, 2015 were identified in the Danish National Patient Register (DNPR). Cases were followed until death, migration or end of follow-up in 2018. The DNPR, which is an administrative registry, has achieved complete nationwide coverage on all non-psychiatric admissions since 1978 and out-patient clinic contacts since 1995 [21]. Registration, which is mandatory and submitted by the treating physician, is used for continuous monitoring of hospital and health service utilization and for billing purposes [18]. Registration includes diagnoses, administrative information and diagnostic procedures, but not results of procedures (e.g., spirometry values, bronchoscopy findings). Primary and secondary diagnoses are classified according to the International Classification of Diseases and Related Health problems 10th Revision (ICD-10). ICD-10 code D86, Sarcoidosis, was applied to identify patients with a primary or secondary diagnosis of pulmonary and/or extra-pulmonary sarcoidosis. 2.3. Incident sarcoidosis Incidence proportion (IP) was assessed yearly from 2001 to 2015 and defined as cases with a first-time diagnosis of sarcoidosis in the DNPR per 100,000 citizens per year. To ensure cases were included on the date of their first-time diagnosis of sarcoidosis, a three-year washout period was applied, to exclude patients with a sarcoidosis diagnosis from 1998 to 2000. To minimize the risk of misclassification, incident cases were restricted to individuals with >1 contact regarding sarcoidosis within the first year from the index diagnosis. Furthermore, because cancer and sarcoid-like reactions (often caused by cancer) are important differential diagnoses to sarcoidosis due to morphological and pathological similarities, individuals diagnosed with cancer (ICD-10 C00-C97) six months prior to or after the time of sarcoidosis diagnosis were excluded, to minimize the risk of misclassification. 2.4. Prevalent sarcoidosis The prevalence proportion (PP) was defined as a contact prevalence and included the proportion of cases with a recorded contact regarding sarcoidosis within one calendar year per 100,000 citizens. To reduce the risk of misclassification, prevalent cases were restricted to cases with at least one more contact during a five-year span, that included the prevalent year and two years prior to/post the prevalent year. By applying this five-year span for a second visit we furthermore ensured the inclusion of cases with a last contact in 2001–2002 and cases with a first contact in 2014–2015 because a second visit was captured given the extended study period from 1998 to 2018. 25. Treatment groups The Danish Register of Medicinal Product Statistics (DRMPS) includes data on all prescription drugs dispensed in Danish community pharmacies since 1995 and consists of high quality data [22]. We did a subgroup analysis in relation to various treatment regimens as a proxy for disease severity. Cases, who redeemed prescriptions of systemic corticosteroid (SC; ATC H02AB), methotrexate (MTX; ATC L01BA01/L04AX03) or azathioprine (AZA; ATC L04AX01) three months prior to, and up to three year after, diagnosis, were identified in the DRMPS. The follow-up period continued until 2018, thus ensuring a minimum of three years follow-up after diagnosis for all incident cases. The sarcoidosis case population was divided into a non-treatment group and a treatment group. The treatment group was then divided into four treatment groups: 1. Short-term treatment group (redemption of ≥1 prescription of SC three months prior to and one year after diagnosis, but no redemption of prescriptions within the following 2 years) 2. Late initiation treatment group (No redemption of prescribed SC three months prior to and one year after diagnosis, but redemption of ≥1 prescription within the following 2 years) 3. Continuous treatment group (Redemption of ≥1 prescription of SC three months prior to and one year after diagnosis and redemption of ≥1 prescription within the following two years) 4. + MTX/AZA treatment group (Cases identified as in the continuous treatment group, but who also redeemed ≥ 1 prescription of MTX or AZA within the three-year follow-up period) 2.6. Diagnostic procedures All diagnostic procedures in hospital settings are recorded in the DNPR. Information on diagnostic procedures both three months before and after the date of the index diagnosis included: spirometry, bodyplethysmography, lung diffusion capacity, chest x-ray, chest CT, PET, bronchoscopy, EBUS/EUS, mediastinoscopy, thoracentesis as well as any procedure involving a biopsy (including skin, liver, lymph node and lung) (Supplementary 2).