A cohort study of Danish patients with interstitial lung diseases: burden, severity, treatment and survival.

Interstitial lung diseases (ILDs) form a heterogeneous group of rare diseases characterised by varying degrees of pulmonary inflammation and fibrosis. We hypothesised that IPF and unclassifiable ILD were common in a Danish ILD cohort and that prognostic factors based on disease characteristics and comorbidities could be identified The aims of the PhD study were to describe the demographics of ILD in Central Denmark, to characterise the distribution of ILD diagnoses, and to assess prognostic factors in IPF and unclassifiable ILD. The study is based on a cohort of 431 ILD patients referred to our department during a 6-year period. All ILD diagnoses were re-evaluated according to current diagnostic criteria. Patients were followed from the time of first visit on suspicion of an ILD to the last visit to the centre, death, transplantation, or discharge from follow-up. The incidence of ILD was 4.1 per 100,000 inhabitants, and the incidence of IPF was 1.3 per 100,000 inhabitants in Central Den-mark. The most frequently occurring ILDs were IPF (28%), unclassifiable ILDs (extensive fibrotic disease and other unclassifiable ILDs) (24%), connective tissue disease-related ILD (14%), hyper-sensitivity pneumonitis (7%) and NSIP (7%). Cardiovascular dis-ease was present in 21% of the patients. The presence of cardio-vascular disease at the time of IPF diagnosis did not lead to increased mortality, whereas cardiovascular disease diagnosed during the course of IPF was a statistically significant predictor of mortality. Our study also showed that diabetes and concomitant anticoagulant therapy were associated with worse outcome in IPF, and that a simple HRCT scoring system could be used in the prediction of outcome in fibrotic ILDs. The study of unclassifiable ILD revealed two disease categories: one group characterised by extensive fibrotic disease and one characterised by more inflammatory features. The latter group was characterised by younger age and significantly better prognosis. We evaluated the pragmatic disease classification based on the clinical disease pattern included in the 2013 revision of the guidelines of diagnosis and treatment of interstitial lung diseases. We found that it was able to separate patients with unclassifiable ILD into categories with highly significant differences in survival. We also evaluated the ILD-GAP model, which is based on gender, age and pulmonary function (physiology), and found that it was a valuable predictor of survival in unclassifiable ILD. In a multivariate model, the two prediction scores showed significant individual contribution to the prognostic assessment. The present study has provided the first estimate of ILD and IPF incidence in the Danish population and has shown that demographics and survival of IPF in this cohort were comparable to what has been reported in other studies. Comorbidities were common among patients with IPF, and the results of the study have led us to believe that careful diagnosis and treatment of comorbidities are important in order to optimise outcome in patients with IPF, although our findings need to be confirmed in larger studies. Unclassifiable ILD is frequent in daily clinical practice but has not been characterised in detail. Our study showed that it was possible to identify predictors of outcome and to validate the ILD-GAP model in this cohort. The study also showed that the Disease Behaviour Classification can be used in the management of patients with unclassifiable ILD.