A cohort study of interstitial lung diseases in central Denmark.

INTRODUCTION: Interstitial lung diseases (ILDs) form a heterogeneous group of diseases with varying degrees of inflammation and fibrosis. Epidemiological data based on the current diagnostic criteria are sparse.

OBJECTIVES: To characterize the incidence rate of ILDs and idiopathic pulmonary fibrosis (IPF) in Danish patients diagnosed at a referral hospital, to evaluate disease severity and survival in these ILD patients and to compare the use of the 2001 and 2011 guidelines to diagnosis of IPF.

METHODS: Single-centre, retrospective, observational cohort study including incident patients diagnosed with ILD at Aarhus University Hospital between 2003 and 2009. All diagnoses were re-evaluated according to current diagnostic criteria. Disease severity in IPF was assessed using the GAP index.

RESULTS: The ILD incidence was 4.1 per 100,000 inhabitants/year. IPF was the most common diagnosis (28%) followed by connective tissue disease-related ILD (14%), hypersensitivity pneumonitis (7%) and non-specific interstitial pneumonia (NSIP) (7%). The GAP index was a strong predictor of survival in IPF. Twenty-three patients who had IPF based on the 2001 criteria had a "possible UIP" HRCT pattern but no lung biopsy, and IPF could therefore not be diagnosed based on the 2011 criteria.

CONCLUSION: ILD and IPF incidence was 4.1 and 1.3 per 100,000 inhabitants/year. The diagnostic re-evaluation raised the number of IPF diagnoses, but a diagnostic "grey zone" was still evident in patients with UIP features not qualifying the patients to be diagnosed with IPF. The GAP index was valuable as a measure of IPF severity in this cohort.