Incidence, comorbidity and survival rate of hypersensitivity pneumonitis: a national population-based study

Hypersensitivity pneumonitis (HP) is a rare disease, which can lead to premature death. Few studies have investigated HP on a national level. The objective of this study was to investigate incidence, survival rate, and comorbidity of HP in Denmark.

Using the Danish National Patient Registry we identified all patients with a first-time diagnosis of HP between 1998 and 2010. Patients with HP were matched 1:4 with controls by sex, age and geography in this case-control study. Comorbidity 3 years prior to diagnosis was explored by the Charlson score index. Survival rates were assessed using Kaplan-Meier curves and hazard ratios.

We identified 753 patients during the observation period equalling an average HP incidence of 1.16 per 100 000 citizens. Patients with HP had a significantly higher Charlson score index when compared with the matched controls and an increased risk of dying (hazard ratio 1.98, CI 1.61-2.58, se 0.14, p<0.001). Survival rates of HP were lower at all time points when compared with the matched control population. The decline in survival was observed for both male and female patients with HP with no clear sex difference. Most deaths were related to diseases of the heart and lungs.

In this Danish longitudinal nationwide observational study we found an increased Charlson score index combined with a higher mortality without sex difference among patients with HP compared with a healthy control group, mainly due to diseases of the heart and lungs.