Long-term Experience in Fibrotic Hypersensitivity Pneumonitis: A Comprehensive Study of Progressive Pulmonary Fibrosis at a Tertiary Center

<bold>Introduction:</bold> Hypersensitivity pneumonitis (HP) is a complex interstitial lung disease (ILD), classified into fibrotic (HPf) and non-fibrotic subtypes. A subset of HPf patients develop progressive pulmonary fibrosis (PPF). The aim of this study is to establish predictors of PPF progression. <bold>Methods:</bold> Patients diagnosed with HPf at the ILD Clinic of Vall d’Hebron University Hospital between January 2010 and December 2022, and with a minimum follow-up period of one year were included. Demographic and clinical data were collected at diagnosis and during follow-up. Date of death or lung transplant was obtained. Patients with HPf were further categorized into PPF and non-PPF according to the definition provided on ATS/ERS guidelines of 2022. <bold>Results:</bold> One hundred fifty-three patients diagnosed with HPf were identified. Seventy-six patients (49.7%) subsequently developed PPF, with 19 (25%) requiring lung transplantation. FVC at the time of diagnosis was 65% in PPF group vs 75% on non-PPF group (p<0.001). The presence of fibroblastic foci and microscopic honeycombing in the biopsy performed at diagnosis were independently associated with the development of PPF in the multivariable logistic model, with odds ratio of 9.02 and 9.63, respectively. The median time to development of PPF was 3.18 years, with a median time to transplant in this group of 4.4 years. <bold>Conclusions:</bold> Half of the patients with HPf developed PPF during the course of the illness, with a median time to diagnosis of 3.18 years. Reduced FVC, the presence of fibroblastic foci, and microscopic honeycombing in biopsy at diagnosis were associated with the development of PPF.