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Background Hypersensitivity pneumonitis (HP) is an immune-mediated interstitial lung disease characterised by lymphocytic infiltration of the airways and, in more advanced stages, fibrotic destruction of lung parenchyma, which worsens prognosis. Avoidance of the antigen (Ag) is crucial for modifying disease progression, yet it remains undetectable in over 50% of cases. The heterogeneity of HP and the lack of robust diagnostic and treatment guidelines complicate patient management.Aim To assess differences between fibrotic HP (fHP) and inflammatory HP (iHP) phenotypes.Methods We conducted a retrospective analysis of 160 HP patients from an ILD cohort at a large UK centre. Patients were categorised as fHP or iHP based on radiological reviews by the ILD multidisciplinary team. We analysed demographic, clinical, and radiological data and applied Chi-square tests for categorical variables and t-tests or Mann-Whitney U tests for continuous variables.Results Amongst 2666 ILD cohort patients, HP prevalence was 6.2%, with 111 (69%) diagnosed with fHP ( table 1). Patients with fHP were older (72.1 vs. 61.1 years, p<0.001) and predominantly male (42.3% vs. 14.2% p≤0.001). Notably, obstructive spirometry was more frequent in iHP (17.0% vs. 6.5%, p<0.04). Reported Ag exposure was similar in both groups (77% iHP vs. 68.4% fHP, p=0.242), with avian exposure being commonest (89.4% in iHP vs. 59% in fHP). Positive serum avian precipitins were more frequently detected in iHP (79.1% vs. 53.5%, p=0.004). Both groups showed diffuse lung involvement, and persistent groundglass (88.2%) and mosaicism (77.4%) was still seen in fHP. There was no significant difference in the cellular composition of bronchoalveolar lavage fluid (BALF) between the groups. Importantly, a BALF lymphocytosis of >30%1 was seen in both fHP and iHP.Abstract P97 Table 1Characteristics of inflammatory and fibrotic HPConclusion Older age and male gender may represent risk factors for fHP. Positive serum precipitins may suggest persistent immune activation and represent an opportunity for immunomodulation in iHP to delay or prevent fibrotic complications. Prospective, longitudinal studies are required to further understand the evolution of iHP and role of serum precipitins and BALF lymphocytosis in disease behaviour.Reference Raghu, et al. Diagnosis of hypersensitivity pneumonitis in adults. Am J Respir Crit Care Med. 2020 Aug 1;202(3):e36-e69.