Characteristics of hypersensitivity pneumonitis diagnosed by interstitial and occupational lung disease multi-disciplinary team consensus

Introduction: The causes of hypersensitivity pneumonitis (HP) in the UK are changing as working practices evolve, and metalworking fluid (MWF) is now a frequently reported causative exposure. We aimed to review and describe all cases of HP from our UK regional service, with respect to the causative exposure and diagnostic characteristics. Methods: In a retrospective, cross-sectional study, we collected patient data for all 206 cases of HP diagnosed within our UK-based regional NHS interstitial and occupational lung disease service, 2002-17. This included demographics, environmental and occupational exposures, clinical features, and diagnostic tests (CT imaging, bronchiolo-alveolar cell count, lung function, histology). We grouped the data by cause (occupational, non-occupational and unknown) and by presence or absence of fibrosis on CT, in order to undertake hypothesis testing. Results: Cases were occupational (n=50), non-occupational (n=56) or cryptogenic (n=100) in aetiology. The commonest causes were birds=37 (18%) and MWF=36 (17%). Other occupational causes included humidifiers and household or commercial waste, but only one case of farmers' lung. Cryptogenic cases were associated with significantly older age, female gender, lower lung function parameters, fewer alveolar lymphocyte counts>20%, and fibrosis on CT; exposure information was missing in 22-33% of cryptogenic cases. Conclusion: MWF is the commonest occupational cause of HP, where workers usually present with more acute/subacute features and less fibrosis on CT; refuse work is an emerging cause. Cryptogenic HP has a fibrotic phenotype, and a full occupational history should be taken, as historical workplace exposures may be relevant.