Hypersensitivity pneumonitis (HP) is a complex syndrome resulting from repeated exposure to a variety of organic particles. It was previously thought to be uncommon, with an incidence of 0.9 per 100,000 person-years,1 but a recent retrospective case-cohort study demonstrated that in a population of patients initially diagnosed with idiopathic pulmonary fibrosis, up to 43% may be reclassified as HP.2 This highlights the challenges in making a diagnosis of HP in patients who have presented with an undifferentiated interstitial lung disease, particularly when the clinical history of allergen exposure cannot be elicited. There is currently no universally agreed upon diagnostic criteria for HP. Vasoka et al has proposed a novel classification system and diagnostic algorithm wherein the diagnosis of HP may be made confidently if there is positive exposure history to a known allergen along with typical radiological and bronchoalveolar lavage findings. In the setting of negative or uncertain history of exposure to a known allergen, serum specific immunoglobulin G (IgG) may be used as a surrogate for said exposure history to support a diagnosis of probable or possible HP.
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