Demystifying fibrotic hypersensitivity pneumonitis diagnosis: it's all about shades of grey

The spotlight on diagnosis in hypersensitivity pneumonitis (HP) intensifies. Over the past 18 months we have seen two pulmonary perspectives proposing diagnostic approaches to HP; an international Delphi survey and several imaging studies focused on clinical and imaging features predictive of HP; and clinical practice guideline initiatives for HP, sponsored by the American Thoracic Society and American College of Chest Physicians, are underway [1–5]. Despite this growing literature, HP continues to resist our best attempts to develop an accurate, reproducible case definition for this disorder. Making a diagnosis of HP is challenging because it is often shadowed by marginal exposure histories, borderline antigen positivity or discordant imaging patterns, and treatment decisions are often based on diagnostic probability rather than certainty [4, 5]. As in idiopathic pulmonary fibrosis (IPF), chest high-resolution computed tomography (HRCT) plays a central role in the initial evaluation of HP and involves the identification of specific patterns. For many years, the presence of mosaic attenuation (MA) on CT has been widelyaccepted as a cardinal imaging feature of HP