Pathologic separation of idiopathic pulmonary fibrosis from fibrotic hypersensitivity pneumonitis.

Accurate separation of idiopathic pulmonary fibrosis from fibrotic (chronic) hypersensitivity pneumonitis is crucial to patient management, but is frequently a difficult problem. Our objective was to identify pathologic variables that help make this separation. Clinical, radiological, and pathologic data were re-reviewed for 23 patients with a fibrotic interstitial lung disease and biopsy suggesting idiopathic pulmonary fibrosis or fibrotic hypersensitivity pneumonitis. Clinical features, high-resolution computed tomography, and surgical lung biopsies were each examined independently using a prespecified approach. This was followed by a multidisciplinary discussion in which the likelihood of an idiopathic pulmonary fibrosis diagnosis was assigned by the clinician alone based only on clinical data, by the clinician and radiologist based on integrated clinical and radiologic data, and by the clinician, radiologist, and pathologist based on all three domains. A higher multidisciplinary discussion-based confidence of idiopathic pulmonary fibrosis was associated with older age at diagnosis, male sex, higher forced vital capacity, and absence of ground glass changes. Pathologic variables associated with a higher multidisciplinary discussion-based confidence of idiopathic pulmonary fibrosis included increased number of fibroblast foci/cm2 and increased subpleural fibrosis. Pathologic variables associated with a higher multidisciplinary discussion-based confidence of hypersensitivity pneumonitis included an increased fraction of bronchioles with peribronchiolar metaplasia, increased foci of peribronchiolar metaplasia/cm2, and presence of giant cells/granulomas. These results provide guidance in separating idiopathic pulmonary fibrosis from hypersensitivity pneumonitis; however, a third of cases could not be confidently classified even when using these pathologic features combined with clinical and radiologic information in a multidisciplinary discussion.