Pathologist's approach to diffuse lung disease.

The pathologic diagnosis of diffuse lung disease is based on the recognition of specific patterns closely correlated with the clinical history and the radiologic findings. The current pathologic classification of the idiopathic interstitial pneumonias is in flux, with the new creation of cellular and fibrotic subtypes of nonspecific interstitial pneumonia and further refinements in the criteria for usual interstitial pneumonia, desquamative interstitial pneumonia, and respiratory bronchiolitis-associated lung disease. The radiologist plays a pivotal role in the diagnosis of these conditions by using high-resolution computed tomography. The radiologist also provides an important service by guiding the clinician or surgeon, based on the site and extent of disease, in obtaining a sample that will provide the pathologist with appropriate diagnostic material. This article reviews the current pathologic criteria for the classification of idiopathic interstitial pneumonias, smoking-related disorders, hypersensitivity pneumonitis, as well as the lymphoproliferative disorder lymphocytic interstitial pneumonitis, with a focus on differential diagnoses.