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Interstitial lung disease (ILD) refers to a heterogenous collection of lung disorders that are grouped together because they share clinical, radiographic, and pathologic features. ILD is also referred to as diffuse parenchymal lung disease (DPLD) to clarify that the interstitium is not the only compartment of the lung affected. For example, organizing pneumonia and pulmonary alveolar proteinosis are alveolar filling processes, whereas respiratory bronchiolitis and chronic hypersensitivity pneumonitis may center on the airway. Occasionally, purely airway-centered disease like bronchiolitis obliterans may be initially identified as an ILD because of overlapping radiographic features. A structured approach is necessary since prognosis and therapy vary considerably depending on the diagnosis. While many ILDs are extremely rare, there are some, such as idiopathic pulmonary fibrosis (IPF) and sarcoidosis, that are seen commonly in general pulmonary practice.

ILD frequently presents with dyspnea on exertion, diffuse bilateral infiltrates on chest imaging, and restriction with diffusion impairment on physiologic testing. When tissue is obtained, the lung parenchyma may contain any combination of inflammation and fibrosis. Masqueraders of ILD, such as infection, pulmonary edema, and malignancy, will be encountered in the assessment of abnormal chest imaging. Distinguishing these from true ILD is essential.

Specific diagnosis is based upon a comprehensive history and careful physical examination, as well as review of laboratory data, physiologic studies, radiography, and, in some cases, pathologic tissue obtained from lung biopsy. Multidisciplinary review is an important part of the process and can have a significant impact on diagnostic and management decisions. For each patient, decisions regarding diagnostic approach and therapy must be individualized based upon the patient’s own personal approach to medical care, in combination with their respiratory status and comorbid medical conditions.


Several proposed classification schemes for ILD include radiographic, histopathologic, and clinical characteristics.1 The American Thoracic Society (ATS)/European Respiratory Society (ERS) consensus panel classification system on the idiopathic interstitial pneumonias was published in 2002 and was updated in 2013 and 2018 with a focus on the diagnosis of IPF.2–4 Additional guidelines on the diagnosis of hypersensitivity pneumonitis and sarcoidosis have further added to the diagnostic armamentarium.5,6

Despite the availability of such guidelines, in the care of any individual patient, it is useful for the clinician to have a methodical diagnostic approach. This process should incorporate a combination of historic, clinical, radiographic, and, when indicated, pathologic features. Distinguishing those patients who have a known cause for their ILD (e.g., connective tissue disease, occupational or environmental exposure, or drug toxicity) from those who do not (e.g., IPF, sarcoidosis) is one helpful diagnostic categorization (Fig. 52-1). Clinical features, such as acuity of onset and specific patient characteristics, may also help to classify disease (Table 52-1). For example, the subacute time course of symptom development in a patient with organizing pneumonia will be quite different than the ...

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