Interstitial lung diseases (ILDs) are a heterogeneous group of disorders characterized by a spectrum of inflammatory and fibrotic changes affecting alveolar walls and airspaces. Clinical manifestations are protean, but progressive cough, dyspnea, parenchymal infiltrates on chest radiographs, and loss of pulmonary function are characteristic. More than 150 causes of ILD are known and include disorders due to specific agents or antigens (e.g. pneumoconioses, asbestosis, silicosis, berylliosis, granulomatous infections, hypersensitivity pneumonia) as well as myriad disorders in which the etiological factors have not been identified (e.g., cryptogenic fibrosing alveolitis, sarcoidosis, etc). Before discussing specific diseases, we review diagnostic strategies to differentiate these diverse ILDs (Table 12.1).

Pulmonary function tests (PFTs) are useful to assess extent of impairment and follow the course of the disease (natural history or response to therapy). Initial testing for patients with suspected ILD should include spirometry, lung volumes, single breath diffusing capacity for carbon monoxide (DLco), and oxygen saturation. Characteristic physiological aberrations in ILDs include: reductions in DLCO and lung volumes (e.g. vital capacity (VC), total lung capacity (TLC)), and impaired oxygenation (either at rest or with exercise). Formal cardiopulmonary exercise tests (CPET) are more sensitive than resting physiological testing to detect aberrations but are expensive, require significant technical support, are modestly uncomfortable, and are logistically difficult (particularly in elderly or debilitated patients). Oximetry is less accurate than direct measurement of arterial blood gases, but is non-invasive and well tolerated.