Diagnosing interstitial lung disease: a practical approach to a difficult problem.

Interstitial lung disease has a variety of causes: environmental, infectious, autoimmune, and drug-related. Accurate diagnosis is essential because the prognosis and treatment of the disease varies widely depending on the cause. However, the respiratory symptoms and pulmonary radiographic picture of these various causes of interstitial lung disease are often similar, making the diagnosis of its cause confusing and frustrating. The practical, algorithmic approach to diagnosis outlined here identifies key diagnostic clues in the patient's history, physical exam, and radiographic findings.