Inhalation challenge in the differential diagnosis of usual interstitial pneumonia.

The authors stress the importance of distinguishing between idiopathic pulmonary fibrosis (IPF) and chronic hypersensitivity pneumonitis in the differential diagnosis, especially in view of the fact that immunosuppressant treatment is suitable for patients with hypersensitivity pneumonitis, but is clearly contraindicated in patients with IPF [2]. Furthermore, the authors note that these diagnoses can easily be confused; indeed, MORELL et al. [3] recently found that up to 43% of patients diagnosed with IPF are actually misdiagnosed chronic hypersensitivity pneumonitis. As WUYTS et al. [1] state: “The diagnosis of hypersensitivity pneumonitis requires proven or suspected exposure associated with exposure-related symptoms; proof of sensitisation (e.g. demonstration of serum precipitins or lymphocytosis in the BAL fluid); and compatible chest radiography/HRCT abnormalities (with or without restrictive ventilatory/gas exchange defects) [4]. However, serum precipitating antibodies to a variety of common causative antigens have low sensitivity and specificity, and are helpful when positive but not if negative. Chronic hypersensitivity pneumonitis may represent a diagnostic dilemma, particularly when a detailed clinical history does not reveal a temporal relationship between symptom onset and antigen exposure”. It is in this context that a specific inhalation challenge (SIC) may be particularly useful in the diagnosis of hypersensitivity pneumonitis, and also in order to differentiate it from IPF.