Farmer s lung (FL) is one of the most frequent types of hypersensitivity pneumonitis. Depending on the diagnostic criteria used in epidemiologic surveys, cumulative prevalence of the disease has been reported to range from 2.5 to 153.1/1,000 farmers.
These figures on the prevalence of the disease support the fact that clinical diagnosis of FL is sometimes difficult to establish. Clinical history is limited by the recognition that active FL may present insidiously, and that symptoms of organic dust toxic syndrome are often indistinguishable from those of acute FL. A large array of nonspecific pulmonary function abnormalities has been described during acute exacerbations of the disease, ranging from severe obstructive defects to an overt restrictive pattern. Twenty percent of patients with acute conditions have normal chest radiographs. Serum precipitins against FL antigens are neither sensitive nor specific for FLS and are merely useful to identify exposed individuals. Finally, evidence of lymphocytic alveolitis on BAL, although highly sensitive for FL, is in no way specific.
The histopathologic characteristics of FL have been well described and consist of nonnecrotizing granulomas, interstitial fibrosis and mononuclear inflammatory infiltrate, and bronchiolitis obliterans. Open lung biopsy is no longer recommended in routine diagnosis and is reserved largely for rare cases of patients with severe disease or an atypical presentation. Transbronchial biopsy (TBB) has been deemed beneficial for the diagnosis of a number of pulmonary interstitial diseases. However, its usefulness in the diagnosis of FL (which most often relies on a constellation of clinical manifestations and laboratory abnormalities in patients with definite exposure to offending antigens) is not known. The objective of this study was to determine the likelihood ratios for TBB for the diagnosis of FL based on simple pathologic criteria and an overall assessment of the biopsy specimens.