Approach Considerations
No single diagnostic or clinical laboratory study is specific to the diagnosis of farmer's lung. The most important diagnostic tool is a detailed environmental history.
Guidelines for diagnosis of farmer's lung include the following:
Major criteria
-
Symptoms compatible with hypersensitivity pneumonitis
-
Evidence of exposure to appropriate antigen by history or detection of antigen-specific antibody in serum and/or BAL fluid
-
Characteristic radiographic changes on plain radiographs or high-resolution CT scans
-
Bronchoalveolar lymphocytes (if BAL was performed)
-
Pulmonary histological changes compatible with hypersensitivity pneumonitis (if lung biopsy was performed)
-
Positive natural challenge findings (reproduction of symptoms and laboratory abnormalities after exposure to the probable environment)
Minor criteria
-
Presence of bibasilar rales
-
Decreased diffusion capacity
-
Arterial hypoxemia either at rest or during exercise
Laboratory Studies
Leukocytosis with neutrophilia (but not eosinophilia) and elevated erythrocyte sedimentation rate (ESR), C-reactive protein level, and quantitative immunoglobulin level are noted. Precipitating immunoglobulin G (IgG) antibodies confirms past exposure but does not indicate active disease.
Precipitating antibodies are present in up to 50% of asymptomatic farmers exposed to the antigen. [19]
In farmer's lung, negative precipitin results have been reported because of a lack of appropriate antigen selection in serologic commercial testing.
Laboratories must select antigens based on knowledge of local climate and agricultural practices rather than reliance on commercially available antigen panels.
Imaging Studies
Chest radiography
Findings are normal between acute attacks. Findings are abnormal during acute and subacute stages of disease.
Diffuse air-space consolidation is typical of acute farmer's lung (with acute antigen exposure). Nodular or reticulonodular pattern is characteristic of the subacute phase. Linear radiodensities may be discovered and indicate areas of fibrosis from previous attacks.
Pulmonary apices are often spared on plain chest radiography.
High-resolution computed tomography
High resolution CT scanning is a superior diagnostic modality compared with plain radiography. A normal finding on high-resolution CT scans eliminates the possibility of active acute or chronic farmer's lung.
Pulmonary fibrosis with honeycombing is observed in chronic disease. Peri-bronchovascular distribution of nodules with ground-glass attenuation may be observed.
Other Tests
Pulmonary function tests
Spirometry findings may be normal between attacks and before the development of chronic disease. Acute, subacute, and chronic forms of farmer's lung have a restrictive ventilatory pattern with reduced forced vital capacity (FVC), reduced total lung capacity (TLC), and preserved airflow.
Mild-to-severe hypoxemia at rest or during minimal exercise may be present with active disease. Decreased diffusion capacity is present with active disease.
Procedures
Bronchoscopy
Bronchoscopy is useful to exclude other disease processes in the acute setting and to obtain bronchoalveolar lavage (BAL) fluid samples for cell counts. Transbronchial biopsy may show evidence of peri-bronchovascular granuloma formation supporting the diagnosis, but its yield is limited because of sampling size.
Open lung biopsy
Open lung biopsy is considered if noninvasive tests are equivocal or inconclusive or if the patient's presentation is atypical in the presence of significant clinical evidence for the disease.
Histologic Findings
Chronic interstitial inflammation is present with infiltration of plasma cells, mast cells, histiocytes, and lymphocytes. Small and poorly organized nonnecrotizing granulomas are present, usually adjacent to bronchioles. Interstitial fibrosis is often present in chronic disease. Changes consistent with bronchiolitis obliterans may be evident.
