Farmer's Lung Workup

  • Author: Laurianne G Wild, MD, FAAAAI; Chief Editor: Zab Mosenifar, MD   more...
 
Updated: Jan 17, 2012
 

Laboratory Studies

  • 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.
    • 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.[11]
    • 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.
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Imaging Studies

  • Chest radiography[12, 13]
    • 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[14]
    • High resolution CT scanning is a superior diagnostic modality compared with plain radiography.
    • Pulmonary fibrosis with honeycombing is observed in chronic disease.
    • Peri-bronchovascular distribution of nodules with ground-glass attenuation may be observed.
    • A normal finding on high-resolution CT scans eliminates the possibility of active acute or chronic farmer's lung.
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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.
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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
    • Consider this procedure if noninvasive tests are equivocal or inconclusive.
    • Consider this procedure if the patient's presentation is atypical in the presence of significant clinical evidence for the disease.
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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. Guidelines for diagnosis of farmer's lung are as follows:

  • 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
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Contributor Information and Disclosures
Author

Laurianne G Wild, MD, FAAAAI  Chief and Professor of Clinical Medicine, Section of Clinical Immunology, Allergy and Rheumatology, Director, Allergy and Immunology Fellowship Training Program, Tulane University Health Sciences Center; Director, Allergy and Immunology Clinic, Southeast Louisiana Veterans Health Care System in New Orleans

Laurianne G Wild, MD, FAAAAI is a member of the following medical societies: Alpha Omega Alpha, American Academy of Allergy Asthma and Immunology, American College of Allergy, Asthma and Immunology, and Association of Subspecialty Professors

Disclosure: Nothing to disclose.

Coauthor(s)

Eduardo E Chang, MD  Fellow, Department of Allergy and Immunology, Tulane University

Disclosure: Nothing to disclose.

Specialty Editor Board

Sat Sharma, MD, FRCPC  Professor and Head, Division of Pulmonary Medicine, Department of Internal Medicine, University of Manitoba; Site Director, Respiratory Medicine, St Boniface General Hospital

Sat Sharma, MD, FRCPC is a member of the following medical societies: American Academy of Sleep Medicine, American College of Chest Physicians, American College of Physicians-American Society of Internal Medicine, American Thoracic Society, Canadian Medical Association, Royal College of Physicians and Surgeons of Canada, Royal Society of Medicine, Society of Critical Care Medicine, and World Medical Association

Disclosure: Nothing to disclose.

Francisco Talavera, PharmD, PhD  Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy; Editor-in-Chief, Medscape Drug Reference

Disclosure: Medscape Salary Employment

Gregg T Anders, DO  Medical Director, Great Plains Regional Medical Command , Brooke Army Medical Center; Clinical Associate Professor, Department of Internal Medicine, Division of Pulmonary Disease, University of Texas Health Science Center at San Antonio

Gregg T Anders, DO is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, and American Thoracic Society

Disclosure: Nothing to disclose.

Timothy D Rice, MD  Associate Professor, Departments of Internal Medicine and Pediatrics and Adolescent Medicine, St Louis University School of Medicine

Timothy D Rice, MD is a member of the following medical societies: American Academy of Pediatrics and American College of Physicians

Disclosure: Nothing to disclose.

Chief Editor

Zab Mosenifar, MD  Director, Division of Pulmonary and Critical Care Medicine, Director, Women's Guild Pulmonary Disease Institute, Professor and Executive Vice Chair, Department of Medicine, Cedars Sinai Medical Center, University of California, Los Angeles, David Geffen School of Medicine

Zab Mosenifar, MD is a member of the following medical societies: American College of Chest Physicians, American College of Physicians, American Federation for Medical Research, and American Thoracic Society

Disclosure: Nothing to disclose.

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