Hypersensitivity Pneumonitis

1) Identified exposure to offending antigen(s) by:

• • Medical history of exposure to suspected antigen in the patient's living environment • Investigations of the environment confirm the presence of an inciting antigen • Identification of specific immune responses (immunoglobulin G serum precipitin antibodies against the identified antigen) are suggestive of the potential etiology but are insufficient in isolation to confirm a diagnosis

2) Clinical, radiographic, or physiologic findings compatible with hypersensitivity pneumonitis:

• • Respiratory and often constitutional signs and symptoms • Crackles on auscultation of the chest • Weight loss • Cough • Breathlessness • Episodic fever • Wheezing • Fatigue

NOTE: These findings are especially suggestive of hypersensitivity pneumonitis when they appear or worsen several hours after antigen exposure.

• • A reticular, nodular, or ground-glass opacities on chest radiograph or high-resolution CT • Abnormalities in the following pulmonary function tests • Spirometry (restrictive, obstructive, or mixed patterns) • Lung volumes (low or high) • Reduced diffusion capacity by carbon monoxide • Altered gas exchange either at rest or with exercise (reduced partial pressure of arterial oxygen by blood gas or pulse oximeter testing)

3) Bronchoalveolar lavage with lymphocytosis:

• • Usually with low CD4:CD8 ratio (i.e., CD8 is higher than normal) • Lymphocyte stimulation by offending antigen results in proliferation and cytokine production

4) Abnormal response to inhalation challenge testing to the offending antigen:

• • Reexposure to the environment • Inhalation challenge to the suspected antigen (rarely done now because of the risk of exacerbating the disease

5) Histopathology showing compatible changes with hypersensitivity pneumonitis by 1 of these findings:

• • Poorly formed, noncaseating granulomas (most often found closer to the respiratory epithelium where deposition of the offending antigen occurs • Mononuclear cell infiltrate in the pulmonary interstitium