The "first hit" in the hypothesis is proposed to be genetic susceptibility and surrounding environmental factors and the "second hit" would be the introduction of the antigen into the respiratory system which causes the exaggerated immune response. Diagnosis
The diagnosis is made through clinical judgement using a combination of findings because there does not exist a single, universal diagnostic criteria for the disease. The diagnosis is most commonly ascertained first with a detailed exposure history followed by a battery of clinical tests including: imaging, histopathology, pulmonary function testing, serology, bronchoscopy, and more. In 2020, official guidelines were published by American Thoracic Society, Japanese Respiratory Society, and Asociación Latinoamericana del Tórax which provides a systematic approach to the diagnosis of HP that relies on high-resolution computed tomography. Exposure History
A detailed occupational, home and environmental exposure history is the first step in diagnosis. Unfortunately, only 60% of inciting antigens are identified in exposure assessment. Re-exposure to the antigen can help aid in diagnosis. Standardized questionnaires have been created to help in obtaining an exposure history although no official questionnaire has been purported. It has been recommended that the questionnaire administered should be relevant to the region in which the exposure has potentially occurred.Detailed exposure assessments are warranted in the cause of damp indoor environments which have the potential to propagate mold throughout the dwelling. The decision to enlist an industrial hygienist should be made based on the answers to evidence based questions during the environmental assessment.
False negatives are often common with serum precipitins because of lack of testing reagents for many antigens.Precipitating IgG antibodies against fungal or avian antigens can be detected in the laboratory using the traditional Ouchterlony immunodiffusion method wherein precipitin lines form on agar plate. The ImmunoCAP technology has replaced this time-consuming, labor-intensive method with their automated CAP assays and FEIA (Fluorescence enzyme immunoassay) that can detect IgG antibodies against Aspergillus fumigatus (Farmers lung or for ABPA) or avian antigens (Bird Fanciers Lung). Differential Diagnosis
Organic dust toxic syndrome presents similarly with fevers, chills a few hours after exposure to bioaerosols with toxins from fungi, however this is not a true hypersensitivity reaction because it occurs on initial exposure without a preceding sensitization In chronic disease, HP must be differentiated from very similarly presenting idiopathic pulmonary fibrosis.Although overlapping in many cases, hypersensitivity pneumonitis may be distinguished from occupational asthma in that it is not restricted to only occupational exposure, and that asthma generally is classified as a type I hypersensitivity. Unlike asthma, hypersensitivity pneumonitis targets lung alveoli rather than bronchi.Similarly, sarcoidosis has noncaseating granuloma formation, however hilar adenopathy is often seen on chest radiographs. Treatment
The best treatment is to avoid the provoking allergen, as chronic exposure can cause permanent damage and acute disease is often self-limiting. The identification of the provoking antigen and its location must be ascertained by conducting an exposure assessment. Home cleaning is one method of antigen avoidance.