Hypersensitivity pneumonitis (HP) is an inflammatory and/or fibrotic disease affecting the lung parenchyma and small airways. It typically results from an immune-mediated reaction provoked by an overt or occult inhaled antigen in susceptible individuals. HP must be taken into account in the process of differential diagnosis of patients with newly diagnosis of interstitial lung disease.
We have two major clinical forms of the disease. The non-fibrotic and the fibrotic one. Based on this categorization, two different diagnostic approaches have been recommended by the experts panel.
For the non-fibrotic, the panel of experts recommend with very low confidence in the estimated effects, to perform serum IgG testing that targets potential antigens associated with HP, to perform BAL for cell counts – particularly lymphocytes -to obtain specimens through the transbronchial forceps lung biopsy. No recommendation for or against lung cryobiopsy. Surgical biopsy when all the other diagnostic tests failed to confirm HP.
For the fibrotic one, there are 2 major differences: No recommendation for or against transbronchial forceps lung biopsy. Suggest with very low confidence in the estimated effects transbronchial lung cryobiopsy.
For both phenotypes a detailed medical history is needed in order to identify potential exposures and sources in the patient’s environment that are known to be associated with HP.
Considering the diagnostic approach using algorithms, the major approach is the evaluation of HRCT by experienced radiologists. Then categorize patients into three categories: Typical HRCT for HP, compatible and interminate for HP. Then combine history of exposute, IgG testing, BAL lymphocytosis with or without histopathology. Based on the above combinations, diagnosis is characterized as definite, low, moderate and high confidence.
For the diagnostic approach a multidisciplinary discussion within experts is recommended.
"The primary goal in the diagnosis of ILD is to make a confident diagnosis using the least invasive approach. HP can be diagnosed with high confidence in patients in whom an exposure has been identified and who have a typical HP pattern at HRCT and have BAL lymphocytosis; such patients do not require additional testing. Patients with any other combination of exposure history, HRCT pattern, and BAL results should undergo an MDD that includes an experienced expert in ILD (pulmonologist), a chest radiologist, and, if transbronchial lung biopsies were performed at the time of BAL, a pathologist familiar with histopathological features of interstitial pneumonias and HP. Additional histopathological sampling should be considered after the MDD in some patients with a high-confidence diagnosis, moderate-confidence diagnosis, or low-confidence diagnosis or in patients for whom an alternative diagnosis has not been established."