Identifying the exact exposure that led to the development of hypersensitivity pneumonitis (HP) can be a challenging task.1,2 The most effective way to find what exposure caused a patient’s HP and the first step recommended in diagnostic workflow is taking a thorough occupational and lifestyle and clinical history.3,4 Several questionnaires have been developed by experts in the field to aid in helping to identify the cause and can be particularly helpful in cases where there is no obvious exposure.5,6
One way for patients to identify the triggering substance is to avoid the most likely antigens.7 There are many sources of antigens that can cause HP, including bacteria, fungi, animal proteins, plant proteins, low molecular weight chemicals, and metals.1,3,4 Research is always ongoing and leads to the discovery of new causes of HP, so the list of known inciting antigens is continually evolving.
A study of 199 patients with HP found that the most common causes were avian antigens (66%), farmer’s lung (19%), and fungi (10%).8 Another study of 85 patients with HP found that common causes were avian antigens (34%), a bacteria found in hot tub water (21%), farm-related mold (11%), and household mold (9%).9 Another study of 142 patients with HP found that common causes were avian antigens (17%), household mold (11%), farm or animal-related exposures (8%), and hot tub water (4%).10
In many cases of HP, the exposure factor cannot be determined.3 Studies have reported varying percentages of antigen-indeterminate HP. One study of 85 patients with HP found that 25% had an unknown antigen cause.9 Another study of patients with ILD, 206 of which had a confirmed diagnosis of HP, found that 60% of patients had an unidentified antigen exposure.11 Another study of 142 patients with HP found that 53% of patients had an unidentified inciting antigen.10
Blood testing for serum immunoglobulin G (IgG) antibodies can help identify potential inciting agents of HP.1,3,4 However, IgG testing only provides supporting evidence of HP and cannot be used to confirm or exclude a diagnosis of HP as often the tests are not sensitive or specific for an exposure. If exposure history is unclear or indeterminant, that does not exclude HP.
High-resolution CT (HRCT) is one of the key diagnostic tools for HP as it can help determine presence of fibrosis, which aids in determining treatment and prognosis.3,4 However, it cannot help determine the inciting agent.
Patients who develop HP have been previously sensitized to the offending antigen.1,3,4 There is a need for more research to explain why only a fraction of people exposed to the harmful antigens develop HP.
References
- Quirce S, Vandenplas O, Campo P, et al. Occupational hypersensitivity pneumonitis: an EAACI position paper. Allergy. Jun 2016;71(6):765-79. doi:10.1111/all.12866
- Lacasse Y, Selman M, Costabel U, et al. Clinical diagnosis of hypersensitivity pneumonitis. Am J Respir Crit Care Med. Oct 15 2003;168(8):952-8. doi:10.1164/rccm.200301-137OC
- Raghu G, Remy-Jardin M, Ryerson CJ, et al. Diagnosis of Hypersensitivity Pneumonitis in Adults. An Official ATS/JRS/ALAT Clinical Practice Guideline. Am J Respir Crit Care Med. Aug 1 2020;202(3):e36-e69. doi:10.1164/rccm.202005-2032ST
- Fernández Pérez ER, Travis WD, Lynch DA, et al. Diagnosis and Evaluation of Hypersensitivity Pneumonitis: CHEST Guideline and Expert Panel Report. Chest. Aug 2021;160(2):e97-e156. doi:10.1016/j.chest.2021.03.066
- Barnes H, Chambers D, Grainge C, et al. Clinical utility of a standardized chronic hypersensitivity pneumonitis exposure questionnaire. Respirology. Apr 2023;28(4):366-372. doi:10.1111/resp.14404
- Barnes H, Morisset J, Molyneaux P, et al. A Systematically Derived Exposure Assessment Instrument for Chronic Hypersensitivity Pneumonitis. Chest. Jun 2020;157(6):1506-1512. doi:10.1016/j.chest.2019.12.018
- Tsutsui T, Miyazaki Y, Okamoto T, et al. Antigen avoidance tests for diagnosis of chronic hypersensitivity pneumonitis. Respir Investig. Sep 2015;53(5):217-24. doi:10.1016/j.resinv.2015.04.003
- Lacasse Y, Selman M, Costabel U, et al. Clinical diagnosis of hypersensitivity pneumonitis. Am J Respir Crit Care Med. Oct 15 2003;168(8):952-8. doi:10.1164/rccm.200301-137OC
- Hanak V, Golbin JM, Ryu JH. Causes and presenting features in 85 consecutive patients with hypersensitivity pneumonitis. Mayo Clin Proc. Jul 2007;82(7):812-6. doi:10.4065/82.7.812
- Fernández Pérez ER, Swigris JJ, Forssén AV, et al. Identifying an inciting antigen is associated with improved survival in patients with chronic hypersensitivity pneumonitis. Chest. Nov 2013;144(5):1644-1651. doi:10.1378/chest.12-2685
- Ryerson CJ, Vittinghoff E, Ley B, et al. Predicting survival across chronic interstitial lung disease: the ILD-GAP model. Chest. Apr 2014;145(4):723-728. doi:10.1378/chest.13-1474
