Approach to the diagnosis of hypersensitivity pneumonitis (HP)

Characteristics that help in the diagnosis of HP in patients with respiratory +/– constitutional symptoms (eg, cough, breathlessness, fever, fatigue) and absence of features to suggest systemic rheumatic disease
Known exposure to offending antigen identified by: History of appropriate exposure: Aerobiologic or microbiologic investigations of the environment that confirm the presence of an inciting antigen The presence of specific IgG antibodies in serum against the identified antigen (serum precipitins, ELISA, or ImmunoCAP)*
Compatible HRCT findings: Acute/subacute: Upper and middle lobe predominant small centrilobular nodules, ground-glass attenuation, and lobular areas of decreased attenuation and vascularity Chronic/fibrotic: Upper and middle lobe predominant fibrosis, peribronchovascular fibrosis, honeycombing, mosaic attenuation, air trapping, centrilobular nodules, and relative sparing of bases
BAL lymphocytosis: Supports diagnosis of HP if lymphocytosis, but nonspecific and not always needed Lymphocytosis >20%, often >50% Mycobacterial stains and culture negative
Positive inhalation challenge testing by: Reexposure to the environment Inhalation challenge to the suspected antigen in a hospital setting (not widely available or standardized) Only performed in selected patients
Confident HP without biopsy:
Criteria 1, 2, and 3 are met – Histopathologic confirmation of the diagnosis is not needed in the majority of such cases Criteria 1 and 2 are met – BAL not needed for patients with clear exposure, typical HRCT, and response to antigen avoidance
Probable HP:
Criteria 1 (A or B) and 3 are met, but HRCT more consistent with UIP, NSIP, CPFE, or OP. Criteria 2 and 3 are met, but no identifiable historical or serologic evidence of exposure.	Lung biopsy^¶ needed to confirm HP or identify alternate diagnosis.
Possible HP
Criteria 1 (A or B) is met. HRCT more consistent with other IIP; BAL not done or without lymphocytosis. Criteria 1 (A or B) and 2 are met, but BAL does not show lymphocytosis.	Lung biopsy^¶ needed to confirm HP or identify alternate diagnosis.
HP unlikely but clinical features atypical for IPF
Criteria 1, 2, 3 not met; HRCT more consistent with UIP, NSIP, CPFE, or OP.	Lung biopsy^Δ may be appropriate to confirm HP or identify alternate diagnosis.
Interpreting results of lung biopsy Histopathologic features of HP include: Poorly formed, noncaseating granulomas, multinucleated giant cells near respiratory or terminal bronchioles Chronic cellular bronchiolitis Chronic cellular pneumonitis with patchy lymphoplasmacytic infiltration Presence of triad: Confident diagnosis of HP for probable or possible HP, need to exclude histopathological mimics, such as connective tissue disease-associated interstitial lung disease. Presence of one or two features of triad: Likely HP in patients with clinical impression of "probable" HP or "possible" HP. Predominant features of UIP, with subtle features of HP (eg, few granulomas or multinucleated giant cell): Could be fibrotic HP or IPF. Multidisciplinary conference may help clarify diagnosis. Predominant features of a different interstitial lung disease (OP, NSIP, CPFE), but subtle features of HP (eg, few granulomas or multinucleated giant cell): Unlikely to be HP unless criteria 1 or 2 is positive. Multidisciplinary conference may help clarify diagnosis.

IgG: immunoglobulin G; ELISA: enzyme-linked immunosorbent assay; HRCT: high resolution computed tomography; BAL: bronchoalveolar lavage; UIP: usual interstitial pneumonia; NSIP: nonspecific interstitial pneumonia; CPFE: combined pulmonary fibrosis emphysema; OP: organizing pneumonia; IIP: idiopathic interstitial pneumonia; IPF: idiopathic pulmonary fibrosis; TBLB: transbronchial lung biopsy; TBCB: transbronchial cryobiopsy.
* A positive precipitin test even in the presence of a clear history of exposure to the identified antigen is merely suggestive of, rather than diagnostic of, a potential etiology.
¶ Lung tissue can be obtained by TBLB, TBCB, or video-assisted surgical biopsy. A multidisciplinary conference can help guide decision-making about whether to obtain TBLB prior to surgical lung biopsy, proceed directly to surgical biopsy, or avoid invasive procedures due to patient frailty or unlikely benefit.
Δ For patients in whom HP unlikely, surgical lung biopsy favored over TBLB or TBCB.

Reference: Vasakova M, Morell F, Walsh S, et al. Hypersensitivity Pneumonitis: Perspectives in Diagnosis and Management. Am J Respir Crit Care Med 2017; 196:680.

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Approach to the diagnosis of hypersensitivity pneumonitis (HP)