Clinical Decision-Making in Hypersensitivity Pneumonitis: Diagnosis and Management

Abstract

This review provides an updated approach to the diagnosis and management of hypersensitivity pneumonitis (HP). The importance of using a multidisciplinary discussion to increase diagnostic and treatment confidence is emphasized. The role of Bayesian reasoning is highlighted throughout, underscoring the importance of hypothesis generation (differential diagnosis) and diagnostic test interpretation based on the probability of HP. Probability estimates of diagnostic certainty (i.e., a confident versus a working diagnosis) and treatment thresholds are carefully examined.Therapeutically, beyond antigen avoidance and newly available antifibrotic therapy for patients with a progressive fibrosing phenotype; the role, timing, and expected response to anti-inflammatory therapy in individual patients are unanswered questions. Since the evidence and validation of testing generally performed during the diagnostic work-up and longitudinal monitoring of HP is feeble at best, the viewpoints discussed are not intended to resolve current controversies but rather to provide a conceptual framework for evaluating discordant information when evaluating and caring for patients with HP.

Hypersensitivity Pneumonitis: Challenges of a Complex Disease

“…It has been established that HP with unknown exposure is associated with poor prognosis [ 10 ], and unfortunately, in nearly half of HP cases, the offending antigens cannot be recognized [ 24 ]. Several measures could contribute to increasing the chances of identifying the exposures: Collecting a thorough environmental history [ 25 ], which could be facilitated by using relevant environmental and occupational questionnaires, which are more likely to identify a potential inciting agent when compared with clinical history [ 23 ]. Several proposed questionnaires were designed for ILD, but the majority included only a limited list of main exposures, and all of them lacked validation.…”

“…Collecting a thorough environmental history [ 25 ], which could be facilitated by using relevant environmental and occupational questionnaires, which are more likely to identify a potential inciting agent when compared with clinical history [ 23 ]. Several proposed questionnaires were designed for ILD, but the majority included only a limited list of main exposures, and all of them lacked validation.…”

“…Thus, a lymphocyte count >30% in the BAL in an patient with ILD may increase the diagnostic confidence for HP to highly probable and make the lung biopsy unnecessary [ 6 , 8 ]. However, the lack of BAL lymphocytosis in the fibrotic type does not exclude it [ 25 ], while the absence of lymphocytosis in the nonfibrotic pattern almost rules out the possibility of HP [ 6 ]. BAL lymphocytosis may also have a prognostic role suggesting more inflammation or less fibrosis, becoming a predictor of treatment response [ 34 ].…”