Impact of antigen avoidance test for fibrotic hypersensitivity pneumonitis in stable phase

Abstract Background The antigen avoidance has been used in the diagnosis and treatment of hypersensitivity pneumonitis (HP); however, its usefulness in stable fibrotic HP is controversial. Objective To investigate the usefulness of the antigen avoidance test in patients with fibrotic HP in stable phase. Methods The antigen avoidance test was conducted during a 2-week hospitalization comparing clinical parameters at admission and before discharge. A retrospective review of patients who underwent surgical lung biopsy or transbronchial lung cryobiopsy, who were diagnosed with fibrotic HP by multi-disciplinary discussion, and whose disease progression was stable for more than two months before the antigen avoidance test was done. Results Between 2016 and 2021, 40 patients met the criteria, and 17 (43%) patients had a positive antigen avoidance test. The patients with positive in the antigen avoidance test had significantly greater annual forced vital capacity (FVC) decline than those with negative before the test (− 6.5% vs. − 0.3%, p = 0.045). The patients with positive antigen avoidance test had less annual FVC decline than those with negative in the year following the test (0.8% vs. − 5.0%, p = 0.048). The differences in annual improvement were found for serum Krebs von den Lungen-6 between the positive and negative patients in the year following the test (− 27% vs. − 5%, p = 0.049). In multivariate Cox hazard regression analysis, a negative result of the antigen avoidance test was a risk factor for death or acute exacerbation of fibrotic HP (HR = 0.26 [95% CI: 0.07–0.90], p = 0.034). Conclusions In fibrotic HP patients in stable phase, the antigen avoidance test under a 2-week hospitalization was valuable in predicting prognosis.