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

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.