Bronchiolitis Obliterans Syndrome After Lung Transplant

A 52-year-old man undergoes bilateral lung transplantation for idiopathic pulmonary fibrosis. His baseline best FEV₁ (average of two measurements ≥3 weeks apart) is 3.2 L (100% predicted). At 18 months post-transplant, he presents with insidious dyspnea and dry cough. Spirometry shows FEV₁ of 2.5 L. Transbronchial biopsy shows no acute rejection, bronchoalveolar lavage is negative for infection, and bronchoscopy reveals no anastomotic complications. The structure marked **B** in the diagram—Bronchiolitis Obliterans Syndrome (BOS)—is suspected. According to ISHLT consensus criteria, what percentage decline in FEV₁ from baseline best FEV₁ is required to operationally define BOS?