Acute exacerbation (AE) of interstitial lung disease (ILD) is an acute respiratory deterioration of unknown etiology, associated with high mortality. Currently, bronchoalveolar lavage (BAL) has been no longer required for the diagnosis of AE-ILD; however, the clinical utility of BAL fluid (BALF) cellular analysis in AE-ILD remains unclear.

A retrospective study of 71 patients who underwent BAL at our institution between 2005 and 2019 and were diagnosed with AE-ILD was conducted. We performed BALF cellular analysis and evaluated its prognostic significance.

There were 26 patients with AE of idiopathic pulmonary fibrosis (IPF) and 45 with AE of non-IPF, including idiopathic interstitial pneumonias/non-IPF (n = 22), ILD associated with collagen tissue disease (n = 20) and fibrotic hypersensitivity pneumonia (n = 3). All patients were treated with high-dose corticosteroids, and the 90-day mortality after AE was 31%. Most patients showed a high percentage of lymphocytes and/or neutrophils in BALF regardless of the underlying ILD. There was a significant negative correlation between BALF neutrophils and the PaO₂/FiO₂ ratio, and patients with UIP pattern or diffuse AE pattern on HRCT had a significantly higher percentage of BALF neutrophils than those with other patterns. Multivariate analysis revealed that lower and higher percentage of lymphocytes and neutrophils, respectively, in BALF were independent poor prognostic factors for 90-day survival. BALF lymphocyte and neutrophil count ≥25% and <20%, respectively, predicted favorable survival after AE.

Cellular analysis of BALF in AE-ILD is a potential biomarker for predicting prognosis after AE.