Clinical Evaluation of Acute Exacerbation of Interstitial Lung Disease in a Single Tertiary Center: Perspectives before and after the Coronavirus Disease 2019 Pandemic

<b>Background/Objectives:</b> Acute exacerbation (AE) of interstitial lung disease (ILD) is a major challenge. This study aimed to retrospectively investigate occurrences of AEs in patients with ILDs, including idiopathic pulmonary fibrosis (IPF), non-IPF (iNSIP: idiopathic nonspecific interstitial pneumonia), and connective tissue disease (CTD)-associated ILDs (CTD-ILDs), at a single tertiary center before and after the coronavirus disease 2019 (COVID-19) pandemic. The study aimed to clarify the seasonal and regional trends of AEs of ILDs, assess the roles of viral and bacterial infections, and identify key prognostic factors for patient outcomes. <b>Methods:</b> We conducted a retrospective review of hospitalized adult patients with AEs of ILDs from January 2019 to February 2024. <b>Results:</b> A total of 93 patients were enrolled: IPF (<i>n</i> = 42), iNSIP (<i>n</i> = 37), and CTD-ILDs (<i>n</i> = 14). The median age was 80 years (interquartile range: 74.0-86.0 years), with males comprising 64.5% (<i>n</i> = 60). AEs of ILDs predominantly occurred in winter and were particularly notable after summer 2023, coinciding with the lifting of COVID-19-related travel restrictions in Japan. Patient referrals from different areas (Northern Tama, East and/or Southern Tama, and other Tokyo metropolitan areas) were evenly distributed throughout the study period. Viral infections were detected in only two patients (SARS-CoV-2), and bacterial infections included methicillin-resistant <i>Staphylococcus aureus</i> and <i>Pseudomonas aeruginosa</i>. The Cox regression analysis identified serum lactate dehydrogenase levels ≥350 IU/L and tachypnea (respiratory rate ≥ 30 breaths per min) on admission as prognostic factors for mortality, with a hazard ratio [HR] of 2.783 (95% confidence interval [CI]: 1.480-5.235, <i>p</i> = 0.001) and an HR of 3.332 (95% CI: 1.710-6.492, <i>p</i> < 0.001), respectively. <b>Conclusions:</b> AEs of ILDs predominantly occur in winter, and viral and bacterial infections are infrequently detected. Elevated serum LDH levels and tachypnea are crucial prognostic markers for mortality. This study highlights the seasonal trend in the AE of ILD and emphasizes the importance of specific prognostic indicators in clinical practice.