Lung Ultrasound in Rheumatoid Arthritis–Associated Interstitial Lung Disease: Association with Radiological Patterns, Lung Function Tests, and Disease Activity—A Single-Center Retrospective Cross-Sectional Study

Introduction: Interstitial lung disease (ILD) represents one of the most relevant extra-articular manifestations in rheumatoid arthritis (RA), and significantly contributes to morbidity and mortality. High-resolution computed tomography (HRCT) is the gold standard for the diagnosis of ILD; however, its use in daily practice is limited. Lung ultrasound (LUS) is a non-invasive, repeatable, and bedside tool for ILD assessment. This study aimed to evaluate the association between quantitative LUS score and HRCT findings in RA patients. Methods: We conducted a single-center, retrospective cross-sectional study in RA patients with and without ILD. Demographic, clinical, lung function, and disease activity (DAS28-CRP) data were collected, together with LUS and HRCT findings at baseline. Correlation and regression analyses were performed to evaluate associations between LUS score, HRCT patterns, pulmonary function, and disease activity. Results: Thirty-seven RA subjects (M/F: 20/17; mean age 68.8 ± 10.4 years) were included; ILD was present in 24 patients (64.8%), including 17 (45.9%) with a UIP pattern. LUS score inversely correlated with DLCO (r = −0.413; p = 0.01) and positively with age (r = 0.47; p = 0.003) and DAS28-CRP (r = 0.366; p = 0.02). Higher LUS score were significantly associated with UIP pattern at both uni- (OR 1.09; 95%CI 1.02–1.16; p < 0.01) and multivariable regression analysis adjusted for age (OR 1.07; 95%CI 1.01–1.15; p = 0.02). Likewise, DAS28-CRP was significantly associated with a UIP pattern on chest HRCT in both univariable (OR 3.05, 95% CI 1.30–7.14; p = 0.01) and multivariable analyses adjusted for age (OR 2.35, 95% CI 1.19–6.86; p = 0.02). Conclusions: Overall, higher LUS score values were associated with reduced pulmonary function, higher DAS28-CRP, and the presence of a UIP pattern on chest HRCT. However, these findings should be considered exploratory and require further confirmation in larger prospective studies.