Diagnostic value of lung ultrasound in interstitial lung disease associated with rheumatoid arthritis: Correlation with HRCT and differentiation of ILD subtypes

Objective: Rheumatoid arthritis (RA) is a systemic autoimmune disease in which interstitial lung disease (ILD) is a major extra-articular complication. Detection of RA-ILD is challenging because chest X-ray and pulmonary function tests have limited sensitivity, while high-resolution computed tomography (HRCT), although the reference standard, is constrained by cost and radiation exposure. Lung ultrasound (LUS) has therefore emerged as a practical bedside imaging tool. This study aimed to assess the diagnostic performance of LUS for RA-ILD using HRCT as the reference standard, examine its correlation with radiological and functional parameters, and evaluate its ability to distinguish ILD subtypes and severity stages. Methods: This prospective observational study included 86 RA patients with respiratory symptoms or auscultatory findings suggestive of ILD. All participants underwent clinical evaluation, CXR, PFTs, TUS, and HRCT. Sonographic parameters included B-line count, spacing, pleural line morphology, and lung sliding. HRCT findings were classified by ILD patterns, and the Warrick score was used to assess ILD severity. Receiver operating characteristic (ROC) analysis was performed to determine optimal B-line cutoffs. Results: Eighty-six RA patients were enrolled, with ILD confirmed by HRCT in 62 (72.1%). LUS demonstrated the highest diagnostic sensitivity (90.0%) among all modalities, outperforming CXR, PFTs, and auscultation. Diagnostic performance improved when stratified by HRCT pattern and disease severity, with higher AUCs for UIP (0.823), NSIP (0.816), and severe ILD (0.986). LUS B-line count showed a strong positive correlation with HRCT Warrick score (r = 0.933, p < 0.0001) and moderate negative correlations with forced vital capacity (FVC) and oxygen saturation. ROC analysis identified ≥ 10.5 B-lines as the optimal threshold for ILD detection (AUC = 0.766), achieving high sensitivity and moderate specificity. Conclusion: Lung ultrasound is an effective, accessible, and non-invasive screening modality for the detection of ILD in RA patients. Its integration into routine clinical practice can facilitate earlier diagnosis, assist in non-invasive differentiation of ILD subtypes, guide timely intervention, and improve patient outcomes. The identification of pattern- and severity-specific B-line cutoffs represents a novel contribution, extends existing evidence beyond a single universal threshold and highlights the added value of tailored ultrasound interpretation.