Correlation of lung ultrasound with HRCT in interstitial lung disease: A cross-sectional study

Dear Editor, Interstitial lung diseases (ILDs) are an increasingly recognized cause of chronic respiratory morbidity in India. Although high-resolution computed tomography (HRCT) remains the diagnostic gold standard, its limitations—cost, radiation exposure, and limited accessibility—restrict repeated use. Lung ultrasound (LUS), based on B-line quantification, represents a practical bedside tool for assessing interstitial involvement. In this cross-sectional study of 51 adults with HRCT-confirmed ILD, we assessed whether LUS severity scores correlate with HRCT abnormalities and physiological markers, including forced vital capacity (FVC), diffusing capacity of the lung for carbon monoxide (DLCO), 6-min-walk desaturation (6MWT), and modified Medical Research Council (mMRC) dyspnoea grade. We found a strong correlation between LUS and HRCT scores (r = 0.866, P < 0.001), supporting the potential role of LUS as a radiation-free surrogate for ILD severity assessment. We conducted a cross-sectional study of 51 adults with HRCT-confirmed ILD between June 2020 and July 2021 at a tertiary care centre. Patients with acute cardiopulmonary instability or contraindications to ultrasonography were excluded. Institutional ethical approval (ECASM-AIMS-2020-110) and informed consent were obtained. Pulmonary function tests (FVC, DLCO), 6MWT desaturation, and dyspnoea grading using the mMRC scale were recorded for all participants. HRCT scans were acquired at full inspiration and evaluated for four ILD-related abnormalities—interlobular and intralobular septal thickening, traction bronchiectasis, ground-glass opacities, and honeycombing. Each lobe was scored using a 0–4 Likert scale based on percentage involvement (0 = absent; 1 = 1–25%; 2 = 26–50%; 3 = 51–75%; 4 = 76–100%). A total HRCT severity score was generated by summing the lobar values. LUS was performed by a trained pulmonologist using a 3.5 MHz convex probe with patients in the supine or sitting position. Ten intercostal zones (five per hemithorax) were systematically examined. Each zone showing ≥3 B-lines was scored as 1, yielding a maximum LUS score of 10. Based on total B-line counts, patients were stratified as mild (3–15), moderate (16–30), or severe (>30), consistent with established ILD LUS protocols.[1] To assess prognostic implications, the GAP index (gender, age, physiology) was calculated, and patients were assigned to GAP stages 1–3. Correlations between LUS scores and HRCT severity, physiological parameters, symptoms, and GAP staging were analysed using Pearson’s correlation coefficient; between-group comparisons used the Kruskal–Wallis test with Dunn–Bonferroni correction. Multivariate analysis was performed to identify independent predictors of LUS severity. The mean age of the cohort was 55.2 ± 11.0 years, and 62.7% were female. Based on LUS scoring, 17.6% of patients had mild disease, 47.1% had moderate disease, and 35.3% had severe disease. The mean HRCT score was 30.9 ± 14.3, and the mean LUS score was 24.5 ± 13. A strong positive correlation was observed between LUS and HRCT scores (r = 0.866, P < 0.001). Patients with higher LUS severity demonstrated significantly lower FVC% (P = 0.006), lower DLCO% (P = 0.033), greater 6MWT desaturation (P = 0.003), and higher mMRC dyspnoea scores (P < 0.001). Fatigue was also more frequent in the severe LUS group (P < 0.001). LUS severity aligned with GAP staging, with the highest LUS scores clustering in higher GAP stages. Table 1 presents the demographic and physiological characteristics across LUS strata. Increasing LUS severity paralleled worsening radiological and physiological impairment, reflecting both structural and functional disease burden. On multivariate analysis, gender (P = 0.024), fatigue (P < 0.001), 6MWT desaturation (P = 0.012), mMRC grade (P < 0.001), HRCT score (P < 0.001), and GAP stage (P = 0.026) independently predicted higher LUS severity.Table 1: Demographic and clinical characteristics of patients stratified by lung ultrasound severity (mild, moderate, severe)Our findings align with international studies. Tardella et al. reported a strong correlation between B-lines and reticular abnormalities on HRCT in systemic sclerosis (r = 0.710; P < 0.01).[2] Yan et al. showed that LUS had a positive predictive value of 94% for ILD diagnosis compared with 97% for HRCT.[3] Man et al. also demonstrated robust correlation between LUS and HRCT (r = 0.64; P < 0.001).[4] Volpicelli emphasized the growing role of LUS for both diagnostic and prognostic stratification in ILD.[5] Milad et al. recently reported that specific LUS features can predict severe HRCT involvement in UIP and NSIP subgroups.[6] Our correlation coefficient (r = 0.866) is comparable or higher, likely reflecting structured scoring and uniform patient selection. The correlation between LUS and HRCT scores is shown in [Figure 1]. The addition of GAP staging in our analysis underscores the prognostic value of LUS in ILD evaluation.Figure 1: Scattered diagram showing the correlation between lung ultrasound score and high-resolution computed tomography scoreLUS is inexpensive, portable, repeatable, and free of ionizing radiation. It has particular utility in peripheral centres and low-resource settings, where HRCT access is limited, and may reduce the need for repeated CT imaging during follow-up.[4] It may also facilitate early referral and disease monitoring, integrating functional and prognostic assessments at the bedside. The present study is limited by its single-centre design and modest sample size. Because only confirmed ILD cases were included, sensitivity and specificity in broader populations could not be assessed. Longitudinal follow-up will be important to validate prognostic value over time. Nonetheless, the strong association with HRCT and GAP staging supports the role of LUS in routine ILD care. In conclusion, LUS scores show a strong positive correlation with HRCT features and physiological impairment in ILD and parallel established prognostic staging. LUS may serve as an effective, low-cost, radiation-free adjunct or alternative to HRCT in appropriate clinical contexts. Standardized protocols and training could enable wider integration of LUS into ILD care pathways. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.