Forced Vital Capacity and Six-Minute Walk Test as Indicators of Interstitial Lung Disease Extent on High-Resolution Computed Tomography in Systemic Sclerosis

Introduction Interstitial lung disease (ILD) is a leading cause of morbidity and mortality in systemic sclerosis (SSc). High-resolution computed tomography (HRCT) is the gold standard for assessing ILD extent but is limited by cost, availability, and radiation exposure. Practical functional tools are thus needed to identify extensive ILD in routine practice. Methods This cross-sectional study enrolled adult patients with SSc-associated ILD at a tertiary referral hospital in Jakarta, Indonesia. All participants underwent spirometry and the six-minute walk test (6MWT). Forced vital capacity (FVC), 6MWT distance, and exertional oxygen desaturation were evaluated. ILD extent on HRCT was classified as non-extensive (<20%) or extensive (≥20%). Associations were analyzed using bivariate analysis, receiver operating characteristic (ROC) curves, and multivariable Poisson regression with robust standard errors. Results A total of 83 patients were included; 21 (25%) had extensive ILD. Six-minute walk test distance did not differ significantly between groups (p = 0.492; AUC 0.550). Exertional oxygen desaturation ≥4% and FVC <70% predicted were each significantly associated with extensive ILD. FVC alone achieved an AUC of 0.795, whereas adding 6MWT distance resulted in only a marginal improvement in discrimination (AUC 0.799; ΔAUC 0.005), with adequate calibration and stable internal validation. Conclusion FVC alone may serve as a practical functional indicator of ILD extent, particularly where HRCT access is limited. Routine addition of 6MWT distance is not justified solely for estimating ILD extent, although the 6MWT remains valuable for assessing functional capacity and exertional desaturation.