Pulmonary Function Test Patterns in Patients with Interstitial Lung Disease

Background: Interstitial lung diseases (ILDs) encompass a heterogeneous group of parenchymal lung disorders characterized by varying degrees of inflammation and fibrosis. Pulmonary function testing (PFT) is integral to the diagnostic evaluation, severity assessment, and longitudinal monitoring of ILD. However, the spectrum of PFT patterns across different ILD subtypes and their relationship with clinical and radiological severity remain incompletely characterized. This study aimed to evaluate the patterns of pulmonary function abnormalities across various ILD subtypes and to correlate spirometric and diffusion capacity parameters with disease severity indices. Methods: A cross-sectional analytical study was conducted at a tertiary respiratory care center. A total of 384 patients with confirmed ILD diagnoses based on multidisciplinary discussion were enrolled. Comprehensive PFTs including spirometry, lung volumes, and diffusion capacity for carbon monoxide (DLCO) were performed. Highresolution computed tomography (HRCT) severity scoring, six-minute walk test (6MWT), and dyspnea assessment using the modified Medical Research Council (mMRC) scale were additionally evaluated. Results: The mean age was 56.8 ± 12.4 years, with 54.7% female predominance. The most common ILD subtype was idiopathic pulmonary fibrosis (IPF) (28.6%), followed by connective tissue disease-associated ILD (CTDILD) (24.0%) and hypersensitivity pneumonitis (HP) (18.8%). A restrictive pattern was observed in 72.4% of patients, mixed obstruction-restriction in 14.6%, isolated reduction in DLCO with preserved spirometry in 8.9%, and obstructive pattern in 4.2%. Mean FVC% predicted was 62.4 ± 16.8%, and mean DLCO% predicted was 52.6 ± 18.4%. IPF patients demonstrated significantly lower DLCO (46.2 ± 16.8% vs. 58.4 ± 17.6% in CTD-ILD, p < 0.001). DLCO showed the strongest correlation with 6MWT distance (r = 0.68, p < 0.001) and HRCT fibrosis score (r = −0.62, p < 0.001). Conclusion: Restrictive ventilatory impairment predominates across ILD subtypes, though significant physiological heterogeneity exists. DLCO emerges as the most sensitive functional parameter, correlating strongly with exercise capacity and radiological severity. Comprehensive PFT assessment beyond spirometry alone is essential for accurate functional characterization of ILD patients.