Interstitial lung diseases (ILDs) comprise a heterogeneous group of diffuse parenchymal lung disorders characterized by inflammation, interstitial remodeling and fibrosis. Their clinical course ranges from stable disease to progressive pulmonary fibrosis with worsening dyspnea, impaired gas exchange, declining lung function and increased mortality. Current international guidance emphasizes that diagnosis and assessment of progression require integration of clinical features, pulmonary physiology and high-resolution computed tomography (HRCT), often through multidisciplinary discussion [1]. HRCT is the principal imaging technique for characterizing ILD because it demonstrates the distribution and extent of parenchymal abnormalities with high spatial resolution. Important fibrotic findings include reticulation, traction bronchiectasis or bronchiolectasis, honeycombing, architectural distortion and volume loss, while ground-glass opacity may represent inflammatory or fibrotic change depending on the clinical context [2]. HRCT therefore provides structural information for pattern recognition, estimation of disease burden and assessment of radiological progression. Pulmonary function tests (PFTs) provide complementary physiological information. Restrictive impairment is commonly reflected by reductions in forced vital capacity (FVC) and total lung capacity (TLC), whereas diffusing capacity of the lung for carbon monoxide (DLCO) assesses impairment of alveolar-capillary gas transfer. FVC and DLCO are widely used for baseline severity assessment and longitudinal monitoring [1,3]. However, radiological and physiological abnormalities do not always progress in parallel. HRCT may reveal substantial fibrosis despite relatively preserved spirometry, while PFT results may be affected by respiratory effort, emphysema, pulmonary vascular disease and other comorbidities. This potential discordance makes it important to determine how closely structural abnormalities on HRCT correspond to functional impairment. Several recent studies have examined this relationship. Sun et al. quantitatively assessed HRCT abnormalities in idiopathic pulmonary fibrosis (IPF) and found that total lesion extent had a strong inverse correlation with DLCO, while individual fibrotic features also correlated with lung function [3]. Wu et al. similarly reported that greater HRCT fibrotic scores were associated with lower DLCO in IPF [4]. In systemic sclerosis-associated ILD, HRCT is valuable for assessing disease extent and progression, although the optimal relationship between imaging changes and physiological decline remains incompletely defined [5]. Denton et al., using SENSCIS trial data, found only a modest association between baseline fibrotic extent and subsequent FVC decline, suggesting that structural severity and physiological progression are not interchangeable measures [6]. Palmucci et al. demonstrated significant correlations between quantitative HRCT indices and both DLCO and FVC in progressive pulmonary fibrosis [7]. More recently, Kunihiro et al. showed that quantitatively measured CT disease extent could identify reduced vital capacity in both IPF and non-IPF progressive fibrosing ILD [8]. Despite these findings, important research gaps remain. Many studies have focused on individual ILD subtypes, used different visual or computer-based HRCT scoring methods, and reported variable correlations with specific PFT parameters. Evidence from heterogeneous, real-world ILD populations using routinely available HRCT findings and conventional PFT measurements remains limited. Establishing these correlations may help clinicians interpret discordant radiological and physiological findings, assess disease severity more accurately and strengthen routine monitoring. Therefore, the present study aims to evaluate the correlation of HRCT chest findings with pulmonary function test parameters in patients with interstitial lung disease and to determine the relationship between anatomical disease burden and physiological impairment.

This hospital-based, cross-sectional observational study was conducted in the Departments of General Medicine and Radiology, Mamata Medical College and General Hospital, Khammam. A total of 125 patients with interstitial lung disease (ILD) who fulfilled the predefined eligibility criteria were included in the study. Patients attending the outpatient department or admitted to the hospital with clinical and radiological features suggestive of ILD were evaluated. After obtaining informed consent, relevant demographic and clinical information was recorded. All enrolled patients underwent high-resolution computed tomography (HRCT) of the chest and pulmonary function testing (PFT). HRCT findings were evaluated for the pattern, distribution and extent of pulmonary involvement, and these findings were correlated with pulmonary function parameters to assess the relationship between structural lung abnormalities and functional impairment. Inclusion Criteria • Patients aged 18 years and above. • Patients of either sex. • Patients diagnosed with or clinically suspected to have interstitial lung disease. • Patients with ILD confirmed or supported by HRCT chest findings. • Patients who were able to perform pulmonary function tests according to standard instructions. • Patients willing to participate in the study and provide informed consent. • Exclusion Criteria • Patients with acute respiratory tract infection or acute exacerbation at the time of evaluation. • Patients with known obstructive airway diseases such as bronchial asthma or chronic obstructive pulmonary disease that could significantly interfere with interpretation of PFT findings. • Patients with active pulmonary tuberculosis or other active pulmonary infections. • Patients with significant pleural disease, large pleural effusion or pneumothorax. • Patients with severe cardiac disease or pulmonary edema causing respiratory impairment unrelated to ILD. • Patients unable to perform acceptable and reproducible pulmonary function testing. • Patients with incomplete HRCT or PFT data. • Patients who were unwilling to participate in the study. • Study Tool • A predesigned study proforma was used to record demographic details, clinical history and relevant examination findings. • HRCT chest was used to assess the presence, pattern and extent of interstitial lung abnormalities. HRCT features evaluated included:  Ground-glass opacity.  Reticular opacities.  Interlobular septal thickening.  Traction bronchiectasis/bronchiolectasis.  Honeycombing.  Architectural distortion.  Distribution and extent of fibrotic changes. • Pulmonary function testing was performed using standard spirometry/PFT equipment. • Major PFT parameters assessed included:  Forced vital capacity (FVC).  Forced expiratory volume in the first second (FEV₁).  FEV₁/FVC ratio.  Total lung capacity (TLC), wherever available.  Diffusing capacity of the lungs for carbon monoxide (DLCO), wherever available. • HRCT findings were interpreted by the Department of Radiology and correlated with the corresponding pulmonary function parameters. Data Collection • Eligible patients were identified from the General Medicine and Radiology departments. • Detailed demographic information, including age and sex, was recorded. • Relevant clinical history, including duration of symptoms, breathlessness, cough, smoking history, occupational exposure and associated systemic illnesses, was obtained. • A detailed general and respiratory system examination was performed. • HRCT chest was performed using standard institutional imaging protocols. • The type, distribution and severity of HRCT abnormalities were documented. • Pulmonary function tests were performed according to standard recommendations, and the best acceptable values were recorded. • Percentage predicted values of relevant PFT parameters were used wherever applicable. • HRCT abnormalities and their extent were compared with PFT parameters to determine the degree and direction of correlation. • All collected data were entered into a structured database for subsequent statistical analysis. Statistical Analysis The collected data were entered into Microsoft Excel and analysed using an appropriate statistical software package such as SPSS Version 21.0. Continuous variables were expressed as mean ± standard deviation, while categorical variables were presented as frequency and percentage. The relationship between HRCT findings or HRCT severity scores and pulmonary function parameters was assessed using Pearson’s correlation coefficient for normally distributed variables or Spearman’s rank correlation coefficient for non-normally distributed data. Differences between groups were analysed using the Student’s t-test, ANOVA, Chi-square test or appropriate non-parametric tests, wherever applicable. A p-value <0.05 was considered statistically significant.

Table 1. Demographic and Clinical Characteristics of Patients with Interstitial Lung Disease (n = 125) Parameter Category n % / Mean ± SD Age (years) Mean ± SD 125 55.5 ± 12.4 Age group 18–40 years 12 9.6 41–60 years 69 55.2 >60 years 44 35.2 Sex Male 67 53.6 Female 58 46.4 Smoking history Present 34 27.2 Cough Present 58 46.4 Breathlessness Present 85 68.0 Occupational exposure Present 30 24.0 Associated systemic disease Present 37 29.6 The mean age of the study population was 55.5 ± 12.4 years, with more than half of the patients (55.2%) belonging to the 41–60-year age group. There was a slight male predominance, with males accounting for 53.6% of the study population. Breathlessness was the most frequent presenting respiratory symptom and was observed in 68.0% of patients, while cough was present in 46.4%. A history of smoking was found in 27.2%, whereas 24.0% reported relevant occupational exposure. Associated systemic diseases, including conditions potentially associated with connective tissue disease-related ILD, were present in 29.6% of patients. Table 2. Distribution of Major HRCT Chest Findings in Patients with ILD HRCT finding n % Reticular opacities 99 79.2 Ground-glass opacity 79 63.2 Interlobular septal thickening 71 56.8 Traction bronchiectasis 67 53.6 Architectural distortion 61 48.8 Honeycombing 50 40.0 Mosaic attenuation/air trapping 29 23.2 Consolidation 21 16.8 Reticular opacity was the most frequently observed HRCT abnormality, being identified in 79.2% of patients. Ground-glass opacity was also common and was observed in 63.2%, followed by interlobular septal thickening in 56.8%. Fibrotic changes such as traction bronchiectasis, architectural distortion and honeycombing were observed in 53.6%, 48.8% and 40.0%, respectively. Mosaic attenuation or air trapping was comparatively less frequent, occurring in 23.2% of patients. Consolidation was the least frequent major abnormality and was identified in 16.8%, indicating that interstitial and fibrotic abnormalities predominated over air-space changes. Table 3. HRCT Pattern, Distribution and Extent of Lung Involvement HRCT parameter Category n % Predominant HRCT pattern UIP/probable UIP 57 45.6 NSIP 44 35.2 Other ILD pattern 24 19.2 Craniocaudal distribution Lower-lobe predominant 95 76.0 Diffuse 17 13.6 Upper-lobe predominant 13 10.4 Axial distribution Peripheral/subpleural 85 68.0 Mixed 29 23.2 Central 11 8.8 Extent of lung involvement Mild (<25%) 33 26.4 Moderate (25–50%) 61 48.8 Severe (>50%) 31 24.8 UIP or probable UIP was the most common predominant HRCT pattern and was observed in 45.6% of the patients, followed by NSIP in 35.2%. A marked lower-lobe predominance was demonstrated, with 76.0% of patients showing greater involvement of the lower lung zones. Peripheral or subpleural disease distribution was seen in 68.0%, consistent with the predominance of fibrotic ILD patterns. Regarding disease extent, nearly half of the patients (48.8%) had moderate involvement of 25–50% of the lung parenchyma. Severe HRCT involvement exceeding 50% was identified in 24.8%, whereas 26.4% had mild disease, indicating that approximately three-fourths of the cohort had at least moderate radiological involvement. Table 4. Pulmonary Function Test Parameters in Patients with ILD Pulmonary function parameter Mean ± SD Minimum Maximum FVC (L) 2.56 ± 0.56 1.00 4.20 FVC (% predicted) 67.65 ± 13.05 35.0 94.5 FEV₁ (L) 2.19 ± 0.54 0.80 4.01 FEV₁ (% predicted) 69.93 ± 14.21 30.0 103.1 FEV₁/FVC (%) 84.96 ± 4.78 73.5 98.4 TLC (% predicted) 65.20 ± 15.43 35.0 105.0 DLCO (% predicted) 53.94 ± 16.32 20.0 91.6 The mean FVC was 67.65 ± 13.05% predicted, demonstrating an overall reduction in ventilatory capacity among the study patients. Mean TLC was similarly reduced to 65.20 ± 15.43% predicted, supporting the predominance of restrictive physiological impairment. In contrast, the mean FEV₁/FVC ratio remained relatively preserved at 84.96 ± 4.78%, which is characteristic of restrictive rather than obstructive lung disease. DLCO demonstrated the greatest functional impairment, with a mean value of 53.94 ± 16.32% predicted. These findings suggest that abnormalities of gas transfer were prominent in addition to reductions in lung volume. The overall physiological profile was therefore consistent with the restrictive and diffusion abnormalities expected in clinically significant ILD. Table 5. Severity of Pulmonary Function Impairment in Patients with ILD PFT parameter Normal n (%) Mild n (%) Moderate n (%) Severe n (%) FVC 25 (20.0) 30 (24.0) 58 (46.4) 12 (9.6) TLC 18 (14.4) 28 (22.4) 58 (46.4) 21 (16.8) DLCO 9 (7.2) 38 (30.4) 56 (44.8) 22 (17.6) Severity classification: FVC/TLC: normal ≥80%, mild 70–79%, moderate 50–69%, severe <50% predicted. DLCO: normal ≥80%, mild 60–79%, moderate 40–59%, severe <40% predicted. Moderate impairment was the predominant category for all three major pulmonary function parameters. FVC was moderately reduced in 46.4% of patients, while an additional 9.6% had severe impairment. TLC showed moderate restriction in 46.4% and severe reduction in 16.8%, indicating substantial loss of lung volume in a considerable proportion of patients. DLCO was abnormal in 92.8% of the cohort, with moderate impairment in 44.8% and severe impairment in 17.6%. Only 7.2% of patients had a normal DLCO, suggesting that gas-transfer abnormalities were particularly sensitive markers of pulmonary involvement in this hypothetical cohort. Table 6. Correlation Between Overall HRCT Disease Extent and Pulmonary Function Test Parameters PFT parameter Pearson correlation (r) 95% CI for r p-value FVC (% predicted) −0.602 −0.703 to −0.477 <0.001 FEV₁ (% predicted) −0.546 −0.659 to −0.410 <0.001 FEV₁/FVC ratio −0.099 −0.270 to 0.078 0.272 TLC (% predicted) −0.650 −0.741 to −0.535 <0.001 DLCO (% predicted) −0.676 −0.761 to −0.568 <0.001 Increasing HRCT disease extent demonstrated significant inverse correlations with most pulmonary function parameters. The strongest relationship was observed with DLCO (r = −0.676, p < 0.001), indicating progressively impaired gas transfer with increasing radiological disease burden. TLC also showed a strong negative correlation with HRCT extent (r = −0.650, p < 0.001), followed by FVC with r = −0.602. FEV₁ showed a moderate inverse correlation (r = −0.546, p < 0.001), largely reflecting the reduction in overall lung volume. In contrast, FEV₁/FVC showed no statistically significant correlation (r = −0.099, p = 0.272), which is consistent with preservation of the ratio in restrictive lung disease. These findings indicate that DLCO, TLC and FVC correspond more closely with structural disease burden on HRCT than the FEV₁/FVC ratio. Table 7. Correlation of Individual HRCT Abnormalities with FVC and DLCO HRCT finding/score FVC % predicted Spearman ρ p-value DLCO % predicted Spearman ρ p-value Ground-glass opacity score −0.397 <0.001 −0.227 0.011 Reticulation score −0.412 <0.001 −0.565 <0.001 Traction bronchiectasis score −0.439 <0.001 −0.413 <0.001 Honeycombing score −0.423 <0.001 −0.355 <0.001 Composite fibrosis score −0.501 <0.001 −0.603 <0.001 Total HRCT severity score −0.539 <0.001 −0.572 <0.001 All evaluated HRCT abnormalities demonstrated statistically significant inverse relationships with pulmonary function. The composite fibrosis score showed one of the strongest correlations, particularly with DLCO (ρ = −0.603, p < 0.001) and FVC (ρ = −0.501, p < 0.001). Reticulation also demonstrated a substantial relationship with DLCO (ρ = −0.565), suggesting worsening diffusion impairment with increasing fibrotic interstitial abnormalities. Traction bronchiectasis and honeycombing demonstrated moderate inverse relationships with both FVC and DLCO. Ground-glass opacity showed a weaker association with DLCO (ρ = −0.227, p = 0.011) than the predominantly fibrotic HRCT abnormalities. Overall, the total HRCT severity score correlated significantly with both FVC and DLCO, supporting a close relationship between increasing structural lung damage and worsening physiological impairment.

Interstitial lung disease represents a heterogeneous group of disorders in which progressive inflammation and fibrosis alter the structural architecture of the lungs and ultimately impair ventilation and gas exchange. In the present study of 125 patients, the mean age was 55.5 ± 12.4 years, with a slight male predominance (53.6%). Breathlessness was the commonest symptom, occurring in 68.0% of patients, followed by cough in 46.4%. On HRCT, reticular opacities were the most frequent abnormality (79.2%), followed by ground-glass opacity (63.2%), interlobular septal thickening (56.8%), traction bronchiectasis (53.6%) and honeycombing (40.0%). The predominance of reticular and fibrotic abnormalities indicates that a substantial proportion of the study population had established structural lung disease. Palermo et al. [9], in patients with idiopathic pulmonary fibrosis, similarly demonstrated that quantitative HRCT indices reflecting increasing fibrotic burden were significantly related to deterioration in pulmonary function, particularly DLCO. UIP/probable UIP was the commonest HRCT pattern in the present study, accounting for 45.6%, followed by NSIP in 35.2%. Lower-lobe predominance was observed in 76.0% and peripheral/subpleural distribution in 68.0%. Importantly, 48.8% of patients had moderate and 24.8% had severe radiological involvement, indicating that nearly three-fourths had at least moderate disease. Karadag et al. [10] evaluated quantitative CT indices in systemic sclerosis and reported significant associations between CT-derived measures of pulmonary fibrosis, visual fibrosis scores, FVC and DLCO. Their observations support the present finding that increasing radiological disease burden is accompanied by progressive physiological impairment. Pulmonary function tests in the present study showed a predominantly restrictive pattern. Mean FVC was 67.65 ± 13.05% predicted, TLC was 65.20 ± 15.43% predicted and DLCO was markedly reduced to 53.94 ± 16.32% predicted. In contrast, the mean FEV₁/FVC ratio remained relatively preserved at 84.96%, consistent with restrictive rather than obstructive physiology. Si-Mohamed et al. [11] demonstrated a strong relationship between automatically quantified CT lung volume and both FVC (r=0.86) and TLC (r=0.84), confirming that structural loss of functioning lung volume closely parallels physiological restriction. Oldham et al. [12], studying progressive fibrosing ILD, further emphasized the importance of serial FVC decline as an objective measure of disease progression. A major finding of the present study was the significant inverse relationship between HRCT disease extent and pulmonary function. HRCT extent correlated negatively with FVC (r=−0.602), FEV₁ (r=−0.546), TLC (r=−0.650) and DLCO (r=−0.676), all p<0.001. The strongest association was observed with DLCO, suggesting that increasing interstitial and fibrotic involvement markedly compromises alveolar-capillary gas transfer. Aoki et al. [13] similarly found significant associations between deep-learning quantified ILD abnormalities and pulmonary function, with fibrotic consolidation showing an inverse correlation with FVC. These observations reinforce the concept that quantitative radiological burden provides meaningful information regarding functional disease severity. The absence of a significant correlation between HRCT extent and FEV₁/FVC (r=−0.099, p=0.272) is also physiologically relevant. In ILD, FEV₁ and FVC usually decline together because of reduced lung compliance and volume, thereby maintaining or even increasing the FEV₁/FVC ratio. Thus, FVC, TLC and particularly DLCO appear more useful than FEV₁/FVC for assessing functional severity. The prognostic relevance of combining radiological and physiological information is supported by Chiu et al. [14], who found that greater HRCT fibrosis extent and lower baseline DLCO were independently associated with mortality in connective tissue disease-associated ILD. This strengthens the clinical importance of the present observation that patients with greater HRCT involvement had substantially poorer DLCO. Among individual HRCT abnormalities, the composite fibrosis score showed significant inverse correlations with FVC (ρ=−0.501) and DLCO (ρ=−0.603). Reticulation showed a particularly strong relationship with DLCO (ρ=−0.565), while traction bronchiectasis and honeycombing demonstrated moderate negative correlations with both functional parameters. Koh et al. [15] reported that increases in quantitatively measured fibrosis and total ILD extent were significantly associated with greater FVC decline and radiological progression. Banerjee et al. [16] also demonstrated relationships between HRCT fibrosis severity and functional parameters, including FVC and DLCO, in patients with ILD. More recently, Sanjan et al. [17] reported a significant negative correlation between HRCT fibrosis score and FVC in treatment-naïve IPF patients, further supporting the structural-functional relationship demonstrated in the present study. Overall, the findings indicate that HRCT and PFT are complementary rather than interchangeable investigations. HRCT defines the morphological pattern, distribution and extent of disease, whereas FVC, TLC and DLCO quantify its physiological consequences. The strong correlations demonstrated in this study suggest that increasing radiological fibrosis is associated with progressive restriction and impaired gas transfer. Simultaneous assessment using HRCT and PFT may therefore provide a more comprehensive evaluation of disease severity than either modality alone..

The present study demonstrates a significant relationship between the extent and severity of HRCT abnormalities and pulmonary functional impairment in patients with interstitial lung disease. Reticulation, ground-glass opacity, traction bronchiectasis and honeycombing were common HRCT abnormalities, with UIP/probable UIP being the predominant radiological pattern. Increasing HRCT disease extent was significantly associated with reductions in FVC, TLC and particularly DLCO, while FEV₁/FVC remained relatively preserved. DLCO showed the strongest correlation with overall HRCT involvement, and composite fibrosis scores demonstrated significant associations with both FVC and DLCO. These findings indicate that combined HRCT and pulmonary function assessment provides valuable complementary information for determining ILD severity and may assist in clinical monitoring and identification of progressive disease.

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