Evaluating the diagnostic and prognostic utility of serial KL-6 measurements in connective tissue disease patients at risk for interstitial lung disease: correlations with pulmonary function tests and high-resolution computed tomography.

Background: Interstitial lung diseases associated with connective tissue diseases (CTD-ILD) necessitate reliable biomarkers for effective management. This study assesses the utility of serial Krebs von den Lungen-6 (KL-6) measurements in predicting disease activity and progression in CTD-ILD patients.

Methods: In a prospective cohort study at a tertiary care center, 50 patients with CTD at risk of or diagnosed with ILD were enrolled. KL-6 levels and pulmonary function tests (PFTs) were measured at baseline, 6, and 12 months, alongside high-resolution computed tomography (HRCT).

Results: Initial KL-6 levels were inversely correlated with PFTs, with mean values starting at 504.96 U/mL (SD ± 508.46), escalating to 739.42 U/mL (SD ± 612.75) at 6 months, and peaking at 1150.27 U/mL (SD ± 1106.70) by 12 months, reflecting disease progression. Higher KL-6 levels were consistently linked with declines in Forced Vital Capacity (FVC) (p = 0.019) and Diffusing Capacity for Carbon Monoxide (DLCO) (p < 0.001). Radiologically, increased KL-6 correlated with subpleural thickening (p = 0.003), septal thickening (p = 0.036), ground-glass opacities (p = 0.018), and other signs of advanced ILD. Sensitivity and specificity of KL-6 for detecting ILD were 86.7% and 71.4%, respectively, at a ≥ 400 U/mL threshold, improving at higher thresholds. Over the study period, patients with elevated KL-6 levels demonstrated more pronounced radiological and functional deterioration.

Conclusion: Serial KL-6 measurements effectively reflect disease activity and progression in CTD-ILD, with strong correlations to functional and radiological outcomes. These findings support the use of KL-6 as a valuable biomarker in the routine clinical management of these complex disorders. Our study demonstrates the significant predictive value of KL-6 for both the diagnosis and monitoring of CTD-ILD, suggesting its integration into clinical practice can enhance patient care and treatment strategies.