Pneumocyte biomarkers KL-6 and surfactant protein D reflect the distinct findings of high-resolution computed tomography in nonspecific interstitial pneumonia

Background: Serum levels of pneumocyte biomarkers KL-6 and surfactant protein D (SP-D) are useful diagnostic markers for interstitial lung diseases. However, associations of serum KL-6 and SP-D with radiologic findings in nonspecific interstitial pneumonia (NSIP) remain unclear. Objectives: To determine whether serum levels of KL-6 and SP-D reflect fibrotic and/or inflammatory processes in NSIP, we investigated the correlation between high-resolution computed tomography (HRCT) findings and serum KL-6 and SP-D levels. Methods: Serum levels of KL-6 and SP-D were measured in 21 patients with biopsy-confirmed NSIP. The radiographic extent of 6 HRCT patterns and total HRCT score, defined as the scored fibrotic index, were assessed. Changes in the levels of serum markers and CT findings during follow-up were also monitored. Results: Serum levels of KL-6 in NSIP positively correlated with the total HRCT score and overall extent of interstitial disease. Serum levels of SP-D in NSIP showed a positive correlation with the area of ground-glass attenuation without traction bronchiectasis and the inflammatory CT pattern, but the levels were inversely correlated with the area of ground-glass attenuation with traction bronchiectasis and the fibrotic CT pattern. The follow-up CT and serum marker changes after treatment showed that percent change of disease extent was reflected in both markers, especially KL-6. Further, the decreased fibrotic pattern correlated with both biomarkers. Conclusions: The results indicate that serum levels of KL-6 in NSIP reflect the overall extent of interstitial lesions, which include both inflammatory and fibrotic lesions, while the levels of SP-D mainly reflect the extent of inflammatory lesions.