KL-6: a serum marker for interstitial pneumonia.

KL-6 is a mucinous high-molecular weight glycoprotein, expressed on type 2 pneumonocytes, which is reported to be elevated in the serum and bronchoalveolar lavage fluid of patients with interstital pneumonia. A total of 118 samples from 112 patients were measured, including 51 samples with three classes of interstital lung disease and 67 samples with 6 classes of noninterstital lung diseases, in order to clarify whether it was a useful marker of pneumonitis activity. The KL-6 level was significantly higher in patients with pneumonitis (1,187 +/- 689 U/mL; range, 224 to 2,656 U/mL) than in patients without pneumonitis (309 +/- 157 U/mL; range, 123 to 855 U/mL). The KL-6 level was also significantly higher in patients with clinically active pneumonitis (1,497 +/- 560) compared with inactive pneumonitis (441 +/- 276) (p < 0.001). The optimal criterion for separating patients with active pneumonitis from patients without pneumonitis was a KL-6 level of 500 to 700 U/mL according to receiver operating characteristic analysis. These results suggest that KL-6 is a useful marker for the clinical diagnosis of pneumonitis and for the evaluation of disease activity.