Serum KL-6 Level as an Indicator of Active or Inactive Interstitial Pneumonitis Associated with Connective Tissue Diseases

Objective To elucidate the cut off levels of serum KL-6 indicating patients with interstitial pneumonitis (IP) and patients with active IP associated with connective tissue diseases (CTDs). Methods CTD patients whose serum KL-6 level was measured were included. IP was diagnosed on the basis of medical records including XP/CT findings, and active IP was assumed in case that intervention for IP was newly added. The cut off levels were determined by receiver operating characteristic (ROC) curve analysis. Results Among 240 (174 females) patients, 67 (42) had IP and 15 (9) had active IP. The ages of patients with and without IP, and with active IP and with inactive IP were 70.3 +/- 9.5 and 62.8 +/- 15.3, and 72.8 +/- 8.1 and 69.6 +/- 9.8, respectively. IP was significantly more prevalent in males and the elderly. The KL-6 levels were 990 +/- 90 and 301 +/- 12 U/mL in patients with and without IP, and 1,905 +/- 236 and 726 +/- 54 U/mL in those with active IP and with inactive IP, respectively. ROC curve analysis showed a cut off level of 509 U/mL for indicating IP, and that of 1,051-1,060 U/mL for indicating active IP. Conclusion A serum KL-6 level of higher than 500 U/mL is a marker of the presence of IP, and a level of higher than 1,000 U/mL is a marker of the presence of active IP associated with CTDs.