Prognostic value of serial serum KL-6 measurements in patients with idiopathic pulmonary fibrosis. 
BACKGROUND: The prognostic significance of serial measurements of serum KL-6 levels in patients with idiopathic pulmonary fibrosis (IPF) is unclear; hence, it was assessed in this study. METHODS: Medical records of 66 patients with IPF, who were not treated with pirfenidone prior to enrollment, were retrospectively reviewed for information on clinical progress, forced vital capacity (FVC), survival, and serum KL-6 levels.  We assessed initial serum levels of KL-6, serial changes in serum KL-6 levels, yearly decline in FVC (ΔFVC), and the rate of decline (%ΔFVC). RESULTS: Patients with increased serum KL-6 levels during follow-up had a significantly steeper decline in ΔFVC than those with no KL-6 increase (-201 vs.  -50.7ml/year; p=0.0001). Patients with both initial serum KL-6 ≥1000U/ml and serial increases in serum KL-6 had the steepest decline, while those with both initial serum KL-6 <1000ml and no serial increases in KL-6 had the least decline  in ΔFVC and %ΔFVC. Relative to the non-increased KL-6 group, survival in the increased KL-6 group tended to be poorer (p=0.0530). Patients with both initial serum KL-6 values <1000U/ml and no serial increase in KL-6 had more favorable prognoses than those with serial increases in KL-6 or initial serum KL-6 values ≥1000U/ml (p<0.0044). Prognosis was significantly poorer in patients with serial  KL-6 changes >51.8U/ml/year than in those with serial KL-6 changes <51.8U/ml/year (p=0.0009). CONCLUSION: Thus, serial serum KL-6 measurements can be useful for assessing prognosis in patients with IPF.