The serum KL‐6 levels in untreated idiopathic pulmonary fibrosis can naturally decline in association with disease progression

BACKGROUND: Serum KL-6 is known to be a useful biomarker for the diagnosis and assessment of the disease activity in idiopathic pulmonary fibrosis (IPF). However, some patients with untreated IPF present with a serial decline in their serum KL-6 levels, despite showing clinical progression. OBJECTIVES: The aim of the present study was to evaluate the association between the time course of the serum KL-6 levels and the clinical course of patients with untreated IPF. METHODS: We conducted a retrospective study of 59 patients who met the diagnostic criteria for IPF, who regularly underwent laboratory tests, including the measurement of KL-6 and pulmonary function tests, and who were untreated during the follow-up period. RESULTS: The rates of decline in comparison to the baseline FVC values were comparable between survivors (n = 30) and non-survivors (n = 29); however, the rates of change in comparison to the baseline KL-6 levels were heterogeneous in all subjects, and the KL-6 levels of the non-survivors were observed to decline significantly more frequently in comparison to the survivors (P = .001). Patients with a higher breathlessness scale, lower pulmonary functions and higher levels of baseline KL-6 were predisposed to demonstrate a decline in the serum KL-6 levels during the untreated period. CONCLUSIONS: A reduction in the serum KL-6 levels of patients with untreated IPF may not mean that the disease activity has subsided. Serum KL-6 levels can naturally decline in association with disease progression.