[Background] IPF is a progressive disease with poor prognosis, including remaining life expectancy of only several years in most cases. However, the disease characteristics and progression at a very early stage are still unknown, largely due to difficulties of detection in the preclinical stage.

[Aims] To evaluate the clinical characteristics of IPF at a very early stage.

[Methods] This was a single center, retrospective cohort study of IPF diagnosed between 2008 and 2015. Cases without any symptoms and without respiratory dysfunction (FVC >90 %pred. and %DLco >80 %pred.) were defined as preclinical IPF.

[Results] Of 419 IPF patients, 28 patients met the criteria for preclinical IPF (median age 68 yo, 27 males, KL-6 595 U/L, FVC 3.69 L (108.2 %pred.), DLco 97.1 %pred.). All cases were incidentally detected to have interstitial lung abnormalities on CT. A pathological usual interstitial pneumonia pattern was detected by surgical lung biopsy in 9 patients, and the remaining patients were diagnosed with IPF clinically and radiologically. The median period of time from the first detection of preclinical IPF to the appearance of either clinical symptoms (apparent cough and/or dyspnea) or respiratory dysfunction (FVC <90 %pred. and/or DLco <80 %pred) was 40.9 months (95% CI: 27.5-54.3). FVC decline at years 1 to 4 was -26.8 mL, -185.4mL, -380.6 mL and -520.6 mL, respectively, which indicates that even those with preclinical IPF may show respiratory function decline.

[Conclusion] This study demonstrates that preclinical IPF can progress to clinical IPF in several years and suggests that asymptomatic interstitial lung disease should not be underestimated when IPF is suspected.