Free-knot spline model for analysis of pulmonary function

The pulmonary function test (PFT) is used to evaluate and monitor respiratory function. The PFT is critical for the care of patients having cystic fibrosis (CF) and adjusting their clinical treatments. We analyzed the percent predicted value of forced expiratory volume in one second (FEV₁%) from PFT of CF patients collected four times a year from 1966. Longitudinal FEV₁% for each patient was fitted with linear free-knot spline (FKS) model. We explained as the time when the PFT trend changes. We classified the patients' pulmonary function trend using eight groups of FEV₁% based on the angle of linear FKS model. The overlapped majority of knots in groups located at 1978, 1991, and 1993 for worsening and at 1983, 1988, and 2000 for not worsening.