Effect of choice of reference equation on analysis of pulmonary function in cystic fibrosis patients

Pulmonary function is an important measure of disease severity and prognosi s in cystic fibrosis (CF). It is generally expressed as a percentage of a p redicted value, calculated using regression equations derived from a refere nce population. A number of reference equations are in widespread use. The purposes of this study were to determine: 1) the extent to which, for a giv en absolute FEV1 value, percent of predicted (PPFEV1) values vary when deri ved by different reference equations; and 2) whether these differences affe ct conclusions of longitudinal and cross-sectional analyses. Subjects were all Caucasians 6-18 years old in the 1990 Cystic Fibrosis Foundation Regist ry. We found clinically important discrepancies in PPFEV1 when calculated by th e methods of Dockery et al. [Am Rev Respir Dis 1983;128:405-412] and Wang e t al. [Pediatr Pulmonol 1993;15:75-78] as compared to Knudson et al. [Am Re v Respir Dis 1983,127.725-734] or Polgar and Promadhat [Pulmonary Function Testing in Children 1971; Philadelphia: W.B. Saunders]. In longitudinal ana lyses, the choice of reference equation resulted in varying apparent rates of decline in FEV1. For example, among subjects ages 12-14 years in 1990, t he decline in PPFEV1 from 1990-1995 varied between 2-11%, depending on the choice of reference equation. In cross-sectional analyses, the choice of re ference equation affected the distribution of subjects classified as having mild, moderate, or severe lung disease. CF physicians should be aware of the impact of choice of reference equation in both clinical care and research.