Predicted values for the forced expiratory flow adjusted for forced vital capacity, a descriptive study

BACKGROUND: The forced expiratory flows (FEFs) towards the end of the expiration may be more sensitive in detecting peripheral airways obstruction compared to the forced expiratory volume in 1 s and forced vital capacity (FVC). However, they are highly variable. A partial solution is to adjust the FEFs for FVC (FEF/FVC). Here we provide reference equations for these adjusted FEFs at 25%, 50%, 75% and 25–75% of FVC, which are currently lacking. METHODS: We included pulmonary healthy, never-smoker adults; 14 472 subjects from Lifelines, a biobank for health research, and 338 subjects from the department's control cohorts (NORM and Fiddle). Reference equations were obtained by linear regression on 80% of the Lifelines dataset and validated on the remaining data. The best model was defined as the one with the highest adjusted R(2)-value. The difference in variability between adjusted and unadjusted FEFs was evaluated using the coefficient of variation. RESULTS: For all adjusted FEFs, the best model contained age, height and weight. The adjustment improved the coefficient of variation of the FEF(75) from 39% to 36% and from 43% to 40%, respectively, in males and females. The highest percentage of explained variance by the reference equation was obtained for FEF(75)/FVC, 32%–38% for males, and 41%–46% for females, depending on the validation set. CONCLUSION: We developed reference equations for FVC-adjusted FEF values. We demonstrated minimally yet significantly improved variability. Future studies in obstructive airway diseases should demonstrate whether it is worthwhile to use these (predicted) adjusted FEF values.