BackgroundThe 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.MethodsWe 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 R²-value. The difference in variability between adjusted and unadjusted FEFs was evaluated using the coefficient of variation.ResultsFor all adjusted FEFs, the best model contained age, height and weight. The adjustment improved the coefficient of variation of the FEF₇₅from 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 FEF₇₅/FVC, 32%–38% for males, and 41%–46% for females, depending on the validation set.ConclusionWe 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.