Selection of reproducible forced expirograms: percentage or fixed-volume criterion

The aim of this study was to evaluate whether the intrasession reproducibility of forced vital capacity (FVC) and forced expiratory volume in 1 s (FEV1) depends on height and lung volume. FVC tracings of 740 subjects (350 males) from a general population sample living in North Italy were analyzed. Subjects filled out a standardized questionnaire and performed three acceptable FVC maneuvers following the American Thoracic Society recommendations. The differences between the largest and the second largest FVC and FEV1 were computed as absolute (DeltaFVC, DeltaFEV1) and as percentage values (DeltaFVC%, DeltaFEV1%). The higher the tertiles of the largest FVC and FEV1 were, the higher were DeltaFVC and DeltaFEV1. Regarding FVC, borderline differences in both sexes for DeltaFVC and in males significant differences for DeltaFVC% were found among the tertiles. Regarding FEV1, in both sexes DeltaFEV1 significantly differed among the tertiles. DeltaFVC and DeltaFEV1 correlated with height and lung volume in both sexes, except for DeltaFVC versus the largest FVC in females. When DeltaFVC and DeltaFEV1 were analyzed with respect to respiratory symptoms/diseases and smoking habit, no significant differences were observed in both sexes, except for DeltaFEV1 between ever- and never-smoking males. It may be concluded that the intrasession within-subject variability of FVC and FEV1 is proportional to lung volume and height, regardless of the sex, presence of symptoms and smoking habit. Thus, our results confirm the usefulness of a reproducibility criterion based on a percentage rather than on a fixed volume.