Defining and grading an obstructive ventilatory defect (OVD): 'FEV1/FVC lower limit of normal (LLN) vs. Z-score' and 'FEV1 percentage predicted (%pred) vs. Z-score'.

An obstructive ventilatory defect (OVD) is defined by a low forced expiratory volume/'forced/slow' vital capacity (FEV₁/FVC) (e.g. 1/FVC < LLN (=local norms value - 1.64 × residual standard deviation)] and GLI (FEV₁/FVC Z-score < -1.64) criteria. The following OVD classifications severity were applied: ATS/ERS (FEV₁%pred): mild (>70%), moderate (60-69%), moderately severe (50-59%), severe (35-49%), and very severe (<35%) and GLI (FEV₁ Z-score): mild (≥ -2.0), moderate (-2.0 to -2.5), moderately severe (-2.5 to -3.0), severe (-3.0 to -4.0), and very severe (<-4.0). The frequencies of OVD were 14.4% (ATS/ERS method) and 10.5% (GLI method) (p < 0.05). Among the 103 participants having an OVD according to the two methods, the severity classification was mild (34.95% vs. 37.86%, p < 0.05), moderate (25.24% vs. 18.45%, p < 0.05), moderately severe (23.30% vs. 15.53%, p = 0.144), severe (9.71% vs. 20.39%, p < 0.05), and very severe (6.80% vs. 7.77%, p = 0.785), respectively for the ATS/ERS and GLI classifications. The two OVD definitions were not exchangeable. Moreover, the two grading severity systems misclassified the OVD grades.