Fixed Ratio versus Z-score/LLN Interpretation in Spirometry: Effects on Ventilatory Pattern Classification

Abstract

Spirometric interpretation depends not only on physiological measurements but also on the diagnostic thresholds used to classify ventilatory patterns. This study aimed to compare fixed ratio- and Z-score/lower limit of normal (LLN)-based interpretation strategies and evaluate their influence on spirometric classification. A total of 378 spirometry records meeting ATS/ERS acceptability and repeatability criteria were retrospectively analyzed. Ventilatory patterns were classified using two interpretation approaches: the fixed ratio method (FEV₁/FVC < 0.70 indicating obstruction; FEV₁/FVC ≥ 0.70 with FVC < 80% predicted indicating a restrictive pattern) and the Z-score/LLN method based on GLI-2012 reference equations (LLN defined as Z = –1.645). Differences in classification were evaluated using the McNemar--Bowker test, and agreement between methods was assessed using weighted Cohen's kappa. According to the fixed ratio approach, 34.9% of cases were classified as normal, 47.4% as obstructive, and 17.7% as restrictive. In contrast, Z-score/LLN-based interpretation increased the proportion of normal classifications to 51.1%, while obstructive patterns decreased to 30.7%, and restrictive patterns accounted for 18.3% of cases (p < 0.001). Overall agreement between the two approaches was moderate (κ = 0.42; 95% confidence interval: 0.33–0.51). Reclassification occurred more frequently among older individuals. These findings indicate that spirometric interpretation strategy substantially influences ventilatory pattern classification and may contribute to clinically relevant differences in respiratory assessment.