Avaliação da função respiratória: Comparação entre valores de referência percentuais fixos e o 5.^o percentil para diagnóstico de obstrução das vias aéreas

Introduction: Fixed criteria have been classically used in the functional characterization of patients with respiratory diseases. International guidelines actually recommend the preferential use of the lower (LNN) or upper limit of normality (below or above the 5th percentile). Objective: To compare the results of lung function (LF) obtained in clinical practice for diagnosis of airway obstruction, using fixed percentage values versus the 5^th percentile (the reference standard) as normality limits. Methods: Retrospective analysis of LF (spirometry and body plethysmography) performed in 2011 by the authors. It has been evaluated the concordance of both methods in the global sample, and divided by age groups. Those with criteria for airway obstruction considering the FEV1/VC (forced expiratory volume in one second / vital capacity) ratio < LLN were selected and among these, we analyzed FEV1, FVC (forced vital capacity), TLC (total lung capacity) and RV (residual volume) considering the 5th percentile and the fixed percentage values. Statistical analysis was performed with SPSS 20.0, using Cohen's Kappa test. Results: During 2011, 1358 subjects underwent LF; 8 were excluded due to incomplete data. Overall, the agreement between the two criteria was kappa = 0.655 ± 0.035. Among the 124 patients who had obstruction diagnosed by LLN, 32 (26%) had a normal test with the 0.70 cut -off, and would be wrongly underdiagnosed. This occurred only in younger age groups, while in older ones a high rate of overdiagnosis (51 subjects - 36%) was observed. Among patients with airway obstruction, the agreement of the 2 criteria for the remaining parameters was good, except for inflation diagnosed with TLC. Conclusion: Using fixed percentage criteria for the diagnosing airway obstruction leads to a high rate of underdiagnosis in younger and overdiagnosis in older ages.