Background and objective: Small airways obstruction syndrome (SAOS) is a particular pulmonary function test (PFT) pattern showing decreased VC and FEV1 but a normal FEV1/VC ratio and TLC. The significance of this syndrome in clinical practice has not been comprehensively investigated. Methods: This study retrospectively identified all patients who had performed PFT that showed a SAOS pattern at a university teaching hospital over 1 year. A simple algorithm for differential diagnosis was developed and validated. Results: Of the 3207 PFT performed, 153 (4.8%) showed a pattern indicating SAOS. Among these, a final diagnosis was confirmed for 85 (63.4%) of the patients. The causes of SAOS included both restrictive and obstructive lung diseases with the leading causes being early interstitial lung disease (n = 20; 23.3%), chest wall deformity (n = 12; 14.1%) and asthma (n = 10; 11.6%). Using a cut-off point of TLC of <95% predicted value to identify restrictive ventilatory defects (P = 0.002) and of ≥95% predicted combined with RV/TLC ≥55% to identify obstructive ventilatory defects (P < 0.001), a simplified algorithm with good accuracy (86.6%) was identified. Validation in an independent group showed accuracy of 91%. Conclusions: The PFT pattern called SAOS is not uncommon. The most common causes of SAOS were early interstitial lung disease, chest wall deformity and asthma. A diagnostic algorithm was proposed, which may help physicians' decision-making in their daily practice.
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