The Interpretation of the Spirogram: How Accurate Is It for 'Obstruction'?

• The accuracy of the spirogram in detecting or excluding airway obstruction based on airflow limitation was assessed prospectively in 200 subjects, 74 with obstruction and 126 without it. The diagnosis of airway obstruction was based on a combination of clinical and body plethysmographic data. The ratio of forced expiratory volume in 1 s to forced vital capacity (FEV 1 /FVC %) had a sensitivity of 0.82 and a specificity of 0.98. A fixed lower limit seemed better than a lower limit based on prediction formulas. Because specificity is so much higher than sensitivity, less precise clinical information is required to confirm the presence of obstruction if FEV 1 /FVC % is abnormal than is needed to exclude obstruction if FEV 1 /FVC % is normal. Using a combination of FEV 1 /FVC % and the ratio of forced expiratory flow (FEF) at 50% of FVC gave a higher sensitivity with a comparable specificity when compared with