Interpretation of spirometric tests in asthmatic patients with reduced forced vital capacity

We have studied 175 consecutive asthmatic patients presenting with: 1) a reversible airflow obstruction, demonstrated by an increase in the forced expiratory volume in 1 second (FEV1) or in the forced vital capacity (FVC) by at least 12% along with an absolute increase of 200 ml versus prebronchodilator values after inhalation of salbutamol; 2) FVC below the lower normal limit before administration of the bronchodilator; and 3) normal FVC or slow vital capacity after bronchodilator. Two different criteria for the lower normal limit of the FEV1/FVC ratio were used to determine whether prebronchodilator spirometric patterns could be considered obstructive or not. The use of the predicted FEV1/FVC ratio as the lower normal limit allowed correct identification of obstruction in 94.9% of the patients, whereas taking the estimated fifth percentile as the lower normal limit of the FEV1/FVC correctly identified obstruction in only 78.9% of the asthmatics. Our results suggest that the predicted FEV1/FVC ratio is an adequate estimate of the lower normal limit in asthmatic patients with reduced FVC in order to distinguish obstructive from nonobstructive patterns.