Auscultated Forced Expiratory Time as a Clinical and Epidemiologic Test of Airway Obstruction

Abstract

Seeking an inexpensive, readily available, clinical, screening, and field surveillance test of airway obstruction, we determined the validity of current dogma that forced expiratory time (FET) is a good clinical test of airway obstruction yet is of no epidemiologic use given excessive intrasubject variability. Two hundred twenty-nine white male plumbers and pipefitters were evaluated by spirometry, chest roentgenography, and a standardized respiratory questionnaire during a union-sponsored asbestos screening program. Subjects were classified as having large airway obstruction (LAO), small airway obstruction (SAO) alone, or no obstruction, on the basis of standard spirometric prediction equations. Two physicians, blinded to clinical and spirometric data, independently measured FET while auscultating the trachea with a stethoscope. The FET was defined as the time taken for an individual to forcefully exhale through an open mouth from total lung capacity until airflow became inaudible. Five such times were recorded for each subject. The mean of the three times having the narrowest range was deemed the FET for calculating test sensitivity and specificity. Based on previous literature, an FET greater than or equal to 6 s was considered abnormally prolonged. Two hundred five subjects completed both spirometry and FET testing; 67 had LAO, 5 SAO, and 133 no obstruction. A total of 83 percent had three FETs reproducible within a range of less than or equal to 1 s. The sensitivity and specificity of FET for LAO were 92 and 43 percent, respectively, while for SAO alone, 60 and 44 percent, respectively. Overall, FET misclassified 56 percent of nonobstructed subjects. Adjusting the normal-abnormal cutoff points for both FET and SAO minimally improved the performance of FET. Although FET is a simple, inexpensive, sensitive, and fairly reproducible clinical test of LAO, it cannot be recommended as a clinical or an epidemiologic tool because of its extremely low specificity.