Objective: To evaluate the test characteristics of the forced expiratory time (FET) in the diagnosis of obstructive airways disease.
Design: A cross-sectional diagnostic test study. The FET of 400 subjects was measured by a physician examiner and was compared with the criterion standard of spirometry. In a second sample of 100 subjects, the FET was measured by pairs of physician examiners to evaluate interexaminer agreement.
Setting: A pulmonary function test laboratory at a tertiary care hospital that receives referrals for preoperative evaluations, acute and chronic pulmonary disease, and occupational lung disease.
Subjects: A consecutive sample of patients who were referred to the pulmonary function laboratory from primary care internists, pulmonary physicians, and surgeons.
Interventions: None.
Main outcome measures: The sensitivity and specificity of the FET in the diagnosis of obstructive airways disease at cutoff values ranging from 2 to 14 seconds. A receiver operating characteristic curve was used to evaluate the diagnostic performance of the FET. Likelihood ratio lines were determined using a logistic regression model adjusting for the subjects age. Interexaminer agreement was evaluated with a kappa statistic.
Results: Using the FET maneuver with a cutoff value of 6 seconds will correctly diagnose the greatest number of subjects with obstructive airways disease. The FET maneuver is more discriminating for subjects 60 years or older compared with younger subjects. The positive likelihood ratio for a subject aged 60 years or older with an FET of 4 to 6 seconds is 0.42 (95% confidence interval [CI], 0.24 to 0.73); of 6 to 8 seconds, 2.19 (95% CI, 1.02 to 4.80); and of greater than 8 seconds, 4.08 (95% CI, 2.54 to 6.79). The kappa statistic for interexaminer agreement is 0.70.
Conclusions: The FET demonstrates moderately good performance as a diagnostic test for obstructive airways disease. The value of the test will depend on the pretest probability of disease and the clinical circumstances in which it is used.