Spirometry can be difficult to perform due to the requirement for complete exhalation to measure FVC. Some have suggested that a 6-second expiratory time may be sufficient for diagnostic spirometry. Swanney and colleagues (AJRCCM 2000;162:917–919) studied whites in New Zealand and found that FEV1/FEV6 had a sensitivity of 95.0% and specificity of 97.4% for the diagnosis of airway obstruction defined by FEV1/FVC. We evaluated the utility of FEV1/FEV6 in a diverse urban population and factors that may influence the sensitivity of this parameter.
We reviewed all spirometric studies performed at Beth Israel Medical Center during 2003-2004. Tests were excluded if they did not meet American Thoracic Society criteria for acceptability and reproducibility, had exhalation times under six seconds, or were done in Asian subjects (the NHANES III reference set provides equations only for white, black and Hispanic subjects). Tests were independently categorized as obstructive or non-obstructive using both FEV1/FEV6 and FEV1/FVC. Sensitivity and specificity of FEV1/FEV6 for diagnosis of obstruction were calculated using FEV1/FVC as the gold standard.
In the 1926 tests that met all study criteria, the sensitivity of FEV1/FEV6 for defining airflow obstruction was 85.6%(95%CI:83.0-88.2) and specificity was 97.1%(95%CI:96.1-98.0). Sensitivity in whites was 88.7%(95%CI:85.3-91.4), in blacks, 78.2%(95%CI:70.2-84.6) and in Hispanics, 83.2%(95%CI:76.7-88.2)(p<0.001 for blacks vs. whites). Sensitivity varied from 100%(95%CI:100) in severe obstruction to 73.7%(95%CI:68.2-79.2) in mild obstruction (p<0.001 for all grades of severity). Sensitivity was inversely related to expiratory time: 100%(95%CI:100) for 6-8 seconds, to 78.0%(95%CI:72.0-84.0) for > 16 seconds (p< 0.001 for all expiratory times).
The overall sensitivity of FEV1/FEV6 in the diagnosis of obstructive airways disease is lower in our diverse urban population (85.6%) compared to the study by Swanney and co-workers of white New Zealanders (95.0%). In our study, factors associated with a lower sensitivity of FEV1/FEV6 included mild airflow obstruction, tests with longer expiratory times and black race.
Practioners should be cautious about using FEV1/FEV6 as a replacement for FEV1/FVC without assessing its use in their patient population.
Mayuko Fukunaga, None.