Sensitivity of different spirometric tests for detecting airway obstruction in childhood asthma.

Objective: The objective of this study was to compare different measures of airflow obstruction by spirometry in childhood asthma. The objectives were; (a) to compare sensitivity of large airway tests (FEV1 and PEFR) with tests at low lung volumes (small airways) (FEF25-75, FEF50 and FEF75); (b) compare within each group which individual tests are more sensitive.

Methods: This was a retrospective analysis of 2307 spirometry tests performed during outpatient visits on 821 doctor-diagnosed asthma patients aged 6-18. Tests were deemed acceptable if they were acceptable and repeatable by American Thoracic Society (ATS) criteria.

Results: In mild obstruction, FEV1 detected 6.8% abnormal tests while FEF75 detected 33% (p < 0.0001). In more severe obstruction, the difference was more obvious (FEV1 14.8%; FEF75 71%). Tests at low lung volumes (small airway tests) were also more sensitive than PEFR. Within groups, FEV1 was more sensitive than PEFR in the large airway tests and FEF75 was more sensitive than FEF25-75 and FEF50 among the tests at low lung volumes (small airway tests). The FEV1/FVC ratio correlated more closely with tests at low lung volumes (small airway tests), than with large airway tests.

Conclusions: (1) Tests at low lung volumes (small airway tests) are more sensitive than large airway tests; (2) Within groups, the FEV1 is better than PEFR and FEF75 is better than FEF25-75 or FEF50.