Significant Bronchodilator Response in FEF 25-75%
FEF 25-75% should not be used as a primary criterion for defining significant bronchodilator response, as it is highly variable and technically problematic for this purpose. 1
Why FEF 25-75% Is Unreliable for Bronchodilator Testing
The European Respiratory Society guidelines explicitly caution against using FEF 25-75% (also called MEF 25-75%) for bronchodilator response assessment due to fundamental technical limitations 1:
- FEF 25-75% depends on FVC, which typically increases with expiratory time in obstructed patients, making pre- and post-bronchodilator values non-comparable when FVC changes 1
- The measurement is inherently highly variable compared to FEV₁ and FVC 1
- Post-bronchodilator FEF 25-75% cannot be accurately compared to baseline if FVC has changed, which commonly occurs after bronchodilator administration 1
Limited Clinical Utility
Even when FEF 25-75% has been studied for bronchodilator response, the results have been disappointing 1:
- Only 8% of asthmatics were identified as responders by FEF 25-75% criteria alone 1
- Only 7% of COPD patients were identified by FEF 25-75% criteria alone 1
- These low detection rates make it an inefficient screening tool
Historical Reference Values (Not Recommended)
While not recommended for clinical use, historical studies have reported FEF 25-75% changes ranging from 20% to 48.3% in various populations 1:
- One placebo-controlled study found the 95% confidence limit for percent change was 45.1% 2
- A general population study showed approximately 20% change 1
- Normal subjects demonstrated 48.3% as the upper 95% confidence limit 1
Recommended Approach
Use FEV₁ and/or FVC instead, with the standard criteria of >12% AND >200 mL change from baseline to define significant bronchodilator response 1, 3, 4:
- These parameters are reproducible, standardized, and clinically validated 1
- FEV₁ alone identifies approximately 88% of true responders 2
- If FEV₁ shows no significant response, assess for decreased lung hyperinflation as an alternative indicator of meaningful bronchodilation 1
Critical Caveat
Laboratory bronchodilator response does not predict clinical response to therapy - patients without significant spirometric changes may still experience symptom improvement and better performance with bronchodilator treatment 1, 3