FEV1/FVC Ratio in Asthma Diagnosis
For asthma diagnosis, the FEV1/FVC ratio should be below the lower limit of normal (LLN) rather than using a fixed cutoff of 0.7, as this more accurately identifies airflow obstruction across all age groups. 1
Spirometric Criteria for Airflow Obstruction in Asthma
- A reduced FEV1/FVC ratio below the lower limit of normal (LLN) indicates airflow obstruction and is supportive of an asthma diagnosis 1
- When LLN is not available, a fixed cutoff of FEV1/FVC <80% can be used as an alternative threshold, not 0.7 1
- Using a fixed ratio of 0.7 (as used in COPD) can lead to overdiagnosis of obstruction in older adults and underdiagnosis in younger patients 1
- The European Respiratory Society strongly recommends using LLN to define abnormal spirometry in children aged 5-16 years with suspected asthma 1
Diagnostic Accuracy and Interpretation
- Spirometry as a one-time measurement has low sensitivity (0.12-0.52) but moderate to high specificity (0.72-0.93) for asthma diagnosis 1, 2
- Normal spirometry does not exclude asthma due to the variable nature of the condition; serial measurements may be required to confirm the diagnosis 1
- In a population with 50% pretest probability of asthma, optimal diagnostic accuracy (68%) is achieved with an FEV1/FVC z-score threshold of -1.0 (16th percentile) 3
- For screening populations with lower pretest probability (5%), a more stringent z-score threshold of -2.0 (2nd percentile) provides better accuracy 3
Additional Diagnostic Considerations
- Bronchodilator reversibility testing is essential - an increase in FEV1 of at least 12% and 200 mL after bronchodilator administration strongly supports asthma diagnosis 2, 4
- A reduced FEV1/FVC ratio with positive bronchodilator response confirms the diagnosis of asthma 1, 2
- FEF25% and FEF75% (forced expiratory flow at 25% and 75% of FVC) may be more sensitive indicators for mild asthma than FEV1 and FVC alone 5
- Even when the FEV1/FVC ratio is in the lower range of normal (not meeting obstruction criteria), it may still be associated with worse clinical outcomes in certain age groups 6
Common Pitfalls and Caveats
- Not all patients can perform adequate FVC maneuvers, which may result in falsely normal FEV1/FVC ratios 1
- There is considerable variability between FEV1 and PEF measurements when expressed as percentage of predicted values; FEV1% predicted may be as much as 35% lower than PEF% predicted 7
- Using the predicted FEV1/FVC ratio as the lower normal limit allows correct identification of obstruction in 94.9% of asthmatic patients with reduced FVC, compared to only 78.9% when using the fifth percentile as the cutoff 4
- Airflow obstruction should be confirmed by repeat spirometry on a separate occasion if the post-bronchodilator FEV1/FVC ratio is borderline 1
In summary, while a fixed cutoff of 0.7 is commonly used for COPD, asthma diagnosis benefits from using the lower limit of normal for FEV1/FVC ratio, with values below 80% (when LLN is unavailable) supporting the diagnosis when clinical symptoms are present.