FEV1 and Peak Flow Variability Criteria for Severe Persistent Asthma
For severe persistent asthma, the FEV1 must be less than 60% of predicted value and peak flow variability must be greater than 20%. 1
Diagnostic Criteria for Severe Persistent Asthma
According to the Mayo Clinic Proceedings guidelines, severe persistent asthma is characterized by specific lung function parameters that help distinguish it from milder forms of asthma:
Lung Function Parameters:
- FEV1: Less than 60% of predicted value
- Peak Flow Variability: Greater than 20% diurnal variation
- FEV1/FVC ratio: Reduced compared to normal values 1
Additional Clinical Features:
- Symptoms occurring throughout the day
- Nighttime awakenings often 7 times per week
- Need for short-acting β2-agonist use several times per day
- Extreme limitation in normal activities
- Two or more exacerbations per year requiring oral corticosteroids 1
Clinical Significance of These Parameters
The American Thoracic Society workshop on refractory asthma confirms that persistent airway obstruction with FEV1 < 80% predicted and diurnal PEF variability > 20% are characteristic of severe asthma. In patients with refractory asthma, airflow limitation is often markedly reduced (≤70% predicted) with wide swings in lung function (>20% PEF diurnal variability). 1
It's important to note that these objective measurements are critical for proper diagnosis, as relying solely on symptoms can lead to misdiagnosis or undertreatment. In fact, studies have shown that in 30% of patients whose symptoms appeared controlled, peak flow measurements still indicated poor control. 2
Measurement Considerations
When measuring these parameters, be aware of several important caveats:
Timing of measurements: Peak flow measurements should be taken at least twice daily to capture variability, though this still underestimates the true circadian variability compared to more frequent measurements. 3
FEV1 vs PEF discrepancies: There is considerable variability between FEV1 and PEF when expressed as percentage of predicted values. FEV1% predicted often results in lower values than PEF% predicted, with studies showing a mean difference of -17.2%. This means relying on PEF alone may underestimate asthma severity. 4, 5
Post-bronchodilator assessment: For patients with refractory asthma, fixed airflow limitation is defined as a post-bronchodilator FEV1 < 80% predicted after a 7-14 day course of oral corticosteroids. 1
Clinical Application
When evaluating a patient for severe persistent asthma:
- Perform spirometry to measure FEV1 (looking for <60% predicted)
- Assess peak flow variability through twice-daily measurements over at least 2 weeks (looking for >20% variability)
- Consider post-bronchodilator response
- Evaluate clinical symptoms and exacerbation history
Remember that some patients with severe asthma may demonstrate poor bronchodilator response, which doesn't exclude the diagnosis but may indicate long-standing inflammation or structural changes in the airways. 1
By using both FEV1 and peak flow variability criteria together with clinical symptoms, you can more accurately classify asthma severity and develop an appropriate treatment plan for patients with severe persistent asthma.