What is the percentage of peak expiratory flow (PEF) variability that characterizes asthma?

Peak Expiratory Flow Rate Variability in Asthma

A peak expiratory flow rate (PEFR) variability of 20% or greater is characteristic of asthma diagnosis. 1

Diagnostic Criteria for Asthma Using PEFR Variability

The British Thoracic Society guideline clearly defines the PEFR variability threshold for asthma diagnosis:

• A 20% or greater variability in amplitude % best (highest minus lowest divided by highest × 100)
• Must have a minimum change of at least 60 L/min
• Ideally measured over three days in a week for two weeks
• This level of variability is highly suggestive of asthma 1

Calculation Method

The recommended calculation method for PEFR variability is:

Amplitude % best = (highest PEF - lowest PEF) / highest PEF × 100

For example:

• Highest PEF = 400 L/min
• Lowest PEF = 300 L/min
• Amplitude = 100 L/min
• Percentage PEF variability = (400 - 300)/400 × 100 = 25% 1

Clinical Significance and Limitations

While the 20% threshold is specific for asthma diagnosis, it's important to note several key considerations:

• Many patients with asthma will demonstrate variability below 20%, making this a reasonably specific but insensitive diagnostic test 1
• The 20% criterion originated from cosinor modeling of four-times-daily PEF data in patients selected for definite circadian variability shortly after asthma hospitalization 1
• For twice-daily PEF measurements, the upper limit of normal for amplitude % mean is actually 8% (9.3% in adolescents) 1
• With more frequent PEF recordings, the upper limit of normal increases to 19% 1

Alternative Methods for Diagnosing Asthma

When PEFR variability assessment is inconclusive, other methods to confirm variable airflow limitation include:

• An increase in PEF of 20% (minimum 60 L/min) after inhalation of a short-acting β2 agonist
• An increase after a trial of oral steroids (prednisolone 30 mg/day for 14 days)
• A decrease after six minutes of exercise 1

Monitoring Considerations

For ongoing monitoring of asthma:

• PEF measurements should ideally be performed at the same time morning and evening 1
• Daily diurnal variation is calculated as the day's highest PEF minus the day's lowest PEF, divided by the mean PEF for the day, averaged over a week 1
• A variability of >10% in adults and >13% in children is suggestive of asthma for monitoring purposes 1

Clinical Application

PEFR monitoring has advantages and limitations compared to spirometry:

Advantages:

• Sensitive
• Easy to perform
• Inexpensive
• Can be done at home 1

Disadvantages:

• Less reliable than FEV1
• Requires patient compliance
• May provide only a snapshot of lung function 1

It's worth noting that PEF monitoring alone may not be sufficient to monitor asthma severity reliably in children, as changes in PEF variation over time may show poor concordance with changes in other parameters of asthma severity 2.

In conclusion, while a PEFR variability of 20% or greater (with minimum change of 60 L/min) is the established diagnostic threshold for asthma, clinicians should be aware that many patients with asthma may demonstrate lower levels of variability, and additional diagnostic methods may be necessary.