How to Calculate Diurnal Variability
The most common and recommended method to calculate diurnal variability of peak expiratory flow (PEF) is amplitude percent mean: (day's highest PEF minus lowest PEF) divided by the mean PEF, averaged over 7 days 1.
The Standard Formula
Amplitude % Mean = [(Highest PEF - Lowest PEF) / Mean PEF] × 100
This calculation should be:
- Performed for each day
- Averaged over 7 days for the most reliable assessment
- Based on twice-daily measurements (morning and evening)
Important Threshold Values
With twice-daily PEF monitoring, the upper limit of normal is 8% (9.3% in adolescents) 1. This represents the 95% confidence limit for a reference population.
Critical caveat: The frequently cited cut-point of >20% diurnal variability for diagnosing asthma only applies to four-times-daily monitoring, not twice-daily monitoring 1. This 20% threshold originated from specialized cosinor modeling of patients selected for definite circadian variability after hospitalization, making it inappropriate for standard twice-daily home monitoring.
Alternative Calculation Method (British Guidelines)
The British Thorax Society recommends amplitude % best for diagnostic purposes 2:
Amplitude % Best = [(Highest PEF - Lowest PEF) / Highest PEF] × 100
For asthma diagnosis, they suggest:
- ≥20% variability using this formula
- Minimum absolute change of at least 60 L/min
- Ideally documented for 3 days per week over 2 weeks
Practical Example from Guidelines 2:
- Highest PEF = 400 L/min
- Lowest PEF = 300 L/min
- Amplitude = 100 L/min
- Percentage variability = (400-300)/400 × 100 = 25%
Number of Daily Readings Required
At least 4 evenly-spaced readings per day are needed to accurately capture true diurnal variation 3. With only 2 readings per day, you risk underestimating true diurnal variability by 1.23-15.10%. This underestimate decreases to just 0.27-3.96% with 4 readings 3.
The timing of peak PEF varies widely (mean 13:56 ± 4:56 hours on rest days, 11:47 ± 5:59 hours on work days), making it impossible to reliably capture maximum variation with just 2-3 carefully timed readings 3.
Between-Day Variability
For assessing variability across multiple days rather than within a single day, calculate: lowest PEF (or lowest morning PEF) over 1-2 weeks divided by highest PEF 1. Note that this index increases as PEF variation decreases (inverse relationship).
When to Measure
- Before taking routine asthma medications
- Before using reliever medications when possible
- At consistent times relative to waking (not clock time, as diurnal changes depend more on hours since waking) 1
- Morning PEF is more responsive to treatment changes than evening PEF 1
Clinical Context
Always state which calculation method you used when reporting diurnal variability 2. The different formulas yield different numerical results and have different normal ranges, making this specification essential for proper interpretation.
Increased PEF variability associates with increased risk of exacerbations 1, making it an important monitoring parameter beyond static lung function measurements. These represent different domains of asthma control and both should be assessed 1.