Peak Flow Diurnal Variability Calculator
Peak Flow Diurnal Variability Calculator
Diurnal peak flow variability by the amplitude-percent-mean method, with the GINA and NICE thresholds side by side, and an honest account of how weak the measurement is.
Peak flow diurnal variability
Amplitude percent meanHighest 420 L/min, lowest 340 L/min, personal best 480 L/min
Formula
Overall variability = total of the daily variabilities ÷ number of days
- the denominator
- the average of the day’s highest and lowest, the amplitude-percent-mean form the PCRS-UK consensus publishes. A second form divides by the day’s highest instead and returns a smaller number for the same readings — two percentage points smaller on the defaults here, enough to change which side of 20% a result falls on
- the thresholds, and they disagree
- GINA asks for average daily diurnal PEF variability of more than 10% in adults and more than 13% in children, over two weeks of twice-daily readings. NICE NG80 asks for 20%. The PCRS consensus declines to choose and places significant variability “in a range between 10% (13% for children) and 20%”
- averaging over days
- both GINA and PCRS specify an average of daily variabilities, not a single day’s. This calculator computes one day; run it for each day of the diary and average the results
- the alternative index
- Reddel and colleagues argued in 1999 that diurnal variability is “too time consuming for normal clinical practice”, that recent beta-2 agonist use causes “large errors” in it even when the peak flows barely move, and that it “may fail to detect important changes in lung function”. Their alternative, the lowest morning peak flow as a percentage of personal best, is the third figure above
- what PEF cannot do
- it is effort-dependent; it is device-dependent, and the PCRS consensus warns of a 20% variation from meter to meter and that the meters “are susceptible to error and are not calibrated as with spirometry”; and it is a large-airway measurement insensitive to small-airway disease. PCRS: PEF variability “should never be used alone to make a diagnosis”
Worked example
Highest 420 L/min, lowest 340 L/min, personal best 480 L/min
Amplitude = 420 − 340 = 80 L/min
Mean of the two = (420 + 340) ÷ 2 = 380 L/min
Daily variability = 100 × 80 ÷ 380 = 21.1%, which is above GINA's adult and paediatric figures and above NICE's 20%
The other denominator, on the same readings. 100 × 80 ÷ 420 = 19.0%, which is below NICE's 20%. Two published forms of one index, two sides of one threshold, from identical blows
Reddel's alternative: 340 ÷ 480 = 70.8% of personal best
Three points across the range, and both band edges. 420 and 380 give 100 × 40 ÷ 400 = 10.0%, which does not meet GINA's criterion because the published wording is more than 10%. 420 and 378 give 10.5%, which does. 420 and 369 give 100 × 51 ÷ 394.5 = 12.9%, just under the paediatric 13%; 420 and 368 give 13.3%, just over
Equal readings — 420 and 420 — give exactly 0%, a perfectly stable day, and the page computes it rather than refusing
Enter a lowest of 450 against a highest of 420 and the page refuses: the two have been swapped. A personal best of 400 below a day's highest of 420 is refused for the same reason
What each body asks for
| Source | Threshold | Period | On the default readings |
|---|---|---|---|
| GINA, 2025 report | average daily diurnal PEF variability of more than 10% in adults, more than 13% in children | two weeks of twice-daily readings | 21.1% — met |
| NICE NG80 | diurnal variability of 20% | not specified in the criterion | 21.1% — met |
| PCRS-UK consensus | significant variability lies in a range between 10% (13% for children) and 20% | an average of daily variabilities | 21.1% — above the range |
One day’s readings, three indices
| Index | Formula | Value | Source |
|---|---|---|---|
| Amplitude percent mean | 100 × (420 − 340) ÷ 380 | 21.1% | PCRS-UK consensus; the form this page computes |
| Amplitude percent highest | 100 × (420 − 340) ÷ 420 | 19.0% | the other form in general circulation |
| Lowest as percent of personal best | 100 × 340 ÷ 480 | 70.8% | Reddel and colleagues, BMJ 1999 |
The weakest measurement here, and what it can still establish
Peak expiratory flow is the only lung function measurement most patients ever make themselves, and that is its entire value: it can be recorded twice a day for a fortnight at home, and variability over time is the one thing a single clinic spirogram cannot show. GINA’s criterion for variable expiratory airflow is an average daily diurnal variability of more than 10% in adults and more than 13% in children, over two weeks of twice-daily readings.
Everything else about it is a weakness, and a page that does not say so is not much use. It is effort-dependent, so a patient who blows harder records a better lung. It is device-dependent: the PCRS-UK consensus warns of a 20% variation from meter to meter, instructs that one meter be used every time, and notes the meters are not calibrated as spirometry is — and they have been sold on more than one scale, so readings from different devices are not interchangeable. It is a large-airway measurement, effectively insensitive to small-airway disease. And the thresholds come from an older asthma literature: Reddel and colleagues argued in 1999 that the index is too time-consuming for routine practice, that recent beta-2 agonist use produces large errors in it even when the flows barely move, and that it may miss important changes altogether.
The index itself is not standardised tightly enough for a threshold to transfer between hands. PCRS divides the day’s amplitude by the average of the highest and lowest readings; a second form in wide circulation divides by the highest. On the readings above those give 21.1% and 19.0% — opposite sides of the 20% NICE asks to be demonstrated, from identical blows. The guidelines disagree too: GINA’s more than 10% against NICE NG80’s 20%, with PCRS publishing the span between them as disputed territory.
So what can it establish? That airflow varies, when it does, and roughly how much — useful corroboration where the history suggests variable obstruction and spirometry on the day was unremarkable. What it cannot do is diagnose: PCRS states that PEF variability “should never be used alone to make a diagnosis”, and low variability over two weeks excludes nothing, because a patient who was well on the days they measured records a flat diary. And a predicted PEF is a reference-equation value like any other: Predicted values, LLNs and z-scores are reference-set dependent — GLI 2012, GLI Global, NHANES III and ECSC disagree for the same person — so use the set your own report names. A lung function pattern is interpreted with the clinical picture and the report’s own reference set, never on its own.
Frequently asked questions
How is peak flow diurnal variability calculated?
The PCRS-UK consensus gives daily variability as the difference between the day’s highest and lowest peak flows divided by the average of the two, and overall variability as the total of the daily variabilities divided by the number of days. A second form in circulation divides by the day’s highest instead, which returns a smaller number for the same readings.
What level of variability is significant?
The guidelines disagree. GINA asks for more than 10% in adults and more than 13% in children, averaged over two weeks of twice-daily readings; NICE NG80 asks for 20%. The PCRS consensus declines to choose and places significant variability in a range between 10%, or 13% for children, and 20%.
Does a normal peak flow diary exclude asthma?
No. The PCRS consensus states that PEF variability should never be used alone to make a diagnosis. A patient on treatment, or well during the fortnight they measured, records little variability; and peak flow is a large-airway measurement that is relatively insensitive to small-airway disease.
Can I compare readings from two different peak flow meters?
Not reliably. PCRS warns of a 20% variation from meter to meter and instructs that one meter be used every time; the meters are not calibrated as spirometers are, and they have been sold on more than one scale. A diary that changed device part way through changed its own baseline.
Is there a better index than diurnal variability?
Reddel and colleagues proposed one in 1999: the lowest morning peak flow as a percentage of the patient’s personal best, on the grounds that diurnal variability is too time-consuming for routine practice, is badly distorted by recent beta-2 agonist use, and may fail to detect important changes. It is the third figure the calculator prints. It is not a guideline threshold.
Related calculators
References
- Primary Care Respiratory Society UK. How to calculate and interpret peak expiratory flow (PEF) readings (PRF2). Solihull: PCRS-UK, 2024.
- Global Initiative for Asthma. Global Strategy for Asthma Management and Prevention: 2025 Report — summary of changes. GINA, 2025.
- Reddel HK, Jenkins C, Woolcock A. Diurnal variability — time to change asthma guidelines? BMJ. 1999;319(7201):45–7.
- Stanojevic S, Kaminsky DA, Miller MR, et al. ERS/ATS technical standard on interpretive strategies for routine lung function tests. Eur Respir J. 2022;60(1):2101499.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/