Peak Expiratory Flow (PEF)
This is usually used for asthma.
It is the maximum rate of air breathed out as hard as possible through a measuring device called a peak flow meter, (after a full breath taken in). The reading is measured in litres/minute (l/min). Three readings are taken and the best of three is recorded.
A measured peak flow of less than 80% predicted indicates that airways obstruction is present but the diagnosis of asthma cannot be made on a single peak flow measurement as PFR may vary throughout the day as well as on a day-to-day basis, a series of sets of readings should be considered. Also, having a normal one-off peak flow reading does not exclude asthma.
Peak flow measurement is done very commonly, and many patients test their peak flows on a daily basis, having their own peak flow meters. They then may record the reading in a graph form, which then easily highlights any change especially morning and evening. It is a very easy test to do. GPs and Specialist Asthma nurses can do an on-the-spot reading to quickly check a patient’s status.
EU Standard for Peak flow Meters
Background
Studies have identified that existing scales on hand held mini-Peak Flow Meters are accurate in the mid range but at the high and low ends of the range they are less accurate, with a result that asthma may be under-diagnosed and treatment may not be optimal.
As a consequence, a new EU standard has been introduced for mini peak flow meters, which is now in force. Any PF meter purchased or prescribed in the UK will from now on be EU standard compliant.
Peak flow meters, which are well maintained, and in good condition can still, be used, since it is possible to apply an algorithm, which converts old to new readings.
It is important to determine the type of meter with which the reading was taken.
Therefore, when documenting Peak Flow, it should be recorded whether an existing “Wright” or new “EU” meter has been used, like this:
PFR 450l/min (Wright) or PFR 450 (EU)
Examples of comparable peak flow values are:
| WRIGHT (value in litres/minute) |
EU (value in litres/minute) |
|---|---|
| 130 | 117 |
| 150 | 130 |
| 200 | 165 |
| 249 | 203 |
| 250 | 204 |
| 299 | 246 |
| 300 | 247 |
| 350 | 294 |
| 400 | 344 |
| 450 | 399 |
| 500 | 457 |
| 550 | 518 |
| 600 | 584 |
| 650 | 652 |
The average range for an adult for peak flow lies between PFR 450 (EU) and PFR 600 (EU), but because the peak flow depends on age, gender, race and height, the measured result is compared to that predicted for a person’s age, gender, and height, using special charts and the measurement is compared with what would be expected.
Below are some approximate result ranges indicating the level of peak flow reduction.
Male Peak Flow Reading range |
|
|---|---|
Mild peak flow reduction | More than 300 litres per minute (l/min) Wright More than PFR 247 (EU) |
Moderate peak flow reduction | 200 – 299 litres per minute (l/min) Wright PFR 165 (EU) to PFR 246 (EU) |
Severe peak flow reduction | Less than 200 litres per minute (l/min) Wright Less than PFR 165 (EU) |
Female Peak Flow Reading range |
|
|---|---|
Mild peak flow reduction | More than 250 litres per minute (l/min) Wright More than PFR 204 (EU) |
Moderate peak flow reduction | 130 – 249 litres per minute (l/min) Wright PFR 117 (EU) to PFR 203 (EU) |
Severe peak flow reduction | Less than 130 litres per minute (l/min) Wright Less than PFR 117 (EU) |
With the new EU values, a new Nomogram has been provided, which should be used to determine whether the claimant has mild, moderate or severe asthma.
“In men, readings up to100 litres/minute lower than predicted are within normal limits.
For women, the equivalent figure is 85 litres / minute.
Values are derived from Caucasian populations”.
Special Peak Flow Monitoring
The patient is asked to measure their peak flow on a peak flow meter, every morning and evening for 1 to 2 weeks, to measure variability.
A variation between the morning and evening readings of more than 15% is diagnostic of asthma.
Amended June 2008
