Peak expiratory flow measures the maximum rate of air blown out of the lungs on expiration. A small portable hand-held device is used with a mouthpiece. The measurement obtained indicates the degree of airway narrowing and obstruction. It is used in patients with asthma to aid diagnosis and monitor the effectiveness of treatment, and determine if the asthma is worsening.

To aid diagnosis, patients may be asked to keep a diary of peak flow measurements twice daily for a week. If the airways are narrowed, the speed at which the air leaves the lungs is reduced, therefore the peak flow would be lower than expected. This is typically in the morning or if the patient is unwell.

Regular peak flow readings are used in addition to a review of asthma symptoms, and they are used as a self-management tool. Readings are taken before and after treatment to open up the airways to monitor its effectiveness. Peak flow readings should improve if the treatment plan is working as the airways are less constricted.

• The child and parents should be given information about the procedure and that it may induce coughing or wheezing.
  
• The marker on the peak flow meter should be reset to zero.
  
• The child should be standing upright.
  
• The child should take a deep breath, seal the lips around the mouthpiece and blow out as hard and as fast as possible.
  
• The marker reading should be noted on the meter.
  
• Reset the marker and repeat this up to three times. The highest of the three values should be recorded.

The correct technique must be observed in order to reduce misleading results. The accuracy of peak flow can be affected by:

• the behaviour of children who have never performed a peak flow before or younger children who are unable to follow the directions of use (usually under 5 years of age);
  
• a poor seal around the mouthpiece or the tongue blocking the mouthpiece;
  
• the child not blowing as hard as possible;
  
• the use of different peak flow meters.

Peak flow is recorded in litres per minute (L/min). It is routinely documented on a graph which predicts normal values based upon a child’s sex, height and age.

The calculation for children below 15 years of age is: