How do you interpret a peak expiratory flow (PEF) test?

From the Guidelines

To interpret a peak expiratory flow (PEF) test, compare the measured value to normative data for sex, age, and height, and consider a 20% increase and absolute improvement of 60l/min after bronchodilator administration as suggestive of asthma. When performing the test, the individual should stand and blow as hard as possible through a disposable filtered mouthpiece into a peak flow meter, taking the best of three attempts 1. The test can be repeated 15 minutes after administration of a bronchodilator such as salbutamol to assess reversibility.

Key Considerations

• The test should be performed with the individual standing and blowing as hard as possible through a disposable filtered mouthpiece into a peak flow meter.
• The best of three attempts should be taken and compared with charts of normative data for sex, age, and height 1.
• A 20% increase and absolute improvement of 60l/min after bronchodilator administration is suggestive of asthma, although diagnosis cannot be made on a single test 1.
• Alternatively, an exercise test such as running for 10-15 minutes followed by peak flow measurements every 5 minutes can also be used to assess for asthma 1.

Monitoring and Recording

• Patients can be prescribed a peak flow meter and chart to record their daily measurements, ideally performed at the same time morning and evening 1.
• Average daily diurnal variation can be calculated as the day’s highest PEF minus the day’s lowest PEF, divided by the mean PEF for the day and then averaged over the course of a week 1.
• A variability of >10% in adults and >13% in children is suggestive of asthma 1.

From the Research

Interpreting Peak Expiratory Flow (PEF) Test

To interpret a PEF test, several factors need to be considered, including the patient's personal best PEF, predicted PEF, and the severity of asthma exacerbation.

• The 1995 Global Initiative for Asthma and 1997 National Asthma Education and Prevention Program guidelines recommend using a patient's "personal best" PEF, if available, as a better measurement than the predicted PEF obtained from population-based nomograms 2.
• However, studies have shown that patients may not always be able to report their personal best PEF accurately, which can lead to inappropriate discharge from the emergency department 2.
• The best PEF may be a better index for the management of patients with asthma, as it can provide a more accurate measurement of pulmonary function than the predicted PEF 3.
• A peak expiratory flow of <80% predicted was found to be the best cut-off to detect airflow limitation, with a sensitivity of 90% and specificity of 50% 4.

Factors Affecting PEF Measurements

Several factors can affect PEF measurements, including the type of peak flow meter used and the patient's respiratory symptoms.

• A study comparing four types of portable peak flow meters found that the correlation coefficients between the readings of each meter and spirometric PEFR did not differ significantly from each other, but the limits of agreement between each meter were very wide 5.
• Another study evaluating 11 peak flow meters found substantial differences in the quality of the adult meters, with the Personal Best and Micro Medical meters having the closest agreement to the pneumotachograph 6.
• Respiratory symptoms, such as breathlessness and cough, can also be important in detecting obstructive airways disease, and a simple questionnaire and peak flow meter measurements can be effectively used in clinical practice for objective detection of asthma and chronic obstructive pulmonary disease 4.

Clinical Applications

In clinical practice, PEF tests can be used to diagnose and manage asthma and chronic obstructive pulmonary disease.

• A peak flow meter with a questionnaire and mini-spirometer can be used as an alternative tool to spirometry for screening of asthma and chronic obstructive pulmonary disease 4.
• The best PEF may be a better index for the management of patients with asthma, as it can provide a more accurate measurement of pulmonary function than the predicted PEF 3.
• A simple questionnaire and peak flow meter measurements can help doctors differentiate between asthma and chronic lung disease, especially in clinical settings where access to specialist equipment and knowledge is limited 4.