🔬 Peak Flow

A simple, portable test that measures the maximum speed of expiration (Peak Expiratory Flow Rate) to assess large airway patency and variability.

Indications include the initial diagnosis of asthma when spirometry is unavailable or results are inconclusive, specifically by looking for diurnal variation. It is used for the ongoing monitoring of patients with known asthma to assess control and identify triggers. It is also a critical component in the assessment and monitoring of patients presenting with acute exacerbations of asthma or COPD.

The patient stands or sits upright, takes a deep breath in to full inspiration, and then performs a short, sharp 'huff' or blow into a peak flow meter (Wright's scale or EU scale). The patient must ensure a tight seal around the mouthpiece and that their tongue does not obstruct the opening. The procedure is performed three times, and the highest (best) of the three readings is recorded as the Peak Expiratory Flow Rate in L/min.

A normal PEF value is determined by the patient's age, sex, and height according to standardised nomograms (e.g., the EU/EN13826 scale). In healthy individuals, there should be minimal diurnal variation (typically <10%) and the values should be consistent with the predicted mean for their demographic.

Peak flow values are compared against the patient's 'personal best' or, if unknown, against 'predicted values' based on age, height, and sex (using the EU scale). Diurnal variability is calculated by taking the difference between the highest and lowest daily readings over a week, divided by the mean. Improvement following a trial of treatment (e.g., 2 weeks of steroids) can confirm an asthma diagnosis. Best of three readings is always recorded.

A low PEFR suggests airway obstruction. Significant variability (more than 20% diurnal variation over two weeks) or a significant increase in PEFR after using a bronchodilator or a trial of corticosteroids is highly suggestive of asthma. In acute asthma, PEFR values are used to grade severity: 'Moderate' (50-75% best), 'Acute Severe' (33-50% best), or 'Life-threatening' (<33% best).

Peak flow is a practical, low-cost tool primarily used in the management and diagnosis of asthma in the UK. It is invaluable for patient self-monitored 'Asthma Action Plans', helping patients recognise when to escalate treatment. In emergency departments, it is the primary metric for assessing the severity of an acute asthma exacerbation and determining the requirement for hospital admission or discharge safety.

The most common pitfall is poor patient technique, such as not taking a full breath or 'spitting' into the meter. Using an outdated 'Wright' scale meter without converting to the modern 'EU' scale can lead to inaccurate comparisons. Clinicians must ensure the patient is not blocking the sliding marker with their fingers during the blow.

PEFR is highly effort-dependent and primarily reflects large airway function, making it less sensitive than spirometry for detecting small airway disease or COPD. It cannot distinguish between obstructive and restrictive defects. A normal peak flow does not exclude asthma, especially in patients who are not currently symptomatic. It is unsuitable for very young children (usually <5 years) who cannot coordinate the manoeuvre.

Students must know how to use PEFR to grade asthma severity (Moderate, Severe, Life-threatening). Understanding the diagnostic threshold for 'significant variability' (>20%) is key for primary care scenarios. PEFR charts are commonly used in OSCEs to assess diagnostic reasoning.