Effort dependent, compared with predicted or personal best
Expansion
Technique: standing if possible, marker at zero, a full deep breath, a tight seal, then a short sharp blast. Take the best of three attempts. It is entirely effort dependent, so a low value may reflect poor technique, weakness or pain rather than airway narrowing.
What it reflects: flow in the large airways, and it is therefore insensitive to small airways disease and cannot distinguish obstruction from restriction. Spirometry is required for diagnosis; peak flow is for monitoring.
Interpretation
- Compare with the predicted value for age, sex and height, or better with the patient’s personal best
- Diurnal variation: normally under 10 per cent, and over 20 per cent supports asthma. It is characteristically lowest in the early morning, the morning dip
- Reversibility: an improvement of 12 per cent and 200 ml in FEV1 after a bronchodilator supports asthma; on peak flow an improvement of 20 per cent or 60 l/min
- Occupational asthma: serial readings at work and away, with improvement on rest days and holidays, which requires at least two weeks of readings including time away
Acute asthma thresholds: 50 to 75 per cent is moderate, 33 to 50 per cent is acute severe, and under 33 per cent is life threatening.
Limitations: it is unhelpful in a patient too breathless to perform it, it can be normal between attacks, and it correlates poorly with FEV1 in chronic obstructive pulmonary disease. A normal peak flow does not exclude asthma, which is why serial measurement and challenge testing exist.