Work out peak flow as a percentage of best or predicted, and variability over a series of readings.
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| % of best or predicted | Severity (BTS/SIGN) |
|---|---|
| Above 75% | Not in the acute range |
| 50–75% | Moderate acute asthma |
| 33–50% | Acute severe asthma |
| Below 33% | Life-threatening asthma |
Variability is the highest minus the lowest reading, as a percentage of the mean. Above 20% supports a diagnosis of asthma (NICE NG245).
Personal best is better than predicted once known. Predicted values come from EU-scale charts by age, height and sex.
It depends on age, height and sex. Use a predicted chart or, better, the person’s own best.
More than 20% variability supports an asthma diagnosis.
Below 33% of best or predicted. Call 999.
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