This measures how much a person's peak flow readings swing between their best and worst values, since large swings are a sign of poorly controlled asthma even when a single reading looks acceptable.
Low variability — stable airway.
Elevated — suggests instability.
High — poorly controlled asthma.
Wide day-to-day swings between high and low readings mark unstable asthma.
Asthma is defined partly by variable airflow obstruction, so the swing between readings captures the disease's episodic nature in a way a single measurement can't.
Day-to-day variability greater than roughly 10–20% suggests poorly controlled or unstable asthma, even if individual readings look acceptable.
Take the highest and lowest peak flow over the period, subtract to get the swing, and divide by the mean of the two, expressed as a percentage.
Typically twice daily (morning and evening) over one to two weeks, since the morning dip is often the lowest point of the cycle.