Serial Peak Flow monitoring has been used as a tool for asthma diagnosis for many years. The aims of this diagnostic tests is to identify objective evidence of asthma; objective evidence of asthma on Serial PEF monitoring is ‘dips’ in PEF on three consecutive days. A ‘dip’ in PEF is a reduction in PEF over a 24-hour period of more than 20%. PEF is also a useful tool in patients who have poor perception of symptoms because it detects deterioration in airway function. However, changes in PEF may not accurately reflect changes in airway function as assessed by more reliable measurements such as forced expiratory volume in one second (FEV1) or forced vital capacity (FVC). In addition, compliance with daily PEF measurements has been shown to be low and decreases with time.

Here is an example of variable PEF measurements, taken on a patient who performed a PEF measurement four times per day for two weeks. As you can see, this patient had a consistently low PEF measurement in the early morning (6am), which appears to rise sharply at lunchtime (12pm), and deteriorate as the day progresses (6pm and 10pm). This is a typical diurnal variation seen in a patient with uncontrolled asthma.