PEF is a simple, objective procedure that can be used to measure the degree of air flow limitation. Although it may not give a full representation of lung function, it can monitor efficacy of treatment and progression of a condition (Cavill and Kerr [36]).

Treatment is often based on PEF measurements and so it is vital that these are as accurate as possible (Wilkinson et al. [215]). Patients should be advised to perform PEF measurements in accordance with their monitoring regimen even if symptom free (NICE [135]) as trends can be more important than isolated results (unless the isolated results reflect an exacerbation) (Dakin et al. [50]). Patients should repeat the procedure three times with the best result of the three being documented (Chapman et al. [38]). Unless the procedure induces an exacerbation, there should be consistency between the three results; if the top two results have a greater disparity than 40 L/min then, as long as the patient is not fatigued, a further two attempts can be made to reach a greater level of consistency (Cavill and Kerr [36]).

While an isolated PEF measurement can indicate a restriction in air flow, in general sequential measurements are of more value, as they display trends, which are essential to understanding the severity and progression of the disease (Hill and Winter [82]). However, there may be significant diurnal variations, with higher values obtained in the evenings and the lowest measurements occurring during the night and first thing in the morning (Dakin et al. [50]). Therefore, it is recommended that measurements should be taken and documented on waking, in the afternoon and prior to going to bed (Frew et al. [65]). If it is suspected that the patient may have restricted air flow due to occupational causes, the patient should take measurements for a minimum of 2 weeks while at work and 2 weeks while not at work to enable a comparison (Frew et al. [65]).