PEF reflects a range of physiological characteristics of the lungs, airways and neuromusculature; the most common disorders that affect PEF are those that increase the resistance to air flow in the large conducting intrathoracic airways, such as asthma (NICE [135], West and Luks [210]). However, PEF may also be impaired by:

PEF readings are subject to individual variation depending on the patient's age, sex, ethnic origin and stature (British Thoracic Society [30]). Therefore, the patient's results should be compared against normal reference values for people of the same age, sex and height (Figure 14.35) and, more importantly, against previous results for that individual (Dakin et al. [50], NICE [137]).

Measuring PEF is similar to measuring forced expired volume in 1 second (FEV₁) but the two are not interchangeable and each measure different aspects of lung function (Cavill and Kerr [36]). FEV₁ measures the volume of air exhaled during the first second of forced vital capacity (FVC), which occurs when an individual exhales forcefully to their maximum capacity following a deep inspiration (Marieb and Hoehn [110]). FEV₁ is usually 80% of FVC in healthy participants and is felt to be more sensitive than PEF in detecting mild airway obstruction as PEF is effort dependent and so can have a greater degree of intra‐subject variability (Marieb and Hoehn [110]). However, FEV₁ and PEF can have similar predictive ability in relation to mortality in patients with chronic obstructive pulmonary disease, although FEV₁ is considered to be a better predictor in patients with asthma (Chapman et al. [38], NICE [137]).