A peak flow meter is a very convenient handheld device that measures the flow of air as it is exhaled. It consists of a plastic tube with a mouthpiece at one end, with a pointer inside which can slide along a scale. When air is blown into the tube, the pointer slides up the scale and indicates the speed and volume of the air exhaled - the peak flow.

Peak flow meters cannot be used to detect asthma, but they can indicate whether the asthma is under control or not. When asthma flares up, inflammation of the air passages causes the airways to constrict. This leads to a smaller than normal amount of air being exhaled. By checking their peak flow, asthma patients can detect whether their medication is working, whether an attack is imminent, or understand what makes their asthma worse.

Each person’s flow rate is different; there is no universal optimum flow rate. Asthma patients use the peak flow meter to measure their ‘personal best’ flow rate, the flow rate they can achieve when their symptoms are fully under control and their inflammation is at a minimum. They then keep track of their condition by comparing all subsequent peak flows to their personal best.