Martin Wright and colleagues pioneered the measurement of peak expiratory flow in the late 1950s and produced small, mechanically based, hand held meters.^1,2 The use of this measure of lung performance has recently come to the fore with the promotion of self management plans for asthma that base advice about treatment on changes in peak flow.*RF 3-5* This has focused attention on the meters' performance, and concern now exists about their accuracy.

The peak flow meters added to Britain's drug tariff in 1990 are robust and give consistent readings. The first evidence questioning their accuracy was presented to the British Thoracic Society more than three years ago,6 and these findings have subsequently been confirmed.^7,8 Although the meters give reasonably accurate readings at low flows, they may overread by about 701/min in the middle of the range and underread by about 50 l/min in the high range. Some people may deem that an error of 15% is acceptable for such low cost meters, but the problem is that the inaccuracy varies across the range of the meters, so that assessing …