EDITOR, - Peter D Sly and colleagues' paper highlights a common mistake in the management of asthma - namely, interchanging devices measuring peak flow.1 In 1990 colleagues and I designed a small study (unpublished) to investigate possible discrepancies between readings obtained with Wright mini peak flow meters and standard Wright peak flow meters. It was undertaken because of the lack of consensus in the literature regarding the comparability of the two devices and after a suggestion that regression analyses commonly used in previous research were inappropriate for comparing the results of two measurement techniques. In addition, earlier comparisons had looked primarily at flow ranges appropriate to adults.

Peak flows were recorded from 186 children, who used both devices. In the laboratory 61 Wright mini peak flow meters were calibrated against known flows with a spirometer. Limits of agreement were then calculated for clinical and laboratory data and showed that the two devices may differ by more than 100 litres/min. The greatest percentage discrepancy occurred at lower flow rates, when the Wright mini peak flow meter tended to read higher flow values than the standard Wright peak flow meter.

As the two meters do not give comparable peak flow measurements …