Correspondence Variation

of peak expiratory flow rate Sir,-In their article, Hetzel and Clark ' suggest an amplitude of 20 % in a 24-hour variation of peak ex-piratory flow rate should be regarded as a useful screening test for asthma. While I entirely sympathise with the view that the measurement of amplitude variation is one of the best ways of demonstrating variability of airway obstruction, this definition cer-tainly includes many patients who would be regarded as bronchitics without any evidence of asthma. In my paper,2 it was demonstrated that the mean amplitude of variation of bronchitis, measured in a different way, was well over 20 % before and over 20 % after bronchodilator. As it could be argued that variation of peak flow rate, at any rate in quite normal subjects, is principally a deviation below a norm, rather than a deviation about a mean, it seems logical to measure amplitude against the norm, rather than the mean. I appreciate that this does not fit the mathematical model of Clark and Hetzel as well, but it could be argued that it does reflect the pathological process more accurately. Calculation of the amplitude against the norm, that is the maximum recorded peak flow rate, results in a lower amplitude than calculation against the mean, as in my papers the calculations of amplitude were made against the minimum achieved. I have therefore made an approximate recalculation of the data on 508 recordings on approximately 400 patients in the study referred to above and another to be published.3 The patients are divided into asthmatics, bronchitics, and patients with features of both conditions. It will be seen (table) that even in pure bronchitics the mean amplitude is above 30% before bronchodilator and remains at approximately 25 % when measured after bronchodilator. Table Amplitude of daytime variation of peak expiratory flow rate measured in asthmatics, bronchitics, and in an intermediate group before and after bronchodilators Before Five minutes Two hours after bronchodilator after bronchodilator bronchodilator n mean n mean n mea