Effect of the number of peak expiratory flow readings per day on the estimation of diurnal variation

Background-The number of peak expiratory flow (PEF) readings required per day to assess diurnal variation accurately is not known; published studies have used between two and seven PEF readings per day. This study compares the diurnal variation calculated using 2-10 PEF readings per day, Methods-All days with 10 readings were selected from a database of PEF records. For each day, diurnal variations calculated using 2-9 of the readings available were compared with that calculated using the full 10 PEF readings. Diurnal variation calculated using all 10 readings was taken as the true diurnal variation, When less than 10 readings were used the readings were evenly spaced over waking hours. Diurnal variation was calculated as maximum - minimum/predicted. Results-Two hundred and 25 days with 10 readings per day were selected from PEF records provided by 63 individuals. When only two PEF readings per day were used, the limits of agreement suggested a possible underestimate of true diurnal variation, calculated using all 10 readings, of 1.23-15.10%. The possible underestimate fell to 0.27-3.96% when calculated using four evenly spaced readings. Analysis of the timing of the highest PEF reading of the day was undertaken for rest and work days, This showed a mean (SD) timing of 13:56 (4:56 hours) for rest days and 11:47 (5:59 hours) for work days, Conclusions-Clinically significant underestimates of true diurnal variation may be seen when only small numbers of PEF readings per day are used in its calculation, At and above four readings the results suggest that the underestimate becomes increasingly insignificant in terms of the diagnosis and treatment of asthma, Analysis of the timing of the highest PEF reading of the day showed a wide variation, precluding the ability to capture the true diurnal variation with just two or three carefully timed PEF readings per day. The authors suggests that at least four readings per day should be performed, evenly spaced during waking hours, to obtain an accurate assessment of diurnal variation in PEF.