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The paper by Kamps et al reported that peak expiratory flow (PEF) diaries kept by asthmatic children were unreliable.1 They found that about 25% of readings recorded by an electronic meter were not identical to those written in the diary. The Vitalograph 2110 meter was used for this study with subjects recording the best of three blows on each occasion. However, the 2110 meter does not necessarily record the highest value indicated. Rather, it records the highest value for good quality blows in preference to poor quality blows, even if the poor blow is a higher value. A good quality blow is one in which PEF is achieved between 40 and 290 ms of starting, a poor blow being one in which the time to achieve PEF is outside this window. Thus, the value recorded by an electronic meter is not necessarily the best value as observed by the subject.
Several members of our department staff have reliably kept serial PEF records using the Vitalograph 2110 electronic meter. We found that, even though the observers were “experts”, 6–20% of readings recorded by the electronic meter were different from the maximum value recorded in the written diary. In one instance the value recorded by the meter was 146 l/min lower than the highest value recorded by the observer. In instances where the electronically stored reading was different from the maximum recorded written value, the value recorded by the meter was still among those noted by the observer. Furthermore, as blows are performed in quick succession, some subjects have reported occasional difficulty in recalling the last one or two digits of the best value. Inaccuracies can also arise when the clock of the logging meter shows the wrong time.
Of the 25% or so recordings that were reported as being incorrect in the study by Kamps et al, it is possible that a significant proportion could have genuinely been observed by the subjects but not recorded as such by the meter. It is wise to be as critical of electronically stored data as the traditional hand written record.
authors' reply We thank Dr Anees and colleagues for their valuable comments on our paper. With regard to their first point, we were aware of the fact that the Vitalograph only records good quality PEF manoeuvres. Because of this, we tested the Vitalograph devices before handing them out to the patients in our study and found that 3.6–10.7% of PEF readings were different from the maximum recorded value in the (reliably kept) written diary. In order to minimise this problem we made sure that all patients were carefully instructed on how to perform “good quality” PEF manoeuvres on the Vitalograph. Although Anees et al are right that the technical performance of the Vitalograph may partly explain the incorrect PEF entries, this cannot fully explain the high prevalence of incorrect entries observed in our patients (22–32%).
Moreover, the large number of missing and invented PEF values (20–40%) were certainly not due to the technical characteristics of the Vitalograph, as these PEF values were simply not blown. We therefore feel that our conclusion that peak flow diaries are unreliable remains valid.
Monitoring of PEF with an electronic PEF meter may not only be preferable for excluding missing and invented PEF values, but also because only good quality PEF manoeuvres are recorded.
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