The optimum duration of PEFRs has not been established, although a duration of several weeks is customary. A prolonged period of testing is important as it may take several days or longer for workplace exposures to affect PEFR or for recovery to occur away from exposure.

A recording period of 4 weeks, including a period of at least 1 week away from work, seems to be the minimum time necessary to reliably identify changes due to work (with optimally at least 2 weeks at work and > 2 weeks off work). It may take repeated recording episodes to capture relevant exposures and changes in PEFRs. The absence of clear work-related changes does not exclude WRA (ie, sensitizer-induced OA or WEA). Evaluating the results

Stability of Underlying Asthma and Use of Medication: The best time to identify changes in PEFRs from workplace exposures is when the patient with underlying asthma is as stable as possible. The use of long-acting bronchodilators or inhaled steroids may mask work-related changes, and a temporary switch to short-acting bronchodilators as needed or a reduced inhaled steroid dose may be required if PEFR monitoring is negative.

Ideally, medication should remain unchanged throughout the recording period except for rescue medication, the use of which should be recorded. Intercurrent chest infections or exposure to asthma triggers away from work should be similarly recorded. Variable shift work may also cause problems in interpretation because of changes in the timing of diurnal rhythms of lung function.

Interpretation of Serial PEFRs: No single universally accepted technique for evaluating the results of serial PEFRs has emerged. Usually, the best of triplicate recordings made at each time point is taken as the value for that time. The results can be plotted (Fig 5) then visually interpreted to determine whether there is a pattern of worsened PEFRs during work weeks compared with days or weeks off work; when undertaken by “experts,” there is relatively good agreement with the SIC in diagnosing sensitizer-induced OA.