Serial peak expiratory flow measurement in the diagnosis of occupational asthma.

Serial measurement of peak expiratory flow (PEF) is the most appropriate initial confirmatory test for occupational asthma suggested by a history of asthmatic symptoms improving on days away from work. PEF recordings need to be taken every 2 h during waking hours, both on days at and away from work. A minimum positive record includes recordings taken over two work periods, and two rest periods, and a minimum negative record will also include a rest period of at least 10 days duration. Exposures at work, medication, and length of time after waking for the first recording should be kept constant. Records may be interpreted by plotting the recordings (usually as maximum, mean and minimum against time, with appropriate shading of days including work). Analysis of cycles and rhythms within the record may also reveal an effect of work. Statistical methods can also be used to detect differences between recordings at and away from work. At present, expert visual assessment of plotted records appears to be the most specific and sensitive method of interpretation. It does require experience, as it involves qualitative judgements. Work is currently being carried out to quantify these judgements. A number of factors may interfere with interpretation by any method. These include; intermittent exposure to suspect agents at work; irregular medication; irregular timing of the first reading after waking; respiratory tract infection; and variable patient peak expiratory flow measurement technique.