Peak flow variability in the SAPALDIA study and its validity in screening for asthma-related conditions. The SPALDIA Team.

We used 3-wk peak expiratory flow (PEF) measurements (twice daily) made in the diary study of the population-based Swiss Study on Air Pollution and Lung Disease in Adults to describe PEF-variability (PEF(var)) (amplitude as a percent of the mean, PEF [i.e., difference between morning and evening values divided by the mean]) in the study population and in five subgroups (physician-diagnosed asthma; current asthma, or physician-diagnosed asthma plus asthma attacks and/or medication; history of wheezing without a cold; hyperreactive; and nonsymptomatic). We assessed the performance of PEF(var) as a potential tool with which to screen for asthma. Alternatively, subjects with a PEF(var) of >/= 20%, >/= 30%, and >/= 50% on at least 2 d were considered to have high variability. The analyses were conducted for subgroups with different pretest probabilities for asthma-related conditions. The median PEF(var) was 4.5%. Among asthmatic subjects, women had nonsignificantly higher PEF(var) values than did men. In all other groups, women had significantly lower PEF(var). Both in the entire population and in subgroups with a higher pretest probability for asthma-related conditions, screening performance of PEF was limited. A PEF(var) of >/= 20% on at least 2 d detected current asthma with a sensitivity of 36% (specificity = 90%; positive predictive value = 16.4%). Results were better among subjects with a history of wheezing without colds (sensitivity = 40.4%; specificity = 83.6%; positive predictive value = 45.2%). PEF(var), a useful measure both clinically and in epidemiology, is of limited value when unselected populations are screened for asthma-related conditions, since the overlap of PEF(var) distributions across subgroups is large.