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

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 Lu ng Disease in Adults to describe PEF-variability (PEFvar) (amplitude as a p ercent of the mean, PEF [i.e., difference between morning and evening value s divided by the mean]) in the study population and in five subgroups (phys ician-diagnosed asthma; current asthma, or physician-diagnosed asthma plus asthma attacks and/or medication; history of wheezing without a cold; hyper reactive; and nonsymptomatic). We assessed the performance of PEF,,, as a p otential tool with which to screen for asthma. Alternatively, subjects with a PEFvar of greater than or equal to 20%, greater than or equal to 30%, an d greater than or equal to 50% on at least 2 d were considered to have high variability. The analyses were conducted for subgroups with different pret est probabilities for asthma-related conditions. The median PEFvar was 4.5% . Among asthmatic subjects, women had nonsignificantly higher PEFvar values than did men. In all other groups, women had significantly lower PEFvar. B oth in the entire population and in subgroups with a higher pretest probabi lity for asthma-related conditions, screening performance of PEF was limite d. A PEFvar of greater than or equal to 20% on at least 2 d detected curren t asthma with a sensitivity of 36% (specificity = 90%; positive predictive value = 16.4%). Results were better among subjects with a history of wheezi ng without colds (sensitivity = 40.4%; specificity = 83.6%; positive predic tive value = 45.2%). PEFvar, a useful measure both clinically and in epidem iology, is of limited value when unselected populations are screened for as thma-related conditions, since the overlap of PEFvar distributions across s ubgroups is large.