Study Objectives.

A recent trend in the treatment of asthma has been the widespread, independent use of peak expiratory flow (PEF). We examined whether PEF monitoring creates inaccuracies in assessment of children with moderate to severe asthma.


We compared the negative predictive value of PEF in relation to the forced expiratory volume in 1 second (FEV1), and to the forced expiratory flow between 25% and 75% of the vital capacity (FEF25–75%) at different levels of air trapping as determined by the residual volume over total lung capacity ratio (RV/TLC).


The study included 244 patients, ages 4 to 18 years with all classes of asthma severity, with FEV1ranging from 28% to 134% of predicted value. We analyzed 367 sets of pulmonary function tests performed throughout a 3-year period. Thirty percent of patients with a normal PEF value had an abnormal FEV1 or FEF25–75%. As air trapping increased, the ability of a normal PEF to predict normal FEV1 and FEF25–75% readings fell from 83% to 53%. The negative predictive value was significantly lower for patients with RV/TLC ratio >30 compared with patients with RV/TLC <30.


The results of this study suggest that it might be possible to identify children for whom the PEF is likely to give false-negative results. As air trapping increases, it causes the PEF to give misleading reassurance of normal pulmonary function. Furthermore, poor predictiveness of PEF is obtained when values 80% of predicted for age are considered normal.

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