Secular trend in the occurrence of asthma among children and young adults: critical appraisal of repeated cross sectional surveys

OBJECTIVES: To review repeated surveys of the rising prevalence of obstructive lung disease among children and young adults and determine whether systematic biases may explain the observed trends. DESIGN: Review of published reports of repeated cross sectional surveys of asthma and wheezing among children and young adults. The repeated surveys used the same sampling frame, the same definition of outcome variables, and equivalent data collection methods. SETTING: Repeated surveys conducted anywhere in the world. SUBJECTS: All repeated surveys whose last set of results were published in 1983 or later. MAIN OUTCOME MEASURES: Lifetime and current prevalences of asthma and current prevalence of wheezing. The absolute increase (yearly percentage) in the prevalences of asthma and wheezing was calculated and compared between studies. RESULTS: 16 repeated surveys fulfilled the inclusion criteria. 12 reported increases in the current prevalence of asthma (from 0.09% to 0.97% a year) and eight reported increases in the current prevalence of wheezing (from 0.14% to 1.24% a year). Changes in labelling are likely to have occurred for the reporting of asthma, and information biases may have occurred for the reporting of wheezing. Only one study reported an increase in an objective measurement. CONCLUSIONS: The evidence for increased prevalences of asthma and wheezing is weak because the measures used are susceptible to systematic errors. Until repeated surveys incorporating more objective data are available no firm conclusions about increases in obstructive lung disease among children and young adults can be drawn.