A 10 year follow up of 180 adults with bronchial asthma: factors important for the decline in lung function.

The interesting study of Dr C S Ulrick and others reported (January 1992;47:14-8) in Thorax provides some food for thought. The findings suggest that the decline in ventilatory function is larger in patients with intrinsic than in those with extrinsic asthma. In addition, the authors suggest that the annual decline in FEVy should be 29 ml for men and 25 ml for women; they base this assumption on predictions for FEVy.' The latter commonly derive from cross sectional studies; the age related findings in cross sectional data need not be representative of longitudinal findings-for example, due to cohort effects. Indeed, longitudinal observations on the same adult subjects show a smaller decline in FEV1 than would be predicted from cross sectional findings2'; they also show that the decline in FEV, is negligible in the second and third decade, increasing gradually with age thereafter. Arising from this, I am concerned about the fact that the subjects with intrinsic asthma, who showed the greatest decline in FEVy in 10 years, had a median age of 43-5 years as compared with 310 years in the group with extrinsic asthma. Hence it is to be expected that the former group would have a greater annual decline in FEV, than the latter, and the findings of the study may be related more to age than to the diagnostic label. Another concern relates to the expression ofreversibility ofairflow limitation, a problem thatmay lead to quite different interpretations of test results.' There is evidence to show that the bronchodilator response should be expressed in absolute rather than in relative terms,' so that a small denominator may artificially inflate test statistics. In absolute terms the median improvements in FEV, in intrinsic and extrinsic asthma are not as different at the start of the study as the percentage improvement; therefore I am concerned that the correlation between initial "reversibility" and subsequent decline may be a spurious one. P H QUANJER Physiology Department, Leiden University, PO Box 9604, 2300 RC Leiden, The Netherlands