Abstract
OBJECTIVES: To compare prediction curves for forced vital capacity (FVC) in children from a large population sample with the standard prediction curves currently in clinical use. A secondary objective was to assess the contribution of weight to the prediction of lung function in preteenagers.DESIGN: The data used for this analysis were from a prospective cohort study of health effects of air pollution on children in 10 Canadian communities, five in Saskatchewan and five in Ontario.PARTICIPANTS: A 96% response rate among a target population of 4935 boys and girls aged seven to 11 years was attained. After exclusion of non-Caucasians and asthmatics there remained 3142 children.ANALYSIS: FVC prediction curves for each sex were compared with the two standard prediction curves most frequently used in children's hospitals in Canada with respect to mean values and the lower limits of normal. A random subsample of 500 subjects was selected and their values were interpreted as normal or abnormal by the three prediction equations. Rates of abnormalcy between prediction methods were compared by the uncertainty coefficient statistic. Regression equations within each sex were examined for the effect of weight in addition to height.MAIN RESULTS: Prediction curves from this large population sample were not statistically different from two widely used prediction curves in the mean or lower limits of normal in boys or girls. In 1575 boys a small but significant effect of weight was demonstrated (r=0.024, P=0.001) after age, height and square of height were added to the model (overall r2=0.725). Similarly, a significant effect of weight was seen in girls (r=0.022, P=0.001, overall r2=0.756).CONCLUSIONS: A large sample of nonsmoking Canadian children offers no further advantage in mean or lower limits of normal over standard predictions based on smaller convenience samples in generating prediction curves for normal FVC. In the youngest children who can perform pulmonary function tests, weight should be taken into account in epidemiological studies of pulmonary function. Although the effect may be small it is of the order of magnitude anticipated due to a number of different pneumotoxins (ie, air pollutants).