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OBJECTIVE: To assess the ability of simple de®nitions of BMI to successfully screen for children with high body fatness. DESIGN: We determined the sensitivity and speci®city of the body mass index (BMI) by testing its ability to correctly identify children with high body fat percentage. Receiver operator characteristic (ROC) analyses were carried out using the top 5% of body fat percentage to de®ne children as obese (true positives). SUBJECTS: Representative sample of 4175 7 y-old (88 ± 92 month-old) children (2120 boys; 2055 girls) participating in the Avon Longitudinal Study of Pregnancy and Childhood (ALSPAC). RESULTS: The current obesity de®nition based on BMI (95th centile) had moderately high sensitivity (88%) and high speci®city (94%). Sensitivity and speci®city did not differ signi®cantly between boys and girls. The ROC analysis showed that lower cut-offs applied to the BMI improved sensitivity with no marked loss of speci®city: the optimum combination of sensitivity (92%) and speci®city (92%) was at a BMI cut-off equivalent to the 92nd centile. Sensitivity of BMI using the new International Obesity Task Force (IOTF) cut-off for obesity was much lower, and differed signi®cantly (P`0.001) between boys (46%) and girls (72%). CONCLUSIONS: Screening for childhood obesity using the BMI is speci®c, and can have moderately high sensitivity if an appropriate cut-off is chosen. New recommendations based on the IOTF approach to de®ning childhood obesity are associated with lower sensitivity, and sensitivity differs between boys and girls.

Previous studies have raised the possibility that the measurement of nitric oxide (NO.) concentrations in expired air may represent a noninvasive measure of lower airway inflammation. To address the question of whether the elevated NO. recovered in mixed expired air from asthmatic subjects is a reflection of the pulmonary airway microenvironment or merely nasopharyngeal contamination, mixed expired NO. determinations were performed in five normal and five asthmatic subjects before and after orotracheal intubation (thereby isolating the lower airway gas from ambient air contamination or gas conditioned in the nasopharynx). The mixed expired NO. concentrations determined in patients with asthma were significantly elevated (p < 0.05 or less) above those of normal subjects in both the pre- and postintubation samples. After intubation, mixed expired NO. levels were 4.7 +/- 1.3 ppb and 13.2 +/- 2.0 ppb in normal and asthmatic individuals, respectively; the difference in these values was statistically significant (p < 0.01). Lower airway gas, sampled through the bronchoscope during a breathhold, was found to contain NO. concentrations of 7.0 +/- 1.2 ppb and 40.5 +/- 5.6 ppb at the tracheal carina of normal and asthmatic individuals, respectively. The asthmatic values were significantly (p < 0.01) elevated above those found in normal subjects. These findings indicate that the difference in mixed expired NO. of normal subjects and asthmatics reflects a difference in NO. concentration present in the lower airway.