Performance of the ISAAC Questionnaire to Establish the Prevalence of Asthma in Adolescents: A Population-Based Study

Background. The epidemiology of asthma has been investigated with questionnaires, such as the International Study of Asthma and Allergies in Childhood protocol. Aim. To investigate the performance of the questions of the International Study of Asthma and Allergies in Childhood questionnaire to diagnose asthma in adolescents. Methods. This is a population-based cross-sectional study of adolescents in the Syndrome of Obesity and Risk Factors for Cardiovascular Disease study. The validity of the asthma symptoms of the International Study of Asthma and Allergies in Childhood protocol was assessed by calculating sensitivity, specificity, positive and negative posttest probabilities, and Youden's Index, taking as a gold standard the history of a medical diagnosis of asthma. Risk ratios (RRs) and 95% confidence intervals (CIs), adjusting for sex and age, were calculated using Cox regression model. Results. In total, 575 adolescents were investigated. Overall, 28.7% reported a lifetime medical diagnosis of asthma, and 40.0% reported at least one episode of wheezing. Ever wheezing had the highest sensitivity (80.6%) for the diagnosis of asthma, compared with the other ISAAC questions. Adolescents who reported ever wheezing were about 8 times more likely (adjusted RR: 8.3; 95% CI: 4.9-14.2) to have ever had asthma, independent of age and sex. Symptoms within the last 12 months (wheezing, cough without cold or respiratory infection, sleep disturbed due to wheezing, wheezing due to exercise, speech limited due to wheezing) had specificity of 92.0% or higher. Dry cough at night without cold or respiratory infection was the strongest independent predictor of asthma (adjusted RR: 8.8; 95% CI: 6.1-12.7). Conclusions. Ever wheezing is the most sensitive indicator of the diagnosis of asthma but falsely identifies a portion of adolescents as asthmatic. Symptoms of asthma in the last 12 months, such as cough without cold or respiratory infection, are rarely positive in the absence of a lifetime asthma diagnosis. The combination of ever wheezing for screening and the presence of other symptoms within the past 12 months to confirm the diagnosis could be an effective strategy to identify the prevalence of asthma in communities.</.