Aim: A questionnaire has been used repeatedly in cross-sectional studies to determine the prevalence of asthma, allergic rhinoconjunctivitis (AR) and eczema among schoolchildren in Nordland County, Norway. The current study was designed to validate the questionnaire against clinical assessment as the diagnostic gold standard and to investigate the extent of possible misclassification.
Methods: A subsample of 801 schoolchildren of 4150, whose parents had answered a questionnaire covering asthma and atopic diseases, underwent a detailed clinical evaluation including a standardized interview, a clinical examination, skin prick tests (SPT), blood samples, spirometry an exercise treadmill test (EIB test) and measurement of exhaled nitrogen oxide (FeNO).
Results: The questionnaire had a sensitivity of 0.96 and a specificity of 0.87 for the diagnosis of asthma ever compared to clinical assessment. The overall agreement (kappa) was 0.80. After clinical assessment the prevalence of asthma ever was adjusted from 17.6 % to 16.9 % (95% CI: 15.8-18.0). The most sensitive and specific questions in identifying asthmatic children by the questionnaire were questions asking about diagnosis ('Has the child ever had asthma?') rather than those covering asthma symptoms such as wheeze, shortness of breath and/or cough. A positive exercise test increased the post-test probability for the asthma diagnosis only to a minimal degree.
Conclusion: Based on the good agreement between the questionnaire responses and the clinical assessments, it is concluded that the questionnaire had good validity and served as a useful epidemiological tool. Detailed clinical testing added little additional information.
Keywords: Atopy; cross-sectional; epidemiology; instrument validation; reliability.