Our asthma research program involves both exposure assessment and prevention components, and focuses on the relationship between exposure to common indoor allergens and asthma prevalence and morbidity. We have worked with investigators at the CDC/NCHS to implement the allergen and asthma component of National Health and Nutrition Examination Survey (NHANES) 2005-2006. We collected bedroom dust, measured serum total and allergen-specific IgE and assessed asthma and allergy prevalence and morbidity in approximately 9000 individuals in the U.S. Analysis of this large dataset will allow us to 1) estimate nationwide prevalence of indoor allergen and endotoxin exposures, 2) estimate nationwide prevalence of allergic sensitization to indoor, outdoor and food allergens, 3) estimate nationwide prevalence of allergic diseases, including asthma, and 4) investigate the complex relationships between allergen and endotoxin exposures, allergic sensitization and allergic diseases. The NHANES 2005-2006 is the first study that enabled both qualitative and a quantitative examination of population-level associations between allergic conditions and sensitization. Among the participants 6 years and older, prevalence of diagnosed hay fever was 11.3%, and 6.6% reported current hay fever. One-third of the participants (33.9%) had diagnosed allergies, and 23.5% suffered from current allergies. The prevalence of doctor-diagnosed current asthma was 8.8%. Presence of plant-, pet-, mold-specific IgEs were most consistently associated with allergy-related outcomes. Although levels of total IgE may overlap between non-allergic and allergic individuals, increase in total IgE appeared to have a small, but independent effect on allergy-related outcomes. Total IgE levels predicted asthma only among allergic subjects, but not among non-allergic subjects. The highest levels of serum markers of IgE-mediated allergy, both total and specific IgEs, were found in asthmatic individuals. Among allergic subjects, the association between total IgE level and asthma became stronger as the number of positive specific IgE test results increased. However, NHANES 2005-2006 data demonstrated that a significant portion of asthma is independent from serum IgE levels, either total or specific. Although standard written questionnaires have been the principal instrument for measuring allergic disease and symptom prevalence in large epidemiological studies, relying on self-reported data may introduce bias. Using the NHANES 2005-2006 data, we evaluated the predictive value of questionnaires to assess atopy (i.e., presence of serum specific IgE) in a nationally representative sample of the U.S. population. We focused on five potentially allergic conditions (hay fever, allergy, eczema, itchy rash, and rhinitis) and recent symptoms. For hay fever, allergy, and eczema, the question required a doctor-diagnosis to be considered positive. Given that it was unlikely any single questionnaire item could capture all individuals with atopy, we generated combinations of variables and assessed their ability to predict atopy. Questionnaire responses were assessed for sensitivity, specificity, and positive and negative predictive values for clinical allergy. Of the NHANES 2005-2006 participants who had specific IgE data available (N=8,334), 44% were atopic and 53% reported one or more allergic conditions. Discordance between clinical allergy and allergic conditions was considerable;37% of atopic individuals reported no allergic condition and 48% who reported an allergic condition were not atopic. Therefore, no combination of self-reported allergic conditions resulted in both high sensitivity and high specificity for IgE. The positive predictive value of reported allergic conditions for atopy ranged from 50% for eczema to 72% for hay fever, while the negative predictive value ranged from 57% for eczema to 65% for any condition. Although questionnaires are often the best, and sometimes the only way to collect medical information on a large sample, their limitations need to be taken into account. To achieve a better understanding of the role that environmental exposures play in allergic sensitization and in the development of allergic diseases, we continue to investigate the complex relationships between allergen exposure, allergic sensitization, and disease in more detail. In addition to providing data on these relationships, the NHANES 2005-2006 will establish a second point-in-time estimate for evaluating allergen and endotoxin exposure trends in U.S. homes, first being established in the National Survey of Lead and Allergens in Housing, which we completed in collaboration with the Department of Housing and Urban Development.
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