World health survey (WHS)
A stratified probability sampling design, where the sampling frame covered 100% of the country's eligible adults ≥ 18 years of age was used in each of the countries. The sample was stratified by sex, age and urban/rural living strata [7]. A multistage cluster design was used in all countries except Australia, China, Comoros, Congo, Cote d'Ivoire, Croatia, India, and Russia, where post-stratification probability weight were not available (simple weights were used), and Austria, Belgium, Denmark, Germany, Guatemala, Netherlands, Slovenia, UK, and Zambia where no probability weights were available.
Each respective Ministry of Health of the 70 member states who elected to participate in the WHS was responsible for designing the local sampling strategy and administering the standardized questionnaires. The sample size for each participating country ranged from 1,000 to 10,000 participants; each country chose a sample size based on their needs, amount of detail required and feasibility/survey costs. Each of the survey modules were pilot tested in 12 countries. Translated surveys were administered by trained personnel either face-to-face or by telephone (Australia, Israel, Luxembourg, and Norway), using both paper and electronic questionnaire formats, depending on feasibility [7]. Training courses for participating countries were run by WHO regional offices, and quality standards set by the WHS Quality Assurance Standards & Guidelines committee were monitored by external peer review. A detailed country report, providing details about the survey, is freely available on the WHS/WHO website.
Definitions of asthma
A strict definition of asthma based solely on doctor diagnosis may be useful in some clinical settings in developed countries; however, in developing countries it may vary greatly depending on the context, availability, and access to health care and medications. A combination of diagnosis and/or treatment for asthma may more accurately classify individuals with active asthma. Since diagnosis and the availability of treatment may be challenging in resource-poor countries, a broader definition which includes respiratory symptoms, in addition to diagnosis and treatment received may yield a higher sensitivity in identifying individuals with asthma. Therefore, in this study, we estimated and compared the global burden of asthma using three definitions of asthma. The first definition was doctor diagnosed asthma which is based on the question "Have you ever been diagnosed with asthma?" The second definition was clinical asthma which was based on doctor diagnosed asthma and/or a positive response in either of two questions "Have you ever been treated for asthma" or "Have you been taking any medications or treatment for asthma during the last 2 weeks?" The third definition, symptoms of asthma, was based on doctor diagnosed asthma, clinical asthma and/or a positive response to "During the last 12 months have you experienced attacks of wheezing or whistling breath?" The WHS survey questions were similar to those used by the ISAAC and ECRHS surveys [3, 5, 6]. To avoid confusion between asthma and Chronic Obstructive Pulmonary Disease (a disease most prevalent amongst older adults); we limited our study population to individuals aged 18 to 45 years.
Definition of smoking
Current smoking was defined based on a positive response to the question "Do you currently smoke any tobacco products such as cigarettes, cigars, or pipes?".
Regional differences
Estimates of asthma prevalence were calculated for each participating country as well as for each WHO region. Six regions using the WHO definitions were included: Africa (includes 18 countries), Americas (7), Eastern Mediterranean (4), Europe (30), South East Asia (5), and Western Pacific (6) [7, 14].
Statistical analyses
All analyses were conducted using STATA statistical software [15]. Country-specific prevalence estimates were obtained by applying survey weights for complex sampling designs. No weights were applied for pooled regional analyses [11, 13]. Given the different age-distributions in each of the participating countries, we age-standardized the country specific prevalence estimates. However, since the standardized estimates were similar to the unadjusted estimates (Mean Difference: -0.04; 95% Limits of Agreement: -0.07; 0.62), we presented the unadjusted results.
Missing data
The risk factor module included questions related to tobacco use and exposure to pollutants and was available for 53 countries [16]. Smoking data was missing for Austria, Australia, Belgium, Denmark, Finland, France, Germany, Greece, Ireland, Israel, Italy, Luxembourg, Netherlands, Norway, Portugal, Sweden, and the United Kingdom. These countries account for only 9,803 of the 181,042 individuals (5.41%) who completed the survey.
Ethics
The study materials and methods were approved by the research ethics board at the Hospital for Sick Children (REB # 1000025091).