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(Chest. 2006;129:602-609.)
© 2006 American College of Chest Physicians

Prevalence of Bronchial Hyperresponsiveness and Asthma in the Adult Population in Thailand*

Wanchai Dejsomritrutai, MD, MSc; Arth Nana, MD; Nitipatana Chierakul, MD; Jamsak Tscheikuna, MD; Suree Sompradeekul, MD; Pimon Ruttanaumpawan, MD and Suchai Charoenratanakul, MD

* From the Department of Medicine, Faculty of Medicine Siriraj Hospital, Mahidol University, Bangkok, Thailand.

Correspondence to: Wanchai Dejsomritrutai, MD, MSc, Division of Respiratory Disease and Tuberculosis, Department of Medicine, Faculty of Medicine, Siriraj Hospital, 2 Prannok Rd, Bangkoknoi, Bangkok 10700, Thailand; e-mail: siwds{at}mahidol.ac.th

Abstract

Objectives: We conducted a nationwide cross-sectional survey of respiratory health in adults aged 20 to 44 years during 2001 to 2002 to determine the prevalence of bronchial hyperresponsiveness (BHR) and asthma in the adult Thai population.

Design: Subjects were selected by a multistage stratified random sampling. The stratification was done on geographic area, age group, and sex. Subjects were interviewed with questionnaires and underwent spirometric testing. Methacholine challenge tests were performed on all subjects without contraindication to determine BHR defined as the provocative concentration of methacholine producing a 20% fall in FEV1 ≤ 8 mg/mL. Definite asthma was defined as BHR present with any asthma symptom within the past 12 months or demonstrated reversible airflow obstruction. Current diagnosed asthma was defined as previous physician-diagnosed asthma and any asthma symptom within the past 12 months or currently receiving asthma medication.

Results: The study population was from 20 provinces of five geographic regions of Thailand and included 1,882 women and 1,572 men. The prevalence of BHR was 3.31% (95% confidence interval [CI], 2.68 to 3.94). However, if subjects with positive reversibility test results were included, the prevalence increased to 3.98% (95% CI, 3.30 to 4.67). The prevalence of definite asthma was 2.91% (95% CI, 2.32 to 3.50), whereas the prevalence of current diagnosed asthma by the questionnaire interview was 2.15% (95% CI, 1.66 to 2.63). The {kappa} index of the agreement between both definitions of asthma was 0.40, indicating poor to fair agreement.

Conclusion: The prevalence of BHR and asthma in the adult Thai population is relatively low as compared with western countries.

Key Words: adult • asthma • bronchial hyperresponsiveness • epidemiology prevalence • Thailand

Asthma prevalence has been rising worldwide over the past 40 years, especially in western countries. However, a wide variation of asthma prevalence among countries has been observed. Epidemiologic data in asthma among Asian countries was limited, particularly in Southeast Asia. Few reports on the prevalence of asthma in Thai children have been published. The studies1234 reported the prevalence of asthma symptoms in Thai children in three cities, yielding the prevalence of 5.5 to 13.6%. There has not been any report on asthma prevalence in the adult population in Thailand.

A consensus on the definition of asthma has not been reached, which creates problems for the epidemiologic study of the disease.56 Although most studies78 of the prevalence of asthma used symptoms such as wheezing as determinants of asthma in the population, the validity of this method is debatable. Data gathered from the questionnaire may be influenced by a wide variety of cultural, psychological, and sociologic factors. The history of diagnosed asthma is also dependent on the degree of awareness of the disease among local physicians. More objective measurements may be useful for determining true asthma prevalence. We therefore conducted a nationwide survey of respiratory health in the adult population in Thailand to determine the prevalence of asthma as well as bronchial hyperresponsiveness (BHR).

Subjects and Methods

The target population was the adult population in Thailand aged 20 to 44 years. The sample size of the study population was calculated from the estimated prevalence of 5%, with precision of 0.05 and acceptable error of 15%. At least 3,244 subjects were needed. The study population was recruited by multistaged stratified randomization, proportional to size. The stratification was done on age group, sex, and geographic region. The age group was stratified to five age groups: 20 to 24, 25 to 29, 30 to 34, 35 to 39, and 40 to 44 years. Geographic regions were classified as Central, Northeastern, Northern, and Southern Thailand, and Bangkok. The number of subjects required in each stratified group was in proportion to the number of the whole population in each comparable geographic area. The multistaged randomization was done in steps from region, province, county and, finally, district. A sampling frame was developed from census report and household registration. The census was done on the year 2000. The distribution of the population stratified by area, sex, and age group is shown in Table 1 .9 The subjects were randomly selected using a random-number table. We used the established networks of local health officers and health volunteers in each district to recruit the selected subjects for the survey. Randomized subjects were invited with a letter that only informed that the survey was about their respiratory health. No specific mention of asthma was made at this stage.


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  		Table 1. The Study Population

 
Each subject underwent a questionnaire interview, spirometry, and methacholine challenge testing. The study questionnaire was modified from the International Union Against Tuberculosis and Lung Diseases questionnaire.10 The interview was done by trained and standardized interviewers. The questionnaire contents were about general respiratory health. No specific mention of asthma was made until all measurements were completed. This translated version of questionnaire was previously validated in 20 known asthma and 20 nonasthma subjects. The results of the sensitivity and specificity for the questions were 90% and 100% for wheeze, 80% and 95% for chest tightness, 80% and 100% for nocturnal dyspnea, 80% and 65% for nocturnal cough, and 95% and 100% for diagnosed asthma, respectively. The spirometric test was done according to the method recommended by the American Thoracic Society (ATS).11 The measurement was performed using a turbine system spirometer (Pony Graphic; Cosmed; Rome, Italy) meeting ATS recommendations. The machines were calibrated twice daily during the survey. The interpretation was based on reference spirometric values in the Thai population.12 A bronchoprovocative test was performed on all subjects, unless contraindicated, using the "five-breath dosimeter" method recommended by the ATS.13 Contraindications to bronchoprovocation included the following: (1) FEV1 < 70% of predicted; (2) heart attack or stroke in the last 3 months; (3) uncontrolled hypertension (systolic BP > 200 mm Hg or diastolic BP > 100 mm Hg); (4) known aortic aneurysm; (5) pregnancy; (6) nursing mother; and (7) current use of a cholinesterase inhibitor. The reversibility test was performed instead of the bronchoprovocation in subjects with low baseline FEV1 (< 70% predicted). The method and interpretation of the reversibility test were according to ATS recommendations.11

Definitions
BHR was defined as the provocative concentration of methacholine producing a 20% fall in FEV1 ≤ 8.0 mg/mL (definition A). However, since a reversibility test was performed on subjects with low baseline FEV1, instead of the bronchoprovocative test as mentioned, patients with reversible airflow obstruction were also assumed to have BHR (definition B). The definition of definite asthma includes subjects with reversible airway obstruction by spirometry or subjects with any symptoms compatible with asthma within the last 12 months in addition to BHR. These asthma-related symptoms include wheezing, chest tightness, shortness of breath, nocturnal dyspnea, nocturnal cough, and having trouble with breathing. Current diagnosed asthma was defined as any previous physician-diagnosed asthma and any asthma symptom within the past 12 months or currently receiving asthma medication.

Statistical Analysis
Descriptive statistics were used to present the prevalence of BHR, definite asthma, and physician-diagnosed asthma. Age and sex-specific prevalence were also revealed. A comparison of the prevalence among groups was done using the {chi}2 test. The agreement between the prevalence of definite asthma and current diagnosed asthma was performed by {kappa} statistics.

Ethical Considerations
This study was approved by the Committee on Human Rights Related to Research Involving Human Subjects at the Faculty of Medicine Siriraj Hospital, Mahidol University, Thailand. All participants were informed that the study was about their respiratory health, and details of the method of each measurement were provided.

Results

The multistaged stratified random sampling was done on the population of subjects aged 20 to 44 years. The overall response rate was 46.9%. A total number of 3,454 subjects were recruited in the study as shown in Table 1.9 All participants underwent a questionnaire interview. Spirometry and bronchoprovocation were performed on 3,245 subjects and 3,141 subjects, respectively.

The questionnaire interview yielded information on the prevalence of asthma-related symptoms within the past 12 months. These included wheezing (16.4%), waking up with chest tightness (15%), waking up by an attack of shortness of breath (7.7%), and coughing (18.3%). Additionally, 3.25% of diagnosed asthma was reported, whereas the prevalence of current diagnosed asthma was 2.15%. The prevalence and 95% confidence intervals (CIs) of asthma-related symptoms and diagnosed asthma stratified by geographic area, sex, and age group are demonstrated in Tables 234 .


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  		Table 2. Prevalence of Asthma-Related Symptoms, BHR, and Definite Asthma Stratified by Geographic Area*

 

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  		Table 3. Prevalence of Asthma-Related Symptoms, Bronchial Hyperresponsiveness and Definite Asthma Stratified by Sex

 

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  		Table 4. Prevalence of Asthma-Related Symptoms Stratified by Age Group*

 
Results of bronchoprovocation with methacholine suggested that the prevalence of BHR in the population was 3.31% (definition A). The prevalence of BHR increased to 3.98% if subjects with reversible airflow obstruction (definition B) were included. Additionally, the prevalence of definite asthma, as defined in subjects with BHR as well as any asthma-related symptoms within the last 12 months or demonstrable reversible airflow obstruction, was 2.91% (Table 5 ). Tables 234 reveal the prevalence of BHR, definite asthma, and current diagnosed asthma in the adult Thai population stratified by geographic area, age group and sex.


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  		Table 5. Prevalence in Adult Thai Population Aged 20 to 44 Years

 
The agreement between both definitions of asthma, ie, definite asthma and current diagnosed asthma, is demonstrated in Table 6 . The methods used to define definite asthma and current diagnosed asthma are independent from each other. The {kappa} index, calculated to determine the level of the agreement, yielded the number of 0.40, which indicated poor to fair agreement.14


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  		Table 6. Agreement Between Definite Asthma and Current Diagnosed Asthma*

 
Discussion

To the best of our knowledge, this cross-sectional survey of the prevalence of BHR and asthma in the adult Thai population is the first study in this country. The information obtained from the study is valuable for determining the burden of the disease in the country. It can be used for priority setting and developing a strategic public health plan for the optimal distribution of resources in disease prevention and treatment.

Since the prevalence of asthma in Thai adults has not been studied, whether the prevalence of asthma in Thailand is increasing is still inconclusive. At present, only a few reports of asthma prevalence in Thai children have been published. The reported prevalence of asthma symptoms (wheezing) in Thai children in Bangkok,1 Khon Kaen,2 and Chiang Mai3 were 11.7 to 13.6%, 10.2 to 11.0%, and 5.5 to 12.6%, respectively. However, apart from age difference, the methods used in these studies were also substantially different. The studies in children used the questionnaire modified from the International Study for Asthma and Allergy in Children study,15 whereas this adult study used the modified International Union Against Tuberculosis and Lung Diseases questionnaire.10

Although all regions in Thailand share common environmental and social factors, a few differences exist. The mean temperature in northern region is generally lower than the others, while more rainfall occurs in central and southern parts. People in northeastern region have lowest mean income and reside in more rural area. These environmental and social factors may influence the difference in the prevalence of asthma. The present study revealed that the prevalence of asthma-related symptoms in the central part of Thailand were relatively lower than other regions (Table 2). There are some disproportions between the study population and the total population in northern (25.1% instead of 19.4%) and northeastern regions (29.8% instead of 34.4%) as shown in Table 1, which was due to some logistic problems. Nevertheless, no significant difference among regions was observed on the prevalence of current diagnosed asthma, BHR, and definite asthma (Table 2).

We found the relatively high prevalence of asthma-related symptoms in older age groups (Table 4). Higher prevalence of BHR was also observed in older age groups with statistical significance, p = 0.008 (Table 4). Higher prevalence of definite asthma was observed in the older age groups, p = 0.002 (Table 4), which may indicate the role of environmental factors on the development of asthma.

Gender variability in the prevalence of BHR has been observed in the present study if subjects with the positive reversibility test were not included (Table 3). BHR is more likely to be present in women than in men (3.92% vs 2.63%, p = 0.04). This gender difference has already been observed in previous studies.16171819 This observation is consistent with the finding that women have a greater susceptibility to environmental exposure, eg, tobacco smoke and air pollutants.2021

The questionnaire used in the present study is similar to that used in the European Country Respiratory Health Survey (ECRHS). The comparison between the results from this study and those from the aggregated data in ECRHS is therefore demonstrated in Table 7 .22 The prevalence of asthma-related symptoms in the adult Thai population is generally lower than the median of the prevalence among 22 European countries in the ECRHS study.


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  		Table 7. Comparison Between the Prevalence From ECRHS and the Present Study

 
A comparison of the prevalence of current diagnosed asthma among countries is demonstrated in Table 8 .23242526272829 The prevalence in the adult Thai population is again substantially lower than that in western countries and much lower than the prevalence in Australia and New Zealand. A study of the prevalence of diagnosed asthma in Mumbai, India,28 and Singapore29 revealed a comparable number to the prevalence in the Thai adult population from this study.


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  		Table 8. Comparison of Physician-Diagnosed Asthma Among Studies

 
Regarding the prevalence of wheezing within the last 12 months, the comparison among countries revealed that the prevalence in Thai adults is relatively low as compared with the United Kingdom, Australia, and New Zealand. However, the figure is comparable to those from the United States, Italy, and Sweden (Table 9 ).172325273031


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  		Table 9. Comparison of Wheezing Within the Last 12 Months Among Studies

 
The comparison of the prevalence of BHR among countries is demonstrated in Table 10 .16172732333435 We found that the prevalence of BHR in the adult Thai population is much lower than that in the population in western countries. Our prevalence seems comparable to that in the population of the southern part of China.35 Nevertheless, the comparison must be done cautiously because the age groups of both populations were different. The discrepancy among studies may result from differences in the method of measurement. This study used the method recommended by the American Thoracic Society, which is substantially different from the method used in the ECRHS studies. Nevertheless, even among ECRHS studies, wide variations of the prevalence of BHR have been observed.32


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  		Table 10. Comparison of BHR Among Studies

 
Defining asthma in epidemiologic studies is still problematic, resulting in the absence of a "gold standard" for the diagnosis of the disease. The issue of the role of the bronchoprovocative test in the survey of asthma is therefore debatable.5678 Some researchers recommended that a symptom questionnaire is better than BHR in epidemiologic studies because of the poor sensitivity of the latter. They compared both methods with physician-diagnosed asthma. However, the use of physician-diagnosed asthma as a reference may not be appropriate because the questionnaire replicates the method that doctors use to reach a diagnosis of asthma during a consultation. Since most of the participants for the questionnaire in the present study (91%) continued to do the bronchoprovocative test, the agreement between current diagnosed asthma and definite asthma can be evaluated. The {kappa} statistics demonstrate poor to fair agreement ({kappa} = 0.40) between the two definitions (Table 6). Which definition is appropriate may depend on the purpose of the study. For instance, research questions about risk factors or etiologic mechanisms should use a more precise definition, eg, definite asthma in this study.

The interpretation of the study results may be limited by the low response rate (49.6%). This may be subject to a selection bias. However, subjects were only informed that the survey was done on respiratory health, and no specific mention of asthma was made. Therefore, the magnitude of the bias, if present, should be minimal. For instance, the response rate for Bangkok (Table 1) is remarkably lower than other parts, which may be due to the differences in the health-care system between the capital and other provinces. Populations in the capital who can easily access to the health-care system are generally not interested to participate in the study. Nevertheless, apart from some subjective evidence, there was no significant difference in the prevalence of current diagnosed asthma, BHR, and definite asthma among regions (Table 2). Moreover, if the bias was present, people without any respiratory problems may not have had the urge to participate in such a study, and conversely. The end result of the bias would tend to lower the actual prevalence of both asthma-related symptoms and definite asthma. The conclusions regarding the low prevalence of asthma according to the present study may still remain valid.

People in Southeast Asia shares considerable common cultural and environmental features. All the countries are in the tropical zone with humid climate. However, there has been only one report29 on the prevalence of adult asthma in Southeast Asian countries; the questionnaire-based prevalence of current diagnosed asthma in Singaporean was 2.4% in men and 2.0% in women. To our knowledge, there has not been a study in the Southeast Asian region that included bronchoprovocation in the survey.

In conclusion, this nationwide cross-sectional survey revealed the prevalence of current diagnosed asthma in 2.15%, definite asthma in 2.91%, and BHR in 3.31 to 3.98% of the adult Thai population. Compared to western countries and Australia/New Zealand, the prevalence of asthma in Thailand is much lower.

Acknowledgements

The authors are grateful to Ministry of Public Health of Thailand for administrative support of the survey.

Footnotes

Abbreviations: ATS = American Thoracic Society; BHR = bronchial hyperresponsiveness; CI = confidence interval; ECRHS = European Community Respiratory Health Survey

Financial support was provided by Mahidol University.

Received for publication February 20, 2005. Accepted for publication July 23, 2005.

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