Arthritis Prevalence and Place of Birth: Findings from the 1994 Canadian National Population Health Survey
Peizhong Peter Wang1,2,
Renee Elsbett-Koeppen1,
Guan-yi Geng2,3 and
Elizabeth M. Badley1,2
1 Arthritis Community Research and Evaluation Unit, The Arthritis and Immune Disorder Research Centre, University Health Network, Toronto, Ontario, Canada.
2 Department of Public Health Sciences, University of Toronto, Toronto, Ontario, Canada.
3 Deceased.
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ABSTRACT
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This paper describes the prevalence of arthritis in Canadians by ethnic origin, including Asians, Europeans/Australians, and North American-born Canadians. Data for this study were derived from the 1994 Canadian National Population Health Survey, a cross-sectional survey with a sample of 39,240 persons aged 20 years and older. Arthritis was defined as a long-term health condition of "arthritis or rheumatism" diagnosed by a health professional. Place of birth was determined according to self-reported country of birth. Unconditional multiple logistic regression models were used to adjust for potential confounding effects. The crude prevalence of self-reported arthritis and rheumatism diagnosed by a health professional as a long-term condition for those aged 20 years and older in Canada was 14.2%. The age-sex adjusted prevalence by place of birth was 6.9% in Asians, 14.2% in Europeans/Australians, and 14.5% in North American-born Canadians. In the multivariate analyses using North America-born Canadians as baseline, the risk for arthritis (odds ratio = 0.56) was significantly lower in Asian-born Canadians after adjustment for age, sex, education, income, occupation, and body mass index. Am J Epidemiol 2000;152:4425.
arthritis; ethnic groups; population; prevalence
Abbreviations:
BMI, body mass index.
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INTRODUCTION
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Studies examining the prevalence of arthritis in people from different racial, ethnic, and geographic backgrounds have the potential to provide insights into disease etiologies. International comparisons suggest that Asian populations have a consistently low prevalence of rheumatoid arthritis (1





8
). However, rheumatoid arthritis accounts for only a small portion of self-reported prevalence of arthritis (9


13
), and studies of osteoarthritis by country, race, and ethnicity have been scant. Direct international comparison of the prevalence of self-reported arthritis in different countries is difficult since the available population surveys have used dissimilar methodologies and study designs as well as disease definitions.
The primary objective of this study was to examine the variation in prevalence of arthritis among Canadian residents classified by the place of birth, which include North America, Asia, Europe/Australia, and others. Immigrants from many different countries have transformed Canada into an ethnically diverse country, which makes it an ideal place for immigration epidemiologic studies.
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MATERIALS AND METHODS
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Data sources
This study analyzed data from the 1994 Canadian National Population Health Survey (NPHS) (14
). This survey was designed to collect information related to the health of the Canadian population, and this household survey used a stratified two-stage design. In the first stage, homogeneous strata were formed, and independent samples of clusters were drawn from each stratum. In the second stage, dwelling lists were prepared for each cluster, and dwellings or households were selected from the lists. For each household, some limited information was collected from all household members, referred to as the general file, and one person aged 12 years and over was randomly selected for a more in-depth interview in each household. These data were combined with the data from the household interview relating to that person to give a combined data set referred to as the health file (14
).
A total of 26,429 households and 59,439 individuals were surveyed across Canada, with a selected person response rate of 96.1 percent at the national level. After application of the age criterion (age 20 years or older), the final sample sizes available for this study were 39,240 for the general file and 15,779 for the health file. All variables used in this study were available in both files except for the body mass index (BMI) (weight (kg)/height (m)2), which was only available in the health file.
Outcome variable and explanatory variables
Arthritis was considered present if an affirmative response was given to the question, "Do you have arthritis or rheumatism diagnosed by a health professional?" A corresponding dichotomous outcome variable was formed. Place of birth was determined by asking for the country of birth, which was further aggregated by Statistics Canada into five broad categories: Canada, United States and Mexico, South America/Africa, Europe/Australia, and Asia. Because respondents born in South America/Africa comprised a very heterogeneous group and accounted for only about 2.0 percent of the total sample, they were combined with those with unspecified birth places (0.4 percent), forming "others" as a separate category. As a result, four nominal categories for birthplace were available: North America, Europe/Australia, Asia, and others.
Explanatory variables consisted of age, sex, level of education, place of residence, occupation, income, and BMI. An ordinal age variable was formed with the following seven categories: 2029 (used as the baseline), 3039, 4049, 5059, 6069, 7079, and 80 or more years. Low education, low income, and place of residence were defined as dichotomous variables. Low education was referred to having some secondary education or less, and not having low education was used as the baseline. The dichotomous income variable was based on Statistics Canada's definition, in which family size and household income had been taken into account (14
). Low income represents an income of lower than $15,000 for a one- or two-person family, $20,000 for a three- or four-person family, and $30,000 for a family with five or more persons. Not having low income was used as the baseline. Place of residence was dichotomized into urban (baseline) and rural location. Occupation was a trichotomized variable derived from Pineo-Porter-McRoberts job classification (15
, 16
), categorizing occupations into 16 homogeneous groups on the basis of skill level and special affinities such as prolonged formal training. The three categories of occupation were professional, skilled, and semiskilled or lower. BMI was derived from the self-reported height and weight, categorized into three groups: no excess weight (BMI <25), some excess weight (BMI 2527), or overweight (BMI >27).
Statistical analyses
All statistical analyses were conducted on data weighted to account for the unequal sampling probabilities and to produce descriptive estimates that are generalizable to the general household population in Canada.
For all statistical analyses, including the calculation of the 95 percent confidence intervals, the adjusted sample size was used. The sample size adjustment was based on rescaled weight for each individual estimate used. This was achieved by dividing the expansion weights by the mean weights so that the average weight was equal to one. Since we were unable to calculate the design effect factor for each individual variable, for the statistical testing the rescaled weights were divided by the average design effect factor provided by Statistics Canada to accommodate the stratified-cluster sampling scheme.
In the logistic regression model, age group was treated as an ordinal variable beginning with age 2029 and ending with age 80 years or older. All other categorical predictor variables were introduced as dummy variables. Since the BMI variable was available only in the health file, a two-stage logistic regression analysis presented by Cain and Breslow (17
) was used. With this approach, a standard logistic regression analysis was initially performed within the health file. To get results that would be applicable to the general file, the odds ratios and the corresponding 95 percent confidence intervals for all explanatory variables (except for BMI) were then adjusted based on sample size ratios between the two files.
Participants with missing values for the outcome variable (1.3 percent) were excluded from the logistic regression analysis.
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RESULTS
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The number of participants aged 20 years and older available for this analysis was 39,240, representing 20.6 million household population in Canada, and among them, 1,510 (3.9 percent) representing 1.2 million (5.8 percent after weighting), reported being born in Asia. The social demographics differed substantially among the three major groups on the basis of place of birth. Compared with people born in North America and Europe, those born in Asia were more likely to be younger, with a higher education and a lower income. The European-born population tended to be older and to be urban dwellers. Compared with the other three populations, people born in Asia had a significantly lower BMI than did their non-Asian counterparts (table 1).
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TABLE 1. Proportion of respondents with selected characteristics by place of birth, 1994 Canadian National Population Health Survey*
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The weighted prevalence of arthritis diagnosed by a health professional in Canadian population aged 20 years and older was 14.1 percent. As shown in table 1, there was a great variation of prevalence of arthritis among the four ethnic groups. Those born in Asian countries had a significant lower prevalence of arthritis than did those born in North America and Europe. The difference in prevalence remained after age and sex adjustment; the age-sex-adjusted prevalences were 14.5, 14.2, and 6.9 percent for participants born in North American, Europe, and Asia, respectively. The findings from the bivariate analysis suggested that female sex, old age, low education and income, and high BMI are associated with increased prevalence of arthritis in all three groups (table 2).
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TABLE 2. Arthritis prevalence, in percent, by selected characteristics and place of birth, 1994 Canadian National Population Health Survey*
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The results of the logistic regression analysis are shown in table 3. Female sex, low income, and older age were significantly associated with increased risk of arthritis. The prevalence of arthritis increased with age exponentially, with an odds ratio of 1.71 (95 percent confidence interval: 1.63, 1.79) for every 10-year increment after age group 2029 years. There was a gradient of association between BMI and arthritis. With no excess weight as the baseline, overweight people were at increased risk of arthritis. However, level of education was not found to be a significant predictor for arthritis.
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TABLE 3. Logistic regression analysis for people with arthritis aged 20 years and older, 1994 Canadian National Population Health Survey
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When North America was used as the comparison group, the odds ratio for a European-born person was not significantly different from one, but the adjusted odds ratio of 0.56 (95 percent confidence interval: 0.48, 0.66) was significantly lower for Asian-born individuals.
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DISCUSSION
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This study using data from a national population health survey suggests that the prevalence of self-reported arthritis is lower in the population born in Asia. The term "Asian" implies diverse nationalities; thus, the results may not be generalizable to any specific race and ethnicity. According to the 1996 Canadian census, more than 80 percent of Asian immigrants were from east and south Asia (18
). It remains unclear what factors have contributed to the prevalence gap between Asian- and non-Asian-born populations and to what extent these results were influenced by biological makeup and culture-related factors. The latter could include the differences in reporting symptoms as well as healthy immigrant effect, whereby immigrants to Canada have better health (19
21
).
The results reported in this study corroborate other recent studies (3
, 22

25
) that indicate that being an Asian may be truly associated with reduced risk of having arthritis. The finding suggests that further etiologic epidemiologic studies might be rewarding.
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ACKNOWLEDGMENTS
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Supported by a grant from the Ontario Ministry of Health through their Health System-Linked Research Unit Grant schemes.
The authors thank Drs. S. Bondy and E. Adlaf for their advice on data analyses.
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NOTES
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Correspondence to Dr. Peizhong Peter Wang, The Arthritis Community Research and Evaluation Unit, The Arthritis and Immune Disorder Research Centre, University Health Network, PMH, 610 University Avenue, 16th Floor, Room 16704, Toronto, Ontario, Canada M5G 2M9.
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Received for publication July 15, 1999.
Accepted for publication October 11, 1999.