National Statistics and Opportunities for Improved Surveillance,
Prevention, and Control

PREVALENCE

Current Overall Prevalence Estimates - 1996/97

Based on 1996/97 Canadian survey data and extrapolations from American sources, the number of Canadians aged 12 and over with diabetes is estimated at 1.2 to 1.4 million (4.9% to 5.8% of the population aged 12 and over), including undiagnosed cases of diabetes. Among Aboriginal peoples where age-standardized diabetes prevalence rates are triple those found in the general population, an estimated 60,000 people have diabetes, including undiagnosed cases.

According to the most recent national survey data from which we have prevalence estimates (NPHS 1996/97), 3.2% of the population aged 12 and over have a diagnosis of diabetes; this represents approximately 779,000 Canadians. In addition, diabetes has been diagnosed in an estimated 27,000 institutionalized Canadians (NPHS 1994/95). NPHS population coverage, however, excludes those living in the territories, on Indian reserves, on Canadian Forces bases and in some remote areas in Quebec and Ontario, which underestimates the true prevalence rate of diagnosed diabetes. Another potential source of underestimation of diabetes prevalence is the reliance on self-reported information that is subject to individual memory and willingness to report. For example, an analysis from the 1996 NPHS of "new cases" of diabetes since 1994 by reported year of diagnosis indicated that approximately 30% to 40% of these cases of diabetes were diagnosed in or before 1994 (NPHS 1996/97, longitudinal file).

Over-reporting of diabetes is also possible (for example, transient hyperglycemia as seen in gestational diabetes could be misinterpreted as diabetes), although NPHS longitudinal data indicate that a very small number of people reported in 1994/95 that they had diabetes but in 1996/97 reported that they either did not have diabetes or that it had disappeared.

Regarding undiagnosed cases of diabetes, American data from the Third National Health and Nutrition Examination Survey (NHANES III) indicate that approximately 35% to 44% of all diabetes cases may be undiagnosed, depending on the test used to diagnose diabetes⁽¹⁾. Note that rates of undiagnosed diabetes are gross estimates and the actual rate in Canada of undiagnosed diabetes could be outside the range given here.

Underestimation of Diabetes Prevalence: Undiagnosed Diabetes

The true prevalence of diabetes in Canada is substantially underestimated by our current self-reported estimates of diagnosed diabetes (at 3.2% of the population) mainly because of large numbers of undiagnosed cases of diabetes, assuming similar rates in Canada as exist in the USA. Note that these are gross estimates of undiagnosed diabetes because they are based on single blood samples, not repeat samples on two separate days as recommended.

According to the Third National Health and Nutrition Examination Survey, 1988-1994 (NHANES III), the prevalence of self-reported diagnosed diabetes is an estimated 5.1% for people aged 20 and over in the USA. In addition to the questionnaire, this American survey obtained fasting blood glucose and oral glucose tolerance test measures to detect diabetes in those people without a medical history of diabetes. Using the 1997 American Diabetes Association fasting plasma glucose diagnostic criteria, approximately one third (35%) of all diabetes cases are undiagnosed, representing an additional 2.7% of the US population aged 20 and over with undiagnosed diabetes. Using the World Health Organization (WHO) diagnostic criteria for the oral glucose tolerance test, which is a more sensitive test than the fasting plasma glucose test, undiagnosed diabetes constitutes approximately 44% of total diabetes cases.⁽¹⁾ US rates of undiagnosed diabetes may have decreased since the 1980s when NHANES II (1976-1980) data indicated 50% of all diabetes cases, using WHO oral glucose tolerance test criteria, were undiagnosed⁽²⁾.

A similar study conducted in the UK from 1990-1992 found a prevalence of undiagnosed diabetes of 4.5% among those 40 to 65 years of age based on WHO criteria using the oral glucose tolerance test⁽³⁾. This finding of high numbers of undiagnosed cases of diabetes in a country with a publicly funded health care system suggests that the high prevalence of undiagnosed diabetes in the USA is generalizable to countries that do not have the American system of largely non-publicly funded health care.

Diabetes Prevalence Rates: Comparison of National Survey Estimates

• Canadian Heart Health Survey (1986-1992) and National Population Health Survey 1996/97

  Estimates of the prevalence of self-reported, diagnosed diabetes in the Canadian population vary widely. A frequently cited prevalence estimate is 5% of those between the ages of 18 and 74, according to the Canadian Heart Health Survey (CHHS) (1986-1992). For the same age group in the NPHS1996/97, the estimated prevalence of diagnosed diabetes is 2.9% of the Canadian population aged 18 to 74 years.

  At first glance, it appears that diabetes prevalence among Canadians aged 18 to 74 years has decreased from 1986 to 1996. However, methodologic differences between the surveys make comparisons inadvisable. For example, the CHHS and NPHS differ in population coverage. The diabetes component of the CHHS was used in nine provinces (excluding Nova Scotia), and included people living on Indian reserves in one province (Manitoba). The NPHS collected data from 10 provinces, and did not include people living on Indian reserves. Another difference between the surveys was the length of time for data collection. The CHHS was carried out over a seven-year period whereas the NPHS 1996 data were collected over approximately a one-year period (June 1996 to March 1997). Also, the question used to identify people with diagnosed diabetes differed between these surveys. The CHHS asked people if they had ever had diabetes: "Have you ever been told by a doctor that you have diabetes?" By contrast, the NPHS asked people whether they currently had diabetes: "Do you have diabetes that has been diagnosed by a health professional?" It is not known whether one question is likely to lead to more accurate responses than the other, but it is known that comparing results of surveys with different interview questions is difficult.

• Comparable National Surveys: Increasing Trends in Diabetes Prevalence

  When surveys with consistent methodologies are compared, the prevalence of diabetes appears to be increasing over time.

  General Social Survey (GSS). The GSS was conducted in 1985 and 1991. GSS results showed a significant increase in the prevalence of diagnosed diabetes over this six-year period, from 2.4% in 1985 to 3.5% in 1991 (p < 0.001). (GSS respondents were asked: "Do you have diabetes?")

  National Population Health Survey (NPHS). The NPHS was initiated in 1994/95 and repeated in 1996/97. Over this two-year period, NPHS results showed a nonsignificant trend toward an increasing prevalence of diagnosed diabetes, from 3% in 1994 to 3.2% in 1996.

  Once again, differences between GSS and NPHS survey methodologies, such as in the question about diabetes, make comparisons of results between surveys inadvisable.

• National Surveys of Diabetes Prevalence: 1978-1996/97

  An in-house LCDC study of existing sources of Canadian national health survey data pertaining to diabetes identified eight surveys conducted from 1978 to 1996: Canada Health Survey, General Social Survey, Campbell's Survey on Well-Being in Canada, Canadian Heart Health Survey, Aboriginal Peoples Survey, National Longitudinal Survey of Children and Youth, National Population Health Survey, National Population Health Survey (institutional component) (Figure 1).

FIGURE 1
Prevalence of self-reported diabetes based on Canadian
health surveys using different methodologies, 1978-1996/97

* gestational diabetes in most recent pregnancy.

Notes: CHS = Canada Health Survey, GSS = General Social Survey, CSWBC = Campbell's Survey on Well-Being in Canada, CHHS = Canadian Heart Health Survey, APS = Aboriginal Peoples Survey, NLSCY = National Longitudinal Survey of Children and Youth, NPHS =  National Population Health Survey, NPHS-I  = National Population Health Survey Institutional, NPHS-HS = National Population Health Survey 1996/97, Health Share File (cross-sectional), preliminary release. NPHS-LS = National Population Health Survey 1996/97, Health Share File (Longitudinal), preliminary release.

• Although it is tempting to compare the prevalence rates of these surveys, the comparisons would not be accurate because of variation in the following methodologic factors (Table 1):

  • geographic coverage;
  • demographic coverage including age, institutional and Aboriginal subpopulations;
  • definitions of diabetes in terms of current versus ever, and who made the diagnosis;
  • method of contact in terms of a face-to-face interview, mailout or telephone survey; and period of data collection.

TABLE 1 Study populations and methodology for national health surveys with diabetes content, 1978-1996/97
Survey	Year	Sample size (weighted pop. estimate)	Geographic/ population coverage	Age (years)	Definition of diabetes
Canada Health Survey	1978	31,668 (23,023,000)	10 provinces	15+	Ever had diabetes?
General Social Survey	1985	11,200 (19,669,000)	10 provinces	15+	Do you have diabetes?
General Social Survey	1991	11,924 (20,980,862)	10 provinces	15+	Do you have diabetes?
Campbell's Survey on Well-Being in Canada	1988	4,345 (22,075,319)	10 provinces	7+	Do you presently have diabetes - insulin dependent?; non-insulin dependent?
Canadian Heart Health Survey	1986- 1992*	23,129 (18,829,203)	9 provinces (no diabetes content -Nova Scotia, 1986); Indian reserves in Manitoba only	18-74	Have you ever been told by a doctor that you have diabetes?
Aboriginal Peoples Survey	1991	36,635 (630,414)	Canada; Aboriginal	15+	Have you ever been told by a health care professional that you have diabetes?
National Longitudinal Survey of Children and Youth	1994/95	22,831 (4,673,390)	Provinces and territories;  diabetes question asked of only women with children under 2 years of age	NA (mothers)	During pregnancy, did you suffer from pregnancy diabetes?
National Pop. Health Survey Institutional	1995	2,287 (227,842)	10 provinces; all long-term residents of selected health care institutions	12+	Do you have diabetes that has been diagnosed by a health professional?
National Pop. Health Survey- cross-sectional	1994/95	17,011 (23,948,603)	10 provinces; no Indian reserves	12+	Do you have diabetes that has been diagnosed by a health professional?
National Pop. Health Survey- cross-sectional	1996/97	77,403 (28,641,735)	10 provinces; no Indian reserves	12+	Do you have diabetes that has been diagnosed by a health professional?
National Pop. Health Survey- longitudinal**	1994/95- 1996/97	14,860 (28,617,694)	10 provinces; no Indian reserves; selected respondents surveyed every two years	12+	Do you have diabetes that has been diagnosed by a health professional?
* The Canadian Heart Health Survey is the only listed cross-sectional survey that collected data over a multi-year period; data were standardized to the 1986 Canadian population. ** National Population Health Survey - longitudinal file was standardized to the 1994 Canadian population. Note: Pop .= population. Non-proxy = respondents report on their own health status. Proxy = a selected respondent reports on the health status of all eligible house hold members.

Each of these differences influences the results to varying degrees, in terms of a tendency to either overestimate or underestimate. For example, most surveys included adult populations but with age cutoffs that varied from 7 to 18 years of age and older. A higher age cutoff will tend to bias the result towards a higher prevalence estimate of diabetes because of the higher prevalence in older age groups. Even though the number of cases in institutional and Aboriginal populations is small, the exclusion of these groups will tend to underestimate the actual prevalence because of their higher diabetes rates.

Diabetes Prevalence by Race/Ethnic Group - 1996/97

The distribution of diabetes prevalence by racial/ethnic group in the Canadian population aged 12 and over is as follows: 3.2% of whites, 3.8% of blacks, and 5.4% of Aboriginal peoples not living on reserves (the last two figures have high sampling variability because of small samples, i.e., n = 39 and 63 respectively). Samples are too small among those with diabetes in other racial/ethnic groups for population estimates to be made. (NPHS 1996)

Data from the 1991 Aboriginal Peoples Survey on diabetes among Aboriginal peoples are reported in a later section of this report.

Diabetes Prevalence by Age - 1996/97

Prevalence rates of diabetes increase with age such that the rate in those aged 65 years and over is three times as high as the rate in those aged 35 to 64 (10.4% vs. 3.2%) (Table 2).

Diabetes Prevalence by Sex - 1996/97

Diabetes prevalence is significantly higher among males than females (3.5% vs. 2.9%, p < 0.001). This difference is due to the higher prevalence in the 35 to 64 age group (3.7% among males vs. 2.7% among females) and the 65+ age group (12.1% vs. 9.1%) (Table 2). As noted in a later section on obesity, the higher level of obesity among males may explain this higher diabetes prevalence.

In terms of the distribution of diabetes cases by sex, 54% of those reporting diabetes are male and 46% female. This difference is due to a larger proportion of males with diabetes in the 35 to 64 age group (58.1% among males vs. 41.9% among females) (Table 3).

TABLE 3 Proportion of males and females among those reporting diabetes, by age group - Canada excluding Territories, 1996/97
Sex	% reporting diabetes
	Age group
	Population (12+)	12 - 34	35 - 64	65+
Male	54	49.6	58.1	50.3
Female	46	50.4	41.9	49.7
Both	100	100	100	100
Note:    Diabetes refers to current diabetes that has been diagnosed by a health professional. Source:    LCDC 1998 - using National Population Health Survey 1996/97, Health Share File, preliminary release.

Prevalence by Province - 1996/97

Prevalence rates of self-reported diabetes by province range from roughly 3% to 4% (Figure 2). Rates for diabetes vary among provinces when surveys are compared, but no consistent pattern is clear at this time.

FIGURE 2
Prevalence of self-reported diabetes by province* -
Canada excluding Territories, 1996/97

* P.E.I. is not included here because the sample size is too small (<30) for reportable prevalence estimate.
** Prevalence estimate has high sampling variability.

Notes: Samples sizes (n) :  BC n=45;   ALB n=384;   SASK n=37;   MAN n=498;   ONT n=1354;   QUE n=77; 
NB n=30  ;  NS n=40;   NFLD n=38 . Diabetes is self-reported. Respondents were aged 12 and over.
Source:LCDC 1998 - using National Population Health Survey 1996/97, Health Share File, preliminary release.

Prevalence of Gestational Diabetes - 1994-1995

Canadian data from the 1994-1995 National Longitudinal Survey of Children and Youth (NLSCY) indicate that 6.5% of women who had children under 2 years of age reported "pregnancy diabetes" in their most recent pregnancy.

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