Diabetes screening and diagnosis

The number of Canadians diagnosed with type 2 diabetes continues to increase. This rise has been attributed to obesity, an aging population, and increasingly sedentary lifestyles. Nowhere is this more apparent than in Canadian Aboriginal communities. The emergence of new evidence, risk prediction tools, and screening options has reintroduced the issue of diabetes screening and diagnosis onto health policy agendas.

Diabetes affects approximately 2 million Canadians, and 6 million more are believed to be at high risk for future diagnosis of diabetes. This risk state, termed “prediabetes,” is defined by blood glucose levels that are abnormally high but not high enough for a diagnosis of diabetes. Prediabetes is predictive of type 2 diabetes in approximately 50% to 70% of cases. It is generally believed that early diagnosis of type 2 diabetes, which is largely asymptomatic, could lead to reduced future morbidity and mortality. This article looks at new evidence and the application of diagnostic and screening tools in use in Canada.

Diabetes complications

It is anticipated that early detection and diagnosis of individuals with prediabetes and type 2 diabetes will lead to earlier detection of microvascular complications such as diabetic nephropathy, neuropathy, and retinopathy. A 2005 national Canadian study of 2,473 type 2 diabetes patients found that 38% of these patients had a diagnosis of one or more microvascular complications.¹

Early diagnosis and treatment is also intended to prevent or delay macrovascular complications such as heart attack, stroke, and clogged arteries.²  Approximately 28% of type 2 diabetes patients in the Canadian study had one or more macrovascular complications.¹

Additionally, 14% of type 2 diabetes patients had a diagnosis of depression, 3% had glaucoma, and, among male patients, 21% had a diagnosis of erectile dysfunction.¹

Diagnostic screening tools

Screening tools for type 2 diabetes typically include risk assessment questionnaires, biochemical tests, or combinations of both. The most widely used biochemical tests and the ones currently recommended by the Canadian Diabetes Association include the fasting plasma glucose (FPG) test and the oral glucose tolerance test (OGTT).³

Another test, the hemoglobin A1C (A1C) test, reflects an average of blood glucose concentrations in the previous two to three months. Unlike the FPG test and OGTT, the A1C test does not require fasting and gives a picture of blood glucose control over a period of time. The A1C test has traditionally been used to monitor long-term glycemic control, adjust therapy, and assess risk for the development and progression of diabetic complications.

A recent assembly of international diabetes experts proposed that the A1C test become the reference standard for the screening and diagnosis of diabetes. The experts agreed that the A1C test demonstrated greater stability, accuracy, convenience, and correlation to the risk of developing retinopathy than either the FPG test or OGTT. This announcement is likely to impact international clinical practices because all North American and most European guidelines currently recommend the FPG test and OGTT.⁴

The recent standardization of the A1C test has been instrumental in focusing attention on its expanded use as a screening test for the diagnosis of diabetes. However, according to Dr. Stewart Harris, the lead investigator of the National Diabetes Management Strategy, the A1C test is not quite ready for widespread use. He noted that because it will be more expensive than conventional tests, studies are required to ascertain its cost-effectiveness and uptake. The A1C test costs between C$6 and C$8. The FPG test, in comparison, costs less than C$1.⁵ Dr. Harris also mentioned that an absence of an A1C value for a formal definition of prediabetes is likely to raise further concerns.

Evidence

A 2007 systematic review that compared the A1C test with the FPG test found both the A1C and the FPG tests to be equally effective as screening tools for type 2 diabetes. The A1C test, however, was reported as being a superior predictor of micro- and macrovascular complications. The higher initial costs of the A1C test were believed to be offset by its ability to predict the costly and preventable complications of diabetes. It was noted that neither the A1C test nor the FPG test adequately identified impaired glucose tolerance, and an OGTT would still be required for its detection.³

A systematic review of screening for undiagnosed diabetes found that the A1C test facilitated wider diabetes screening implementation. The screening for and treatment of undiagnosed impaired glucose tolerance with an OGTT was also recognized as a potentially cost-effective alternative, particularly when lifestyle modifications were used to manage identified cases.⁶

A 2009 study on type 2 diabetes testing in an Ontario population showed that the A1C test is already being used to screen for type 2 diabetes, in spite of its absence from Canadian Diabetes Association recommendations. An analysis of laboratory tests over 10 years revealed the A1C test to be the most commonly performed assay for people without diabetes. The study, by the Institute for Clinical Evaluative Sciences, proposed that the A1C test become the test of choice for diagnosis.⁷

Canadian Screening Strategies

Canadian screening efforts are mostly limited to passive case findings at family physician visits. Targeted screening strategies with the intent to promote early detection are, however, being piloted in regions across Canada: CANRISK⁸ consists of a 16-item survey that is designed to predict an individual’s 10-year risk of developing type 2 diabetes. During the piloting phase, an OGTT is also required. The Canadian risk prediction tool is modelled on a validated Finnish questionnaire called FINDRISC. The tool does not require laboratory tests unless a person is identified as being at high risk.

Because rates of diabetes among Aboriginal people in Canada are three to five times higher than those of the general Canadian population, more aggressive screening strategies are used. Through the Aboriginal Diabetes Initiative, a federally funded program, a range of culturally relevant health screening initiatives are supported. These include increased and regular screening for early diagnosis using population-based and opportunistic screening methods. Screening is carried out by mobile detection programs, through local health care providers and through partnerships with neighbouring provincial health care services.⁹

According to the Canadian Diabetes Association guidelines, population-based screening is unlikely to be cost-effective, because of the relatively low prevalence of the disease. However, overall cost saving may be realized in screening people with risk factors for type 2 diabetes or with diabetes-associated conditions. This is in keeping with the findings of a 2007 systematic review that concluded that screening for undiagnosed diabetes is cost-effective for people at high risk, specifically those between the ages of 40 and 70 years, and for people who are overweight or have other indicators of metabolic risk.6 A comparison of North American clinical practice guideline screening criteria is presented in Table 1.

Although there is evidence that the A1C test can better predict micro- and macrovascular complications and may facilitate wider diabetes implementation, evidence is missing on the costs and consequences of detecting type 2 diabetes by either targeted or population-based screening strategies. There is potential for health care savings where diabetic complications have been delayed or averted. Application of the A1C test and the refinement of the CANRISK prediction questionnaire may be additional tools for the early detection of type 2 diabetes in Canada.

TABLE 1: Comparison of Guideline Criteria for the Screening of
Type 2 Diabetes

IFG = impaired fasting glucose; IGT = impaired glucose tolerance.

¹ Canadian Diabetes Association: http://www.diabetes.ca/files/cpg2008/cpg-2008.pdf.

² Canadian Task Force on Preventive Health Care: http://www.ctfphc.org/Full_Text/Ch50full.htm.

³ US Preventive Services Task Force: http://www.annals.org/cgi/content/full/148/11/855?maxtoshow=&HITS=25&hits=25&RESULTFORMAT=1&searchid=1&FIRSTINDEX=0&sortspec=date&resourcetype=HWCIT.

⁴ American Diabetes Association: http://care.diabetesjournals.org/content/27/suppl_1/s11.full.

References

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