The rising prevalence of type 2 diabetes globally and its associated morbidity and mortality call for early preventive screening and case finding of high-risk individuals. At first presentation, 10% to 12% of cases have retinopathy and 5% have neuropathy, suggesting that diabetes was present long before diagnosis. Hypertension and accelerated atherosclerosis may also precede the diagnosis of frank diabetes by many years. Surveys indicate that diabetes is often underdiagnosed, and that future cases of diabetes may be predicted by abnormal glucose tolerance.

Impaired glucose tolerance (IGT) and impaired fasting glucose (IFG) are now recognized prediabetic states, with the potential to progress to frank type 2 diabetes. In Europe, the prevalence is ~10% for IGT and ~12% for IFG.

IGT is a more robust predictor of future diabetes, although rates of progression to diabetes vary with age, race, and degree of obesity. Generally IGT-to-diabetes progression is 3.6% to 8.7% per annum in middle-aged populations. Rates of progression are higher with positive repeat tests for IGT and inclusion of other risk factors (eg, body mass index). In combination, IGT and IFG increase the probability of progression to diabetes.6 Also, IGT, but not IFG, is an independent predictor of future CV disease events (hazard ratio ~1.3).

These data emphasize the need for early case-finding of type 2 diabetes and screening for IGT, when treatment of the metabolic abnormalities could reduce the severity of diabetic complications, or at least enable management of the disease.

The rationale for early screening
The philosophy behind screening is to prevent or delay disease onset and complications. The main evidence in support of screening was summarized by the Australian National Health and Medical Research Council:

• type 2 diabetes is a serious and costly health problem
• it has an asymptomatic phase prior to clinical presentation (prediabetes)
• treatment is available that reduces morbidity from complications.

How to screen
Despite the serious public health implications of type 2 diabetes, its overall prevalence does not justify universal testing. The potential for widespread intervention in the community-dwelling population has not yet been validated, and in developing countries, where the prevalence rates are very high, the costs of blanket screening would be prohibitive. Selective screening of high-risk groups is thus common policy, taking into consideration a number of risk factors for the development of diabetes (Table I).

TABLE I: High-risk candidates for regular and early diabetes screening. Adapted from the guidelines of the National Health and Medical Research Council of Australia, National Health Advisory Committee

An algorithm for the detection of undiagnosed type 2 diabetes and the prediabetes categories (IGT and/or IFG), shown in Figure 1, has been field-tested in Australia and Canada with positive outcomes.

FIGURE 1: Algorithm for the detection of undiagnosed type 2 diabetes and the prediabetes categories (IGT and/or IFG).

There are now clinically reliable, generally accepted diagnostic cut-off points for fasting and post-glucoseload blood glucose levels (Table II).

TABLE II: Diagnostic venous plasma glucose values for diabetes mellitus and other categories of hyperglycemia, using fasting and 2-hour post-glucose load samples.

IGT is a firm indication to implement preventive measures, but with IFG, a glucose tolerance test should be performed. Ideally, this should include fasting and 2-hour post-load values.6 Of note is that IFG may be present with normal 2-hour post-load blood glucose values, and in individuals with IGT or frank diabetes.

Screening in the primary health care setting

Clearly, most case detection is best conducted in the primary health care setting by general practitioners (GPs). They have the resources to identify high-risk cases, arrange laboratory tests, and supervise follow-up clinics for advice and treatment. Importantly, GPs can explain the screening procedure and obtain informed consent. The diagnosis of diabetes or its precursors may cause anxiety and there is the possibility that such a diagnosis could interfere with employment and insurance prospects. Hence, there is a need for thorough discussion and the patient’s approval. It is also important that screening programs should not be initiated unless structured follow-up and aftercare plans are in place.

Frequency of screening

Where possible, subjects in high-risk groups should be checked every 3 to 5 years, depending on available resources. Individuals with IFG or IGT should be screened annually. Information on the prevention of type 2 diabetes should be given to all patients identified by screening, and this advice should be reiterated at regular checkup appointments.

Conclusion

Screening programs are the essential first step in reducing the serious health care burden posed by the steep global rise in diabetes. The identification of individuals prone to developing frank diabetes in the future must be a key priority in clinical practice today. Early pharmacotherapeutic intervention will allow prevention or amelioration of the diabetic complications that are so costly both to individuals and health care systems. Initiation and support of screening programs is therefore of the utmost importance.