Volume 23 Supplement 2
Data, Results, and Consequences of Major Trials With Focus on Type 2 Diabetes
Proceedings from a Symposium
Type 2 Diabetes Worldwide According to the New Classification and Criteria
Jonathan E. Shaw, MRCP
Two major reports have recently revised the classification of and diagnostic criteria for diabetes. Classification was previously based on the need for insulin (insulin-dependent or noninsulin-dependent), but this has become increasingly confusing. Now, the type of diabetes is determined by the etiological process rather than the treatment modality. Type 1 diabetes is thus characterized by islet cell destruction and type 2 diabetes by a combination of defects in insulin secretion and action. An individual with either type of diabetes may be on any treatment modality. This classification should prove to be more logical and, for example, allow latent autoimmune diabetes in adults, which typically does not require insulin at presentation, to be classified as type 1 diabetes. The fasting plasma glucose diagnostic threshold for diabetes has been lowered to 7.0 mmol/l (126 mg/dl), and impaired fasting glucose (fasting plasma glucose 6.16.9 mmol/l [110125 mg/dl]) has been introduced as a new category of intermediate glucose metabolism. These changes recognize that the old fasting threshold did not match the 2-h (postload) threshold well and that both micro- and macrovascular disease develop at lower fasting glucose levels than previously recognized. Although the prevalences of diabetes according to the new fasting and 2-h criteria are now similar in most populations, the actual individuals identified as having diabetes are often different. Over 30% of all those with diabetes have a nondiabetic fasting glucose but still have increased cardiovascular mortality. Thus, it is important to retain the oral glucose tolerance test for the diagnosis of diabetes.
Diabetes Care 23 (Suppl. 2):B5B10, 2000
Type 2 diabetes affects large numbers of people from a wide range of ethnic groups and at all social and economic levels throughout the world. Currently, at least 120 million people suffer from type 2 diabetes, but by the year 2010, 220 million people are projected to have the disease (1). Diabetes is among the five leading causes of death in most countries (2), yet mortality statistics greatly underestimate the true rate of diabetes-related mortality because diabetes is frequently underreported on death certificates (2,3). Therefore, diabetes is often ignored when public health priorities are set, and quite apart from the health impact, the economic cost of diabetes and its complications is enormous for health care, and loss of productivity to society is great (2).
The extraordinary high prevalence of type 2 diabetes reported over 20 years ago in the Pima Indians (4) and the Micronesian Nauruans in the Pacific (5) and then later in other Pacific and Asian island populations (68) suggested the potential for a future diabetes epidemic. This scenario proved to be correct, and type 2 diabetes has now reached epidemic proportions in many developing nations (9) as well as in disadvantaged minorities in the developed countries. Examples of the latter include Australian Aboriginals and Torres Strait Islanders (10); Native, African-, and Mexican Americans (11); migrant Asian Indians (8); and some groups in the Middle East Arab states (12).
There is every reason to expect that over the next decade, the epidemic of type 2 diabetes will continue to escalate (1). Inevitably, diabetes and its complications will emerge as one of the major threats to future public health resources throughout the world at a huge economic and social cost, particularly in developing countries (13).
Against this global epidemic, changes proposed for the classification and diagnostic criteria for diabetes become very important not only for individual epidemiological studies, but also for global comparisons of incidence and prevalence. Therefore, the recommendations of the 1997 American Diabetes Association (ADA) report on classification and diagnostic criteria (14), followed by the 1998 provisional World Health Organization (WHO) recommendations (15), will have important implications. The outcomes of the changes should be better management of diabetes, due both to earlier diagnosis and better classification of cases for more appropriate therapy.
CHANGES IN CLASSIFICATION In 1979 and 1980, both the U.S. National Diabetes Data Group (16) and WHO (17) tackled the task of classification and criteria of diabetes. The terms "juvenile-onset diabetes" and "maturity-onset diabetes" were replaced by insulin-dependent diabetes mellitus (IDDM) or type 1 diabetes and non-insulin-dependent diabetes mellitus (NIDDM) or type 2 diabetes. The category impaired glucose tolerance (IGT) was also created. This classification was based on the patient's need (or lack thereof) of insulin for survival and potential etiologic mechanisms. Because heterogeneity exists in both type 1 and type 2 diabetes, the general applicability of the WHO classification has had limitations. The advantage of the latest ADA/WHO classification (Fig. 1), which results from new knowledge of the causes, is that it combines both clinical stages of hyperglycemia and the etiological types. The actual staging reflects that any etiological type can pass or progress through several clinical phases (both normoglycemic and hyperglycemic) during its natural history. Moreover, individuals may move in either direction, from stage to stage. The type of diabetes, however, is now determined by the etiological process rather than the treatment modality.
The etiological types represent processes that may result in diabetes (e.g., -cell destruction results in type 1 diabetes) where insulin is required for survival to prevent the development of ketoacidosis. Type 2 diabetes results from insulin resistance and/or abnormal insulin secretion, either of which may predominate. In many instances, the specific molecular or metabolic causes are not yet known. However, as with diabetes resulting from specific gene mutations, e.g., glucokinase or mitochondrial DNA, when eventually determined, they will be reclassified under "other specific types," such as genetic, endocrine, drug-induced, etc. (15).
The new classification proposes that hyperglycemia, regardless of the underlying cause, can be subcategorized as follows:
The designation of diabetes type is no longer dependent on which of these categories an individual is in, although the majority of people with type 1 diabetes require insulin for survival, and the majority of those with type 2 diabetes fall into the other two categories. This classification will hopefully remove the confusion between insulin-dependent and insulin-treated.
IGT and fasting
diabetes in adults
CHANGES IN DIAGNOSTIC CRITERIA The diagnostic criteria for diabetes have changed on a number of occasions over recent decades. In 1985, the WHO published diagnostic thresholds based on the 75-g oral glucose tolerance test (OGTT), which became the gold standard over subsequent years (20). These diagnostic cut points were based on the threshold for development of retinopathy, the glucose values that divide a bimodal distribution of glucose, and the glucose levels at which insulin levels start to decline. However, it has always been recognized that the glucose values that are chosen as diagnostic cut points are to a certain extent arbitrary because the distribution of blood glucose is continuous. Epidemiological data collected over the last decade recently led both the WHO and the ADA to review the diagnostic criteria (14,15). The new information related to the fasting plasma glucose (FPG) threshold (previously 7.8 mmol/l [140 mg/dl]) rather than to the 2-h plasma glucose (PG) threshold and indicated the following:
These changes update the criteria to match the relevant data, although some data from Asian populations have suggested that the FPG equivalent to a postglucose loading PG of 11.1 mmol/l (200 mg/dl) may be even lower than 6.0 mmol/l (108 mg/dl) (27). Consequent on these amendments to the fasting criteria, the ADA (but not the WHO) indicated that FPG rather than the OGTT should be the diagnostic test of choice both for clinical and epidemiological purposes. The fasting glucose has the advantage that it is considerably more reproducible than the 2-h PG and is simpler to perform than the OGTT.
The requirement to confirm an abnormal test on a different day in an asymptomatic individual is retained by both the ADA and WHO. The impact of the changes is complex. They affect both diabetes and the intermediate states of carbohydrate metabolism: IGT and IFG.
Furthermore, even when the total prevalence is similar by the two methods, the actual people identified by screening may be different. The percentage of individuals classified as having diabetes by the new FPG cutoff who also have diabetes according to the old criteria varies from only 41% (Diabetes Epidemiology: Collaborative Analysis of Diagnostic Criteria in Europe [DECODE]) to 91% (Japanese-Brazilians and residents of Hong Kong). It is not clear what drives this variation in the degree of classification disagreement, but in the DECODE Study of 16 different European populations (29), obese diabetic individuals were more likely to satisfy the fasting criterion, and nonobese diabetic individuals were more likely to satisfy the 2-h criterion. It is possible that ethnicity is also important, but there are not yet enough data to assess this. This change in the individuals who are identified raises the further question of whether changing to the new fasting threshold will alter the phenotype of diabetes or the associations between risk factors (such as obesity) and the subsequent development of diabetes. These issues are currently unresolved, although the Third National Health and Nutrition Examination Survey (NHANES III) shows that the mean HbA1c is higher in the group diagnosed by the new fasting criterion than in the group diagnosed by the 1985 WHO OGTT criteria (31).
The association of hyperglycemia and cardiovascular disease is a crucial one on which to test the validity of the new criteria. The Hoorn Study has shown that people with an FPG >7.0 mmol/l (126 mg/dl) (i.e., diabetic) but a nondiabetic 2-h PG have an abnormal cardiovascular risk profile (28), and data from two large cohorts of men with a nondiabetic 2-h glucose value at baseline showed an increased 20-year mortality when the baseline FPG was >7.0 mmol/l (126 mg/dl) (38). Similarly, evidence would now indicate that people whose only abnormality is in the postload state have elevated blood pressure and lipids (28) and have a higher cardiovascular mortality than their nondiabetic counterparts (39). Thus, isolated abnormalities in either the fasting or the postload state have important associations with cardiovascular disease, and while lowering of the fasting threshold will help to identify one group, abandonment of the OGTT may prevent or delay diagnosis in the other group. Cardiovascular risk, however, demonstrates again the arbitrary nature of glucose cut points. Blood glucose represents a continuous cardiovascular risk factor extending well into the normal range (40) and interacts with other risk factors.
Will the new criteria influence the total number of people in whom diabetes is actually diagnosed (rather than the prevalences estimated by epidemiological studies), and will an increased detection rate reduce morbidity and mortality? The simple lowering of the fasting threshold alone should increase the number of people who are classified as having diabetes. However, when lowering of the threshold is combined with a switch from using the OGTT to just using the FPG, it can be seen from Table 1 that the overall effect can be either to increase or decrease the prevalence as determined by epidemiological studies.
In routine clinical practice, the OGTT is rarely used, and in this setting, the number of people diagnosed should increase as a result of lowering the fasting threshold. However, the degree to which it will increase is unclear and probably depends more on increasing the number of people screened (by whichever diagnostic test) than it does on altering the parameters of the screening test. It is worth noting in this context that the proportion of diabetes that is undiagnosed is directly influenced by the diagnostic test and the thresholds used. NHANES III data show that when using the 1985 WHO criteria, this proportion was 44%, but was only 35% when using the new fasting criterion (41). Thus, future diagnostic rates will need to be analyzed and interpreted with caution.
Diagnosing type 2 diabetes earlier in its natural history can only be beneficial if therapy reduces mortality and morbidity. No study has specifically addressed this question of the long-term benefit of screening, but in a computer-simulated model, opportunistic screening for diabetes was shown to be cost-effective (42). Furthermore, the UKPDS has recently shown that intensive glucose-lowering therapy reduces the risk of microvascular disease (43) and that in obese diabetic patients, metformin reduces mortality and myocardial infarction (44).
IFG and IGT
In a further twist to the complexity of changing the diagnostic criteria, the introduction of IFG requires that the prediction of subsequent diabetes among people with either IFG or IGT be reassessed. One of the original reasons for introducing IGT was that as well as increasing the risk of cardiovascular disease, it carried a high risk of progressing to diabetes, and this has been confirmed in a number of different studies (46). Therefore, IGT serves as an important risk factor for an individual. In addition, its prevalence can be used to suggest future changes in the prevalence of diabetes within a population (47), and it is being used to identify subjects suitable for diabetes prevention trials. Can IFG perform a similar role?
We have recently reported data from a prospective population-based study in Mauritius (48) addressing this issue. From a baseline population of 3,229 subjects, 609 had IGT and 267 had IFG, with only 118 of these subjects having both IFG and IGT. After 5 years of follow-up, 297 subjects had progressed to diabetes, of whom 148 had IGT at baseline, but only 77 had IFG at baseline (Fig. 2). Therefore, the ability of IFG to identify those who would progress to diabetes (sensitivity) was considerably less than that of IGT. However, for those people with IFG, the risk of progression (positive predictive value) was slightly higher for IFG than for IGT (29 vs. 24%). This difference between IFG and IGT should not be seen as necessarily indicating an absolute difference between the predictive properties of FPG and 2-h PG, but merely between the two categories as currently defined. Further work in other populations will be needed to assess the prognostic implications of IFG and to determine what advantage, if any, can be gained from changing the diagnostic limits of IFG.
SUMMARY An improved understanding of the pathophysiology of diabetes has now allowed its classification to be based on etiological processes, not the form of therapy. Although a precise classification may not always be possible in clinical settings, the new system should lead to less confusing and more logical terminology.
Lowering the FPG threshold for diagnosing diabetes recognizes recent advances in the understanding of glycemic thresholds with respect to the risk of complications. It also makes the diagnosis of type 2 diabetes easier by putting less emphasis on the OGTT. However, this change (especially if the OGTT is abandoned) will have unpredictable effects on measurements of diabetes prevalence, will identify different individuals, and will make it difficult to compare old studies with new ones. In addition, IFG requires further study before its significance is fully understood. More prospective data from a variety of populations looking at both microvascular and macrovascular outcomes in relation to fasting and postload glucose values are still needed to clarify the diagnostic thresholds.
Acknowledgments This work was partially supported by U.S. National Institutes of Health Grant DK-25446. J.E.S. is supported by a grant from the Institute for Diabetes Discovery, Branford, Connecticut.
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From the International Diabetes Institute and World Health Organization Collaborating Centre for the Epidemiology of Diabetes Mellitus, Melbourne, Victoria, Australia.
Address correspondence and reprint requests to Jonathan E. Shaw, MRCP, Specialist Registrar, Department of General Medicine, Wythenshawe Hospital, Southmoor Road, Wythenshawe, Manchester M25 9LT, England. E-mail: firstname.lastname@example.org.
Received for publication 8 July 1999 and accepted in revised form 19 November 1999.
Abbreviations: ADA, American Diabetes Association; DECODE, Diabetes Epidemiology: Collaborative Analysis of Diagnostic Criteria in Europe; FPG, fasting plasma glucose; ICA, islet cell antibodies; IFG, impaired fasting glucose; IGT, impaired glucose tolerance; LADA, latent autoimmune diabetes in adults; NHANES III, Third National Health and Nutrition Examination Survey; OGTT, oral glucose tolerance test; PG, plasma glucose; UKPDS, U.K. Prospective Diabetes Study; WHO, World Health Organization.
A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances.
This article is based on a presentation at a symposium. The symposium and the publication of this article were made possible by an unrestricted educational grant from Aventis Pharma.
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