New classification and criteria for diagnosis of diabetes mellitus

Position Statement from the Australian Diabetes Society^*, New Zealand Society for the Study of Diabetes^†, Royal College of Pathologists of Australasia^‡ and Australasian Association of Clinical Biochemists^§.

Peter G Colman^*, David W Thomas^‡, Paul Z Zimmet^*, Timothy A Welborn^*, Peter Garcia-Webb^§ and M Peter Moore^†.

First published in the Medical Journal of Australia (MJA 1999, 170: 375-378). Reprinted with permission.

Introduction

Recently, there has been major growth in knowledge about the aetiology and pathogenesis of different types of diabetes and about the predictive value of different blood glucose levels for development of complications. In response, both the American Diabetes Association (ADA) and the World Health Organization (WHO) have re-examined, redefined and updated the classification of and criteria for diabetes, which have been unchanged since 1985. While the two working parties had cross-representation, they met separately, and differences have emerged between their recommendations.

The ADA published its final recommendations in 1997¹, while the WHO group published its provisional conclusions for consultation and comment in June 1998².

The WHO process called for comments on the proposal by the end of September 1998, with the intention of finalising definitive classification and criteria by the end of December 1998 and of publishing these soon thereafter. However, WHO publications need to go through an internal approval process and it may be up to 12 months before the final WHO document appears.

A combined working party of the Australian Diabetes Society, New Zealand Society for the Study of Diabetes, Royal College of Pathologists of Australasia and Australasian Association of Clinical Biochemists was formed to formulate an Australasian position on the two sets of recommendations and, in particular, on the differences between them. This is an interim statement pending the final WHO report, which will include recommendations on diabetes classification as well as criteria for diagnosis. We see it as very important to inform Australasian health professionals treating patients with diabetes about these changes.

What are the new diagnostic criteria?

The new WHO criteria for diagnosis of diabetes mellitus and hyperglycaemia are shown in Box 1. The major change from the previous WHO recommendation³ is the lowering of the diagnostic level of fasting plasma glucose to ≥7.0 mmol/L, from the former level of ≥7.8 mmol/L. For whole blood, the proposed new level is ≥6.1 mmol/L, from the former ≥6.7 mmol/L.

This change is based primarily on cross-sectional studies demonstrating the presence of microvascular⁴ and macrovascular complications at these lower glucose concentrations. In addition, the 1985 WHO diagnostic criterion for diabetes based on fasting plasma glucose level (≥7.8 mmol/L) represents a greater degree of hyperglycaemia than the criterion based on plasma glucose level two hours after a 75g glucose load (≥11.1 mmol/L)⁶. A fasting plasma glucose level of ≥7 mmol/L accords more closely with this 2 h post-glucose level.

Recommendation: The ADA and the WHO committee are unanimous in adopting the changed diagnostic level, and the Australasian Working Party on Diagnostic Criteria recommends that healthcare providers in Australia and New Zealand should adopt it immediately.

Clinicians should note that the diagnostic criteria differ between clinical and epidemiological settings. In clinical practice, when symptoms are typical of diabetes, a single fasting plasma glucose level of ≥7.0 mmol/L or 2 h postglucose or casual postprandial plasma glucose level of ≥11.1 mmol/L suffices for diagnosis. If there are no symptoms, or symptoms are equivocal, at least one additional glucose measurement (preferably fasting) on a different day with a value in the diabetic range is necessary to confirm the diagnosis. Furthermore, severe hyperglycaemia detected under conditions of acute infective, traumatic, circulatory or other stress may be transitory and should not be regarded as diagnostic of diabetes. The situation should be reviewed when the primary condition has stabilised.

In epidemiological settings, for study of high-prevalence populations or selective screening of high-risk individuals, a single measure – the glucose-level 2 h post glucose load – will suffice to describe prevalence of impaired glucose tolerance (IGT).

Box 1: Values for diagnosis of diabetes mellitus and other categories of hyperglycaemia²

What about the oral glucose tolerance test?

Previously, the oral glucose tolerance test (OGTT) was recommended in people with a fasting plasma glucose level of 5.5–7.7 mmol/L or random plasma glucose level of 7.8–11.0 mmol/L. After a 75g glucose load, those with a 2 h plasma glucose level of <7.8 mmol/L were classified as normoglycaemic, of 7.8–11.0 mmol/L as having IGT and of ≥11.1 mmol/L as having diabetes.

The new diagnostic criteria proposed by the ADA and WHO differ in their recommendations on use of the OGTT. The ADA makes a strong recommendation that fasting plasma glucose level can be used on its own and that, in general, the OGTT need not be used¹. The WHO group² argues strongly for the retention of the OGTT and suggests using fasting plasma glucose level alone only when circumstances prevent the performance of the OGTT.

There are concerns that many people with a fasting plasma glucose level <7.0mmol/L will have manifestly abnormal results on the OGTT and are at risk of microvascular and macrovascular complications. This has major ramifications for the approach to diabetes screening, particularly when the Australian National Diabetes Strategy proposal⁷, launched in June 1998 by Dr Michael Wooldridge, then Federal Minister for Health and Aged Care, has early detection of type 2 diabetes as a key priority.

Recommendation: The Australasian Working Party on Diagnostic Criteria has major concerns about discontinuing use of the OGTT and recommends that a formal recommendation on its use in diabetes screening be withheld until the final WHO recommendation is made. However, in the interim, the OGTT should continue to be used.

Diabetes in pregnancy

The ADA has retained its old criteria for diagnosis of gestational diabetes¹.These differ from those recommended by both WHO² and the Australian Working Party on Diabetes in Pregnancy⁸ and are generally not recognised outside the United States. The new WHO statement retains the 1985 WHO recommendation that both IGT and diabetes should be classified as gestational diabetes. This is consistent with the recommendations of the Australasian Diabetes in Pregnancy Society, which recommended a diagnostic 2 h venous plasma glucose level on the OGTT of ≥8.0 mmol/L. In New Zealand, a cut-off level of ≥9.0 mmol/L has been applied⁸.

How has the classification of diabetes changed?

The proposed new classification encompasses both clinical stages and aetiological types of hyperglycaemia and is supported by numerous epidemiological studies. The classification by aetiological type (Box 2 above) results from new knowledge of the causes of hyperglycaemia, including diabetes. The terms insulin-dependent and non-insulin dependent diabetes (lDDM and NIDDM) are eliminated and the terms type 1 and type 2 diabetes retained. Other aetiological types, such as diabetes arising from genetic defects of β-cell function or insulin action, are grouped as ‘other specific types’, with gestational diabetes as a fourth category.

The proposed staging (Box 3 below) reflects the fact that any aetiological type of diabetes can pass or progress through several clinical phases (both asymptomatic and symptomatic) during its natural history. Moreover, individuals may move in either direction between stages.

Impaired glucose tolerance and impaired fasting glycaemia

Impaired glucose tolerance (IGT), a discrete class in the previous classification, is now categorised as a stage in the natural history of disordered carbohydrate metabolism. Individuals with IGT are at increased risk of cardiovascular disease, and not all will be identified by fasting glucose level.

Box 3:

In reducing the use of the OGTT, the ADA recommended a new category – impaired fasting glycaemia (IFG) – when fasting plasma glucose level is lower than that required to diagnose diabetes but higher than the reference range (<7.0 mmol/L but ≥6.1 mmol/L). Limited data on this category show that it increases both risk of progressing to diabetes⁹ and cardiovascular risk⁵. However, data are as yet insufficient to determine whether IFG has the same status as IGT as a risk factor for developing diabetes and cardiovascular disease and as strong an association with the metabolic syndrome (insulin resistance syndrome).

IFG can be diagnosed by fasting glucose level alone, but if 2 h glucose level is also measured, some individuals with IFG will have IGT and some may have diabetes. In addition, the number of people with OGTT results indicating diabetes but fasting plasma glucose level <7.0 mmol/L is unknown, but early data suggest there may be major variation across different populations¹⁰. A number of studies, including the DECODE initiative of the European Diabetes Epidemiology Group, have reported that individuals classified with IFG are not the same as the IGT group^11-15. The European Group believes that, on available European evidence, the ADA decision to rely solely on fasting glucose level would be unwise.

Recommendation: The Australasian Working Party on Diagnostic Criteria recommends immediate adoption of the new classification. However, clinicians should be aware that some cases of diabetes will be missed unless an OGTT is performed. Thus, if there is any suspicion or other risk factor suggesting glucose intolerance, the working party continues to recommend use of an OGTT pending the final WHO recommendation.

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