Evolution In Diagnostic Criteria For Diabetes

The question of diagnostic criteria for type 1 diabetes does not usually give risk to much debate because of its clear acute-onset phenotype, and the logical link between aetiology (lack of insulin) and treatment modality. However, the recognition of a non-insulin-dependent form of diabetes, in which there was a much less clear distinction between normality and disease, created a need for diagnostic criteria. Classification with only the help of symptoms and clinical signs was soon regarded as unsatisfactory. Another major impetus for the development of diagnostic criteria was the recognition that the absence of standardisation was an obstacle to epidemiological and clinical research.

In 1964, the World Health Organisation (WHO) convened an Expert Committee on Diabetes Mellitus which attempted to provide a universal classification of the diabetes syndrome. But it was not until 1980 that an international accepted classification was established.

Two international work groups, the National Diabetes Data Group (NDDG) of the National Institutes of Health, USA in 19795, and the World Health Organisation (WHO) Expert Committee on Diabetes in 19806 proposed and published similar criteria for diagnosis and classification. They both recognised that diabetes was an aetiologically and clinically heterogeneous group of disorders with hyperglycaemia shared in common.

The NDDG/WHO classification system incorporated data from research conducted during the previous decades and set the path for unifying nomenclature, diagnostic criteria and requirements such as the amount of oral glucose load used. The inclusion of impaired glucose tolerance (IGT) into the diabetes classification followed the recognition that a zone of diagnostic uncertainty existed in the oral glucose tolerance test (OGTT) between what was clearly normal glucose tolerance and diabetes.

However, controversy continued around the criteria, and minor changes to the classification took place in 1985, letting the WHO slightly modify its criteria to coincide more with the NDDG values, whereas NDDG later modified the diagnostic requirements by dropping the intermediate sample during the OGTT, to be identical with the WHO recommendations. With the 1985

WHO Study Group classification7, a number of clinical classes of diabetes were agreed upon, which included the major two as insulin-dependent diabetes mellitus (IDDM) and non-insulin dependent diabetes mellitus (NIDDM). The terms type 1 and type 2 diabetes were omitted in the 1985 revision and Malnutrition-related Diabetes Mellitus (MRDM) was introduced as a new class. The other important classes were retained from the 1980 document as Other Types and Impaired Glucose Tolerance (IGT) as well as Gestational Diabetes Mellitus (GDM). These classifications were widely accepted for the next decade and represented a compromise between a clinically and an aetio-logically based system. This had the advantage that cases where the specific cause or aetiology was unknown could still be classified according to their clinical presentation.

Recent changes to the classification of diabetes

Data from genetic, epidemiological and aetiologic studies continued to accumulate throughout the 1980s and 1990s, and the understanding of the aetiology and pathogenesis of diabetes improved. With the application of universal criteria for diagnosis and standardised testing procedures, estimation of the global burden of diabetes became possible8.

Nevertheless, calls continued to revisit the NDDG and WHO recommendations to further fulfil the aims set out by the NDDG in 1979 in order to take into account the dynamic phasic nature of diabetes9. Advances such as the use of immunological markers of the type 1 diabetes process suggested that the clinical classification of diabetes into IDDM and NIDDM was unsatisfactory. Equally, age of diabetes onset was increasingly regarded as a confounding factor in the classification, rather than its basis. Frequently clinicians observed autoimmune forms of diabetes among adults and diabetes with features of NIDDM in adolescents. In addition, many adult patients with NIDDM were well controlled for several years with diet and oral hypoglycaemic agents but needed insulin later in the course of their disease. At a time when auto-antibody measurement was not available, it would have remained uncertain if these patients had type 2 with a progressive insulin insufficiency or if they had slowly progressing type 1 diabetes. With the help of the immune markers many patients previously classified as type 2 were then reclassified as having type 1 diabetes10. As an indicator of this transitional time, terms such as type llA entered the literature.

A thought-provoking attempt at keeping staging of glucose intolerance separate from (sub-) classification according to aetiological type was proposed by Kuzuya and Matsuda for a new classification of diabetes mellitus11. Their concept sought to separate the criteria related to aetiology and those related to the degree of deficiency of insulin or insulin action and to define each patient on the basis of these two criteria. This concept was taken on by the American

Diabetes Association's expert group, which has convened since 1995 to review the literature and determine what changes to the classification were necessary. WHO also convened a Consultation in December 1996 to consider the issues and examine the available data, and a provisional report was published in 199812, which was adopted with minor modifications in 199913.

This latest classification is based on stages of glucose tolerance with a complementary sub-classification according to the aetiological type. Diabetes mellitus is defined as a group of metabolic diseases characterised by hyper-glycaemia resulting from defects in insulin secretion, insulin action, or both. Common to all types of diabetes mellitus is chronic hyperglycaemia, which is associated with long-term damage, dysfunction and failure of various organs, especially the eyes, kidneys, nerves, heart and blood vessels. Hyperglycaemia can be sub-categorised regardless of the underlying cause, into:

• Insulin required for survival (includes the former IDDM).

• Insulin required for control - i.e. for metabolic control, not for survival (includes the former insulin treated NIDDM).

• Not insulin requiring, i.e. treatment by non-pharmacological methods or drugs other than insulin (includes NIDDM on diet alone /or combined with oral agents).

• Impaired Glucose Tolerance (IGT) and/or impaired fasting glycaemia (IFG).

• Normal glucose tolerance.

In this new classification, stages reflecting the various degrees of hyper-glycaemia are set across the disease processes which may lead to diabetes mellitus in an individual (Figure 2.1). In all circumstances it should now be possible to categorise each individual with diabetes mellitus according to clinical stage. The stage of glycaemia may change over time depending on the extent of the underlying disease processes, and impact of therapeutic glucose control. The presence of hyperglycaemia is not an essential consequence of the underlying disease process because in certain situations this may not have progressed far enough to cause high levels of blood glucose. Reframing the classification in terms of aetiology allows identification of the defect or process which leads to diabetes. For instance, the presence of islet cell antibodies makes it likely that a person has the type 1 autoimmune process even at a time when they are normoglycaemic. In contrast to type 1 and type 3 diabetes (Other Specific Types) there are still few sensitive or highly specific indicators of the type 2 process at present, although these are likely to become apparent as aetiological research progresses.

The possibility that individuals may change over time across glycaemic stages in both directions reflects the observation that in some individuals with diabetes, adequate glycaemic control can be achieved with weight reduction, exercise and/or oral agents. These individuals, therefore, do not require insulin

Stages

Types

Normoqlycaemia Normal glu cose tolerance

Hyperglycaemia

IGT and/or IFG

Diabetes Mel ¡tus

Not insulin requiring

Insulin: for control

Insulin: for survival

Diabetes Sustenance

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