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Cohort studies of patients with type 1 diabetes are rarely large unless they are compiled from more than one centre. The earliest report of patients with type 1 diabetes alone was from Pittsburgh in 1972 (Sultz etal., 1972) and since then there have been a number of further studies published from Pittsburgh, including a cohort study of 1966 patients with type 1 diabetes in 1984 (Dorman etal., 1984; Krolewski etal., 1987; Lloyd etal., 1996a). There have also been a number of studies of a similar size from Scandinavian countries (Deckert etal., 1979; Borch-Johnsen etal., 1986; Lounamaa etal.,1991; Laakso and Kuusisto, 1996).

To date, the largest study of patients with type 1 diabetes has come from the UK (Laing etal., 1999a, 1999b). The Diabetes UK Cohort Study (formerly British Diabetic Association Cohort Study) has followed over 23 000 patients with insulin-treated diabetes, recruited from separate registers across the UK. Both prevalent and incident cases were recruited. All had been diagnosed under the age of 30 years and were treated with insulin, and were therefore presumed to have type 1 diabetes. The first patients were recruited into the study in 1972, and recruitment continued until 1993. Although insulin treatment rather than evidence of absolute insulin deficiency was the criterion for inclusion, this cohort was considered to be essentially one of patients with type 1 diabetes. From the age-specific percentages of diabetic patients with type 1 diabetes (Laakso and Pyorala, 1985) it was estimated that at least 94% will have had type 1 diabetes.

A few international studies have compared complications and outcomes between countries. A four-country comparative study run by the Diabetes Epidemiology Research International Study Group has followed patients with type 1 diabetes from the USA, Finland, Israel and Japan (Diabetes Epidemiology Research International Study Group, 1995), and the WHO Multinational Study of Vascular Disease in Diabetes (which follows patients with both type 1 and type 2 diabetes) continues to report from 10 centres worldwide (Fuller etal., 2001; Morrish etal., 2001).

As it is more usual nowadays to distinguish between the two types of diabetes rather than group them together, it is tempting to draw comparisons. However, there are a number of difficulties in comparing studies of patients with type 1 and type 2 diabetes. Factors that must be taken into consideration include the relative ages of the two groups, the calendar period during which the data were collected, the endpoint chosen, together with the measurement used, and the population from which the cohort was selected.

As the patients with type 2 diabetes are diagnosed at an older age, usually over 45 years, there are very few age-specific studies of these patients and the patients are generally grouped together. As mortality is known to vary with age a comparison of type 1 and type 2 patients without any reference to age group would be flawed. To complicate things further, in a number of the type 1 studies there may be insufficient numbers to subdivide by age. Mortality is also known to vary with calendar period as lifestyles change or medical treatments improve and it would be difficult to draw any comparisons between results from two studies conducted 20 or 30 years apart. Studies may also differ in the type of endpoint that is measured, for example some may report mortality, others morbidity or a combination of the two. In addition these may be reported as a rate, a proportion, or a ratio relative to the underlying general population. The variation in mortality between countries further complicates international comparisons.

Despite these difficulties, it is only by drawing comparisons that the similarities and differences in CVD risk between type 1 and type 2 diabetes can be understood, which in turn might lead to a better understanding of the mechanisms by which CVD complications develop.

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