We diagnose diabetes on the basis of a Fasting Plasma Glucose greater than 7.0 mmol/L (125 mg/dL) or a Casual Plasma Glucose over 11.0 mmol/L (200 mg/dl). The reason for choosing these levels is that above these levels there is an increasing incidence of microvascular complications of diabetes.
The complications of diabetes involve the vascular system and are of 2 types microvascular (small vessel) complications and macrovascular (large vessel) complications.
The microvascular complications are those which lead to Retinopathy (eye disease), Nephropathy (kidney disease) and Neuropathy (nerve disease). The microvascular complications of diabetes do not occur at glucose levels below the diagnostic level for diabetes but with fasting glucose over 7 mmol/L or casual glucose over 11 mmol/L, the occurrence and severity of microvascular complications is proportional to the degree of glucose elevation and the time that this elevation has persisted. The goals of treatment are therefore to maintain glucose levels below a fasting level of 7 mmol/L or a post meal less than 11 mmol/L (this latter goal is likely to be lowered to 7.8 mmol/L at the next revision of the Canadian Diabetes Association's Guidelines in 2003).
Macrovascular complications are those involving the large blood vessels, leading to stroke, heart attacks and Peripheral Arterial Disease. While tight glucose control reduces the frequency & severity of macrovascular complications, we have been less successful in reducing the frequency of these complications with glycemic control alone. The Honolulu Heart Study and the San Antonio Study have shown that in order to significantly reduce the macrovascular complications we need to get glucose values down to fasting levels around 4.0 mmol/L and post meal levels of 5.2 mmol/L. Since we can rarely achieve these levels, diabetics remain at very high risk for heart attack or stroke. Steven Haffner from San Antonio Texas showed (NEJM, 1998) that the risk of an MI in a diabetic patient with no previous history of heart disease is higher that the risk in a non-diabetic who has already had a heart attack Because of the risk which is 2-4 times the risk in non diabetics, we treat hyperlipidemia to secondary intervention criteria which are the highest standards of prevention. Since macrovascular disease starts at much lower glucose levels than microvascular disease up to 50% of patients already have cardiovascular disease by the time that they are diagnosed with diabetes.