GLYCATED HEMOGLOBIN ASSAYS FOR THE ASSESSMENT OF DIABETIC CONTROL

THOMAS HIGGINS, MD

Since the DCCT Trial, a target average blood glucose of 155 mg% has been generally accepted as a desirable target for most persons with diabetes. It is important to realize, however that this target was set to be a level at which microvascular disease (neuropathy, nephropathy & retinopathy) was significantly impacted. It has been found that macrovascular disease (heart attacks, strokes & hardening of the arteries) continues to decrease significantly if the average blood sugar is further reduced toward normal.

Glycated hemoglobin assays are commonly used to asses the level of diabetic control and are directly related to the average glucose concentration in blood over a two to three month period of time. Glycated hemoglobin assays are generally reported as either % HbA1c which is one specific type of glycated hemoglobin or as % Glycosylated Hemoglobin which is made up of HbA1c and other glycated hemoglobin components. Since these tests are not measuring the same things, it is important to know which reporting format is being used.

The table below shows the relationship between average blood glucose, %HbA1c and % Glycosylated Hemoglobin.

% Hb A1C	Average Blood Glucose	% Glycosylated Hb
4	60 mg%	4.2
5	90 mg%	5.7
6	120 mg%	7.2
7	150 mg%	8.8
7.2	159 mg%	9.1
8	180 mg%	10.2
9	210 mg%	11.7
10	240 mg%	13.1
11	270 mg%	14.6
12	300 mg%	16.1

In general, the maximum normal average blood glucose is considered to be around 100 mg% and is not associated with diabetic complications. There is a small increase in the rate of diabetic microvascular complications from normal average glucose values to an average glucose of 155 mg%. The incidence of diabetic microvascular complications then rise at an increasing rate at higher average glucose values. Macrovascular complications begin increasing at levels of average blood glucose which are minimally elevated & progressively rise with increasing glucose. In fact, macrovascular disease is increased at blood glucose levels that are not high enough to be called diabetes. The association of insulin resistance in patients with glucose intolerance & Type 2 diabetes probably accelerates this progression because of direct effects of insulin resistance on other comorbid conditions associated with macrovascular disease.

Average blood glucose levels in the 155 range have traditionally been considered excellent. More recently, lower values are being recommended because of the significantly increased incidence of macrovascular disease at average glucose levels well below 155. In addition, more aggressive treatment of other comorbid conditions associated with macrovascular disease such as cholesterol levels & hypertension is also being recommended.

Glycated hemoglobin tests can be used to asses the level of your control during the testing period and also any trends in your control when compared serially. In general these tests are not recommended any more frequently than every 3 months.