Hemoglobin Ac

The Big Diabetes Lie

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A single blood glucose measurement is necessary to tell you how much insulin to give for the next meal and whether the insulin you gave your child previously was sufficient. However, it doesn't give you the big picture. Even four blood glucose measurements a day say little about overall diabetic control because blood glucose can and does change within minutes; you're looking at four points in time, but the blood glucose may be very different the rest of the time, especially during the long period of sleep when no measurements are made at all. The test that gives you the big picture is called the hemoglobin A1c test. It looks at the last three months and gives you an average of all the ups and downs in blood glucose levels. (To understand how it does this, see the nearby sidebar, "What the hemoglobin A1c test measures.")

At the beginning, the hemoglobin A1c tells you just how out-of-control your child's glucose was when the disease began. Every three months after that, it tells you how close he's coming to a level of control associated with avoiding the complications of diabetes that I discuss in Chapters 4 and 5.

Targeting certain levels

Figure 7-1 compares the hemoglobin A1c with the whole blood glucose and the plasma glucose (which is important to know because the plasma glucose is what you're measuring multiple times a day, not the hemoglobin A1c).

Figure 7-1:

Hemoglobin A1c compared to whole blood glucose and plasma glucose.

Blood Glucose Control Chart

Hemoglobin A1c (%) 5 6 7 8 9 10 11 12 13 14

Normal

Good

Fair

Poor

90 120 150 180 210 240 270 300 330 360 Average Whole Blood Glucose (mg/dl)

90 120 150 180 210 240 270 300 330 360 Average Whole Blood Glucose (mg/dl)

100 140 170 210 240 280 310 350 380 420 Average Plasma Glucose (mg/dl)

What the hemoglobin A1c test measures

Hemoglobin A1c is the largest of three subfractions of the protein hemoglobin, the carrier of oxygen in the bloodstream. As hemoglobin is being formed, it binds to glucose to form glyco-hemoglobin. Glycohemoglobin ordinarily makes up 6 percent of the total hemoglobin in the blood. The glycohemoglobin takes three forms: hemoglobin A1a, A1b, and A1c. Because hemoglobin A1c is the largest subfraction of the three, it's the easiest to measure. The more glucose there is in the blood, the more glycohemoglobin will be formed. As it's formed, hemoglobin is transferred into red blood cells where it remains for 60 to 90 days. So by measuring the hemoglobin A1c in a sample of red blood cells, you can determine how much glucose was circulating in the blood over the last two to three months.

A hemoglobin A1c of 6 percent, considered normal, corresponds to an average whole blood glucose of under 120 mg/dl. A good hemoglobin A1c reading of 7 percent corresponds to an average whole blood glucose of 150 mg/dl. A reading of 8 percent, corresponding to an average whole blood glucose level of 180 mg/dl, is considered fair; anything above that figure is considered to be poor. In terms of plasma glucose, a good hemoglobin A1c of 7 percent corresponds to a plasma glucose of 170 mg/dl. A reading of 8 percent means the plasma glucose is 210 mg/dl.

For very young children, whose brain cells are still developing, it's advisable that the hemoglobin A1c be kept a little higher, perhaps up to 8 percent, until they've reached the age of 2. That's when low blood glucose levels will no longer damage their brains.

During puberty, as a result of the brain's increased production of growth hormone, it's extremely difficult to keep the hemoglobin A1c below 8 percent. Your teenager is considered to have poor control of his glucose if his hemoglobin A1c is 9 percent or higher.

The American Diabetes Association recommends that the goal of therapy for adults and adolescents with diabetes should be a hemoglobin A1c of 7 percent or less. This was the goal of the Diabetes Control and Complications Trial that was associated with a tremendous decrease in microvascular complications like eye disease, kidney disease, and nerve disease (see Chapter 5). However, for long-term prevention of macrovascular complications including heart attacks and strokes, a hemoglobin A1c of 6.5 percent is recommended.

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