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Diabetes Myths #10 - HbA1c is an Average |
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Written by Nige
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Thursday, 03 June 2010 07:21 |
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Its quite common for people to refer to the HbA1c as an average of their blood glucose but this isn't strictly true and can be quite misleading.
The first hint of this is that the HbA1c is reported as a percentage e.g. 6.0% wheras blood glucose readings are reported as mmol/l.
For example, if my Blood Glucose (BG) is 5.9mmol/l for an hour, then 6.1mmol/l for an hour, then my average over the 2 hours would be 6.0mmol/l. It would not be 6.0%. In the same way, if you have 2 apples weighing 49 grams and 51 grams, the average weight is 50 grams, not 50%.
Recently there was a change to reporting HbA1c in two ways - the old percentage figures and a new figure in mmol/mol. So if your HbA1c were 6.0% then it would also be reported as 42mmol/l.
So does that mean my average blood glucose is 42mmol/l, when I usually see numbers betwen 4.0 and 8.0 on my meter most of the time? No.
The reason for this difference is that they are measuring two different things and the HbA1c is not an average at all, but the two do follow very roughly in parallel.
What the HbA1c measures is the amount of red blood cells which have a glucose molecule attached to them. In non-diabetics this is normally around 5% to 6% of all red blood cells. thats perfectly normal. In diabetics it can be higher because when your blood glucose is higher, more glucose attaches to red blood cells.
But the higher your blood glucose, the more chance there is of a glucose molecule attaching to a red blood cell. So if you are running higher blood glucose, then your HbA1c will be higher. Thats what I mean by them running roughly parallel.
The reason it may be misleading is that it appears from research that damage may be done to the body before any glucose attaches to red blood cells. If your BG is 10.5 for a while then some small amount of damage may be done to the eye for example. But although some damage has been done, your blood glucose was not at 10.5 for long enough to affect your HbA1c.
So your HbA1c might look fine, but its the peaks of your blood glucose that do the damage.
Small amounts of damage (microdamage) happens all the time and the body can repair some of it automatically. If you keep your BG under control then this should not be too much of a problem. However if you are having a lot of short high peaks, more microdamage is caused and may not be reflected in your HbA1c.
Looking again at averages, when we test blood glucose it tends to be before or after meals. These are the times when the blood glucose changes the most, so your BG may be higher or lower at those times than the rest of the day. The other approach is to use your fasting BG to work out roughly what your HbA1c should be. The problem with that is that I can change my fasting BG by at least 1.0 mmol/l by having a snack before bed. That throws the figures out considerably. The only way to get a true average of your BG is to use a continuous glucose monitor which are very expensive and rare.
While there are calculators on various sites online, they are really just a rough guess about what you can expect your HbA1c to be. They will nearly always be out to some degree, although often they will be out by the same range every time if you are consistent in the way you live your life.
But they also miss the point in that the real problem is the blood glucose peaks
This is a series of blog entries about diabetes myths. Diabetes Myth #1 - Its all your own fault Diabetes Myths #2 - Its all about sugar
Diabetes Myth #3 - Calories Count Diabetes Myth #4 - Cures Diabetes Myth #5 - The Diabetic Diet Diabetes Myths #6 and #7 - "Diabetes is progressive" and "Pills or Insulin mean failure" Myth #8 - Glycemic Index is the answer. Now whats the question? Diabetes Myths #9 - Cholesterol Diabetes Myths #10 - HbA1c is an Average
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Last Updated on Tuesday, 07 June 2011 12:15 |
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