Is it possible that a normal A1C (in this instance, 5.1) might still mask high blood sugar? Or are the two basically inseparable?
In the vast majority of people, the two are inseparable as you say.
However, if someone has what is called a hemoglobin variant (essentially the same as a hemoglobinopathy), the A1c result may be falsely low or high.
The word hemoglobinopathy is not quite synonymous with hemoglobin variant since the latter doesn’t have to be pathologic, it may simply be a normally functioning variant.
Recall that A1c is formed by glucose binding to the hemoglobin molecule. In the presence of a variant hemoglobin, both the binding of glucose to it and the half-life of the red blood cell carrying it can be different than normal, thus leading to unexpected A1c results.
I know that the HbA1C is thought to be better than a fasting glucose test, because the HbA1C shows post-prandial effects and is a better estimate of glucose levels over time. I had some of what I consider paradoxical results recently where my average glucose based on my HbA1C was lower than my fasting glucose.
Everything was still normal, and no problems, but it seems to me that blood glucose levels after 12 hours fasting would almost have to be lower than a 3 month average. Are there conditions that can raise fasting blood sugar that wouldn’t raise the average blood sugar?
The thing to remember is the A1c is an average. Someone could be having extreme highs and lows and their A1c could still be an acceptable number. I’ve worked with diabetics with some wide swings in BG levels who still had good-looking A1c numbers. The fasting glucose number, while useful, also does not tell the whole story and should not be used as the only number to determine if treatment is effective or if someone is in good control. When you put the A1c and the home spot checks (preferably at different times of day) together, then you begin to get a good picture of how someone’s control is and if their treatment is working. The A1c is useful, but it’s only part of the big picture.