I got my new A1c result yesterday, the first in six months.
It was 5.5%.
During this period, my blood sugars have been significantly better than they had been for years. My fasting blood sugars have dropped about 20 mg/dl and my post-meal values have dropped by about 30 mg/dl. These changes have been measured many times using reference meals with known blood sugar outcomes.
In the past, my A1cs were almost always 5.7%.
The drop in A1c doesn't seem to capture the significant lowering of my blood sugars over this period.
The usual formula to estimate the relationship between A1c and mean plasma glucose was derived from the DCCT study.
That formula is:
Mean Blood Glucose = (A1c * 35.6) - 77.3
Applying this formula, an A1c of 5.5% is supposed to correspond to a mean plasma glucose of 118.5 mg/dl and an A1c of 5.7% is supposed to correspond to a mean plasma glucose of 125.6 mg/dl.
Neither of these values correspond to anything I have ever seen in my testing, and I test a lot and at many different times of day. My 30 day meter average, based on 150 measurements, has ranged between 98 and 103 during this period.
There is another, less cited formula that works better, at least for me. It is called "The Nathan Formula" it is:
Mean Plasma Glucose = (A1c * 33.3) -86
This formula yields a mean plasma glucose value of 97 mg/dl, which comes much closer to what my meter's 30 day average has looked like throughout this period.
Here's a calculator that will give you the Nathan Formula A1c/Mean Plasma Glucose equivalents:
However, I think the most important point is this: despite doctors' reliance on A1c it is not a particularly accurate measurement of what your blood sugars have been over the past three months. And not only that, the DCCT formula seem to work a lot better in people with very high blood sugars which is the group from which it was derived than they do those of us with near normal ones.
Here's a study published in the Journal of Family Practice that looked at the evidence connecting observed glucose measurements and A1c.
Does daily monitoring of blood glucose predict hemoglobin A1c levels?
The conclusions of this review include the following statements:
The relationship between HbA1c and blood glucose levels is such that blood glucose levels from the preceding 30 days determine about 50% of the total HbA1c. (10) This relationship may be altered by uremia, intake of vitamins C or E, and conditions that affect erythrocyte turnover. (11)
It remains unclear whether management strategies that focus on minimizing HbA1c levels are optimal for prevention of diabetic complications.
Although HbA1c levels correlate with the risk of some complications, aspects of glycemia not reflected in the HbA1c level, such as the heights of glycemic "excursions" from the mean, may independently affect the risk of complications of diabetes. (12) If so, quantitative analysis of day-to-day blood glucose levels might yield a better estimation of the risk of diabetic complications than HbA1c levels.
So what this means is this. The improvement I've made in my blood sugar by keeping my "excursions" (i.e. post meal values) around 110 instead of 140, is probably a lot more significant, healthwise, than the measily .2% improvement in A1c.
Keep this in mind if your A1c results don't match your observed daily testing results, and trust what you see on your meter not a cheering (or baffling) A1c.
UPDATE OCT 31, 2007
The American Association of Diabetes Glucose Trials has come up with a new and improved equation to relate A1c to mean glucose developed using "hundreds of thousands of readings" and CGMS data.
It is the A1C-Derived Average Glucose (ADAG) formula:
Measured in MMOL/L: Average Glucose = 1.583 * A1c - 2.52
Measured in mg/dl: Average Glucose = (1.583 * A1c - 2.52)*18.05
Using this formula:
4% = 69 mg/dl
5% = 97 mg/dl
6% = 126 mg/dl
7% = 155 mg/dl
8% = 182 mg/dl
9% = 211 mg/dl
10% = 239 mg/dl
In short, 1% of A1c equates to a difference in mean glucose of 29 mg/dl.
I've put together a calculator that will convert A1c and average blood glucose using the new ADAG formula.
You'll find it at http://bloodsugar101.com/A1Ccalc.php