The hemoglobin A1c test, as we all know, is supposed to give a sense of your average blood glucose levels over the past three months. But here’s a question for you: have you ever tried to figure out what those average blood glucose levels actually are? Say you have an A1c of 6.5% — what, in mg/dl, does that translate to?
Try searching Google — it’s hard to find an answer. To quote from a post I wrote a few years ago (see entry from 4:45), that’s partially because:
“Not only is there no one standardized definition as to the correlation between A1c and mean glucose levels (JDRF says 1% = 24.4 mg/dl, ADA says 28.7), but different people have different correlations. For example, if you are a ‘high glycolator’ (more glucose sticks to your hemoglobin than the average) you can have a relatively high A1c but a low mean glucose. The speaker gave the example of a patient who had a 8.2% A1c, but a mean glucose of 159 mg/dl (he was speaking using the generally accepted idea that 7% roughly equals a mean of 154 mg/dl). Treat him more aggressively, and you’ll end up with hypos. And if you’re a ‘hypoglycolator,’ it’s the opposite.”
Well, just this week, a new paper was published in the American Diabetes Association’s Diabetes Care journal that provides a more solid answer to this question than I’ve seen — even though, as I must warn you, personal variability (as described above) means there’s still no precise answer. In the study, researchers wanted to find out what your average blood sugar would have to be in three situations — fasting, after meals and before bed — in order to achieve a particular A1c.
Here are their results:
A1c test results of 5.5-6.49% were associated with an average fasting blood glucose level of 122 mg/dl.
A1c test results of 6.5-6.99% were associated with an average fasting blood glucose level of 142 mg/dl.
A1c test results of 7-7.49% were associated with an average fasting blood glucose level of 152 mg/dl.
A1c test results of 7.5-7.99% were associated with an average fasting blood glucose level of 167 mg/dl
A1c test results of 8-8.49% were associated with an average fasting blood glucose level of 178 mg/dl.
The abstract only reveals two values for the post-meal and pre-bed glucose values:
An A1c test result of 6.5-6.99% was correlated with an average post-meal blood glucose of 139 mg/dl, and an A1c of 7.0-7.45% with an average post-meal reading of 152 mg/dl (unfortunately I can’t tell from the abstract how long after meals these numbers were taken).
An A1c test result of 6.5-6.99% was associated with an average pre-bed blood glucose of 153 mg/dl, and an A1c of 7.0-7.49% represented an average pre-bed value of 177 mg/dl.
More information on the study is here.