A1C, GMI, eAG, oh my!

I spent quite some time figuring how different numbers you might see in your blood test results and in your continuous glucose monitoring (CGM) application relate to each other. The confusing part here is that some of these values are directly measured, others are calculated using formulas, and also different countries use different scales. Let's try to figure this out. I'll also provide calculators for all glucose related conversions: A1C to glucose conversion, blood glucose to GMI conversion, and the corresponding A1C and GMI charts.

A1C

A1C, also known as HbA1C, is a number you receive after the lab test. It measures percent of hemoglobin that has glucose attached to it. My non-scientific understanding is that the red blood cells (those which contain hemoglobin) live for about 3-4 months, so A1C effectively corresponds to an average glucose level in your blood for the past few months (they say 2 or 3 months).

Estimated average glucose

If we know the A1C percentage, there is a formula that helps estimate an average amount of glucose in your blood:

eAG = 28.7 × A1C - 46.7

So, after you got your lab tests results, you can use this formula to convert A1C value to an estimated glucose. Here's the A1C to estimated blood glucose (eAG) calculator:

A1C: %  Convert eAG:

Actual average glucose

If you use a CGM, the things are working in an opposite direction. Your CGM monitors your glucose level by checking it every few minutes, and knows the real average glucose number. In the United States, this number is normally shown in mg/dL.

Glucose management indicator

Now here the things get weird. For some reason, looking at the average glucose is not enough, and the CGM applications like to show you another metric: a thing they call glucose management indicator, which–just like A1C–is a percentage. I think the idea was to give you something that would kind of maybe correspond to your real A1C, but... no, not really. If AG is your average glucose (as measured by your CGM), then the formula for this GMI thing is

GMI = 3.31 + 0.02392 × AG

Here is the average blood glucose to GMI calculator:

AG: mg/dL  Convert GMI:

Now, if you remember a little bit of your school algebra, you can figure out that both formulas are linear functions, and they have different slopes, which means that they actually intersect in exactly one point: around 7.0% of A1C or GMI, which corresponds to about 154 mg/dL of glucose. For all other values of the average glucose (estimated or actual), these formulas inevitably give different results, so no, you cannot think that GMI is an estimated A1C. Not at all.

This actually matches my last A1C test experience pretty well: my CGM app showed both 3 months GMI of 6.3% and 1 month GMI of 6.1%, and the A1C result was 5.5%, which nicely matches the formulas, and shows that these numbers do indeed show very different things.

Other scales

To make everything more confusing, there is one more scale for glucose, and one more scale for A1C. In Europe they prefer to use mmol/L for glucose (instead of mg/dL); to convert from mg/dL to mmol/L, divide the number by 18.

Here is the calculator to convert blood glucose between mg/dL and mmol/L:

mg/dL  Convert

mmol/L  Convert

Instead of A1C or GMI percentages, sometimes they use mmol/mol, which converts using another linear formula:

GMI(%) = 0.09148 × GMI(mmol/mol) + 2.152

Here is the calculator to convert between GMI or A1C percentage and mmol/mol:

%  Convert

mmol/mol  Convert

Let's graph it

Here's the graph that shows how the line converting A1C to estimated average blood glucose differs from the line converting actual average blood glucose to GMI. I'm showing both US mg/dL and European mmol/L scales, and hope it makes sense for you!

Look at trends, not at the values

Last but not least: I believe that the absolute values of the numbers you see are not as important as the trends. I always find that a CGM glucose value differs from a finger prick value, the difference is sometimes as much as 15%. Changing an expired sensor to a new one changes the reading quite often too. Also, CGMs report glucose in so called "interstitial fluid", not directly in your blood, and it's at least 15-20 minutes delayed compared to the finger prick test. So... don't stress too much about the specific numbers, but follow the trends.

Next, we'll talk about my experience with different foods and activities and how it all affects my glucose levels according to my CGM. Keep reading to learn about my CGM findings!