Last week, I talked about the process behind the A1C readings that all us Diabetians know so well — what it is we’re actually measuring when we get an A1C result, what kind of process is used to get that measurement, and what the limitations are of that number. This week, I’m going to follow up on that and go into some of the detail behind our meter and continuous glucose monitor (CGM) readings, and then talk about how to integrate all this information in order to understand our own diabetes management.
I’ve written a more detailed blog entry on how our meters and CGM devices work, so I’m going to give you an abbreviated summary here — the goal is to give you a basic idea of what it is we’re measuring, and then talk about how all the information fits together.
Let’s start with conventional meters. A conventional fingerstick meter is typically a bit more accurate than a CGM, as it directly measures our blood. (I’ll elaborate more on this when I talk about CGMs.) Here’s the basics of what it’s measuring — a meter takes in our blood sample, and through a chemical process receives an electrical signal based on the amount of glucose present in that sample. It then converts that electrical signal into a number, which is displayed on our screen. It’s simple enough. But while it is directly measuring a sample of our blood, it can still have some wiggle room. Meters must meet the following requirements: that 95% of all tests be within 20% of the actual blood glucose level for results greater than 75 mg/dl, and within 15 mg/dl 95% of the time for values below 75 mg/dl. That little “mg/dl” is milligrams per deciliter, by the way. So it’s not “laboratory level” perfection. And, the glucose level in whole blood (the sample given to the meter) tends to be about 10–15% lower than the glucose level in plasma, which is what would be used for a lab test of glucose. There are other factors that can affect the accuracy of readings, but I’ll leave it at this: While the fingerstick is our most accurate home measurement, it’s still not ironclad.
Now let’s talk about CGM readings. CGMs have become more accurate over the last several years, and earlier this year, the FDA gave clearance for people to use the Dexcom G5 Mobile CGM for treatment decisions (that is, using the number from the CGM to determine the amount and timing of an insulin injection). That was a huge decision, brought on in part because of the increasing accuracy that CGM devices have shown in recent years.
But many CGMs still lag behind the fingerstick. Here’s why: These devices measure interstitial fluid (the fluid between cells), and while this fluid does respond to changes in glucose, it lags behind the response in our blood by roughly 10–15 minutes. This is why treating a low blood glucose with a CGM requires a little patience — you don’t want to look at it 15 minutes later, see the number hasn’t shot up yet, and take MORE sugar. Instead, that’s when you need to fall back on your trusty fingerstick, which WILL measure your blood directly and tell you if the sugar you took is taking effect. And of course, beyond the time lag, it needs to be understood that our interstitial fluid response won’t always be the same as our blood response to sugar. It’s not a direct, one-to-one thing — this is why CGMs need to be calibrated with fingerstick measurements several times a day, and why the CGM doesn’t just present the sensor’s data to us directly, but takes the measurements it’s getting and runs them through algorithms designed to “smooth out” that data and present something that is as close as possible to what our blood glucose really is at any given moment.
Putting everything together
So now that I’ve pointed out the shortcomings of every diabetes test we regularly use, let me take a step back for a moment. Like I said last week, just because we’re getting impartial or “approximated” data does NOT mean we can’t use all this information to get a very clear picture of our control. It simply means we can’t rely on any ONE set of numbers.
If our A1C is low, but our CGM graph line is a jagged line of rapid rises and rapid falls, and our fingerstick numbers confirm those high peak numbers and drops into severe low numbers, we’ve got some work to do. Similarly, if we tend to test at the same time every day and the numbers we’re getting are solid, but we don’t have the benefit of a CGM and don’t get our regular A1C, we might assume we have great control when we really don’t. We might always be 100 in the morning before we eat and 130 by 1 PM when it’s time for lunch. That doesn’t mean we didn’t shoot up to 265 during our post-breakfast peak and slowly come back down. We might very well need to adjust our insulin dose or timing, but never realize it if we’re merely going by those two numbers. But if we had a CGM graph to view, and if we had an A1C number to reference, it could give us a much clearer picture. Of course, we should have also been testing a few hours after breakfast, but the point is that having all of the information gives us a clearer picture.
Lastly, it’s important to remember that managing diabetes is about the big picture. Our meter might read 107 when the actual plasma glucose is 124. Our CGM might be lagging behind our true blood glucose level. Our A1C might be pushed in one direction or another by fluctuations in the lifespans of our individual red blood cells. That’s OK — we’re shooting for a good range of glucose readings, not an exact number. We’re not trying to get a perfect “100” on our meter all day, every day. We’re never going to have the exact same A1C every time, either. Life happens, we get sick, we deal with stress, life keeps on dancing forward. We have plenty of good information, even if none of it is perfect by itself. If we use it wisely, it can guide us and we can live good, healthy lives for many years to come.