Pretty Little Liars


According to my continuous glucose monitor, I just had one of the best diabetes days ever– between 98 and 102 the entire day. The trend line on my monitor was gorgeous, a distant ocean horizon with nary a boat to be seen.

There was only one problem with this: my CGM throughout the day was not correctly reporting blood glucose measurements. It wasn’t entirely inaccurate or uncorrelated, though; when my blood glucose levels went up, the CGM’s output showed an increase as well, and when I went down, it followed. The problem was one of scale. Whatever rise in current was being detected in the interstitial fluid resulted in a reported rise in blood glucose levels that was only about a fraction of the corresponding finger-prick-measured rise in blood glucose levels. So, though I was centered pretty well around 100, I learned quickly to interpret the CGM’s “102” as “150” and the CGM’s “98” as “60.”

I have experience this before to a lesser degree– sometimes a sensor’s conversion of interstitial fluid current to blood glucose levels reflects a dampened function of the actual blood glucose values, such that I can be 200 according to a finger-stick, and the CGM will rise proportionately, but only ever reach 140. Usually, though, the scale results in numbers limited to a range of 70 – 140; 98 – 102 was impressive in how tight the scaled range of values was.

Now, I’m not sure what causes this dampening. Anecdotally, it seems to be related to the location of the sensor. I spoke to a Minimed representative about it, once, early on in my experience with the CGM; I was near tears in the bathroom at work, bleeding profusely as I tried to replace the sensor while waiting on hold, inserters and alcohol pads spread across paper towels on the counter. The representative insisted I was calibrating the device incorrectly– too frequently or at points of too much change. I tried to explain that I had been tracking as the day went by the current measured (this is displayed as “Isig” in one of the Minimed screens), and I could see that a 20 point change in blood glucose corresponded with no displayed change of Isig at all, so unless there were orders of magnitude more specificity available to the device than it was displaying, it couldn’t just be a problem of calibration. The call turned somewhat combative, and eventually I gave up. (It probably wasn’t entirely his fault; I was, after all, bleeding from a stomach wound in a public restroom, crying in frustration with my broken immune system, and not happy to be told, the problem is you, not our sensor. Not one of my prouder moments.)

I’ve had some success avoiding sensors with dampened trend lines by inserting the sensors higher and more towards the outside of the abdomen. Unfortunately, I have found that many of the places that seem most able to take the sensor are also the ones that have the least viable sensor performance. And even more unfortunately, there’s only so much space available, so sometimes I must start annexing territories with questionable promise as sensor sites.

And indeed, the sensor with a four-point range? In dangerous territory. So, though part of me wanted to keep that sensor in forever, reporting beautiful numbers, the loss of granularity was too much for me. I changed the sensor when I got home, cursing and crying at the pain and the blood and the scarred expanse of my abdomen.

Sigh. We can cure this disease, right? Like, soon?

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Dr. Margaret A. Morris
11 years ago

If the CGM were always off, it would not be too useful. But most of the time, it is pretty close, and even when there is a dampened curve, it is extremely useful; as soon as I recognize I have a dampener inserted, I make sure to check if it says I’m at 130, since that might mean 160. The extremely tight range of 98 – 102 was not very useful, as I noted– but that was a rare case, and, further, it still proved more useful than nothing, since I knew 102 meant, “Watch out! You could be high,”… Read more »

David Parker
David Parker
11 years ago

OK. I hear you. But if the device doesn’t give sufficiently accurate results and you must resort to standard finger pricks, etc., what is gained? Slightly fewer finger pricks? Less frequently? I guess you’d say that it reduces your uncertainty and stress. Wouldn’t that same reduction in uncertainty come with one or two more finger pricks? How can you have less stress if you don’t know whether the device is working properly or just flat lining so to speak? That seems quite stressful to me, to the point where I’d do more finger sticks. I don’t think I’m alone in… Read more »

Dr. Margaret A. Morris
11 years ago

@David: Oh, to be clear, despite all my complaining, the CGM is worth it, a hundred times over! I will probably write a more thorough explanation eventually, but the short version is: One of the worst parts about diabetes for me is the not knowing– where my blood sugar is, where it’s going, and what I should be doing to counteract it. It’s incredibly stressful to be in such a state of needing to know, yet not knowing. The CGM takes much of that stress away– it’s a little window into my body, letting me know more or less where… Read more »

David Parker
David Parker
11 years ago

So, tell me please, Karmel, what is it about a CGM system that makes you put up with all the grief?

11 years ago

I am with you on this one.  I have a two year old with type 1 and we’ve recently started using a CGM.  It’s maddening for us to even find a time when we can calibrate it.  If we are suppose to wait until there are no changes than we can ONLY calibrate when he’s sleeping.  ONLY.  Which adds to the already sleep deprived state of a parent of a CWD.  I dream daily for a cure, but sadly often my husband (a t1d for over 20 yrs) brings me back to reality of what he’s been promised for the… Read more »

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