CGMs and Interstitial Fluid

Most of us know that our Dexcom CGMs give us an “estimate” of our blood glucose as the sensor is inserted in interstitial fluid as opposed to blood. The CGM then uses an algorithm and gives us a proxy BG reading. [In addition, supposedly it is 10-20 minutes behind the actual BG reading especially if BG is rapidly rising or falling.]

Per the URL below, " Interstitial fluid acts as a kind of fueling station in terms of nutrients for our cells. Interstitial fluid contains glucose, salt, fatty acids and minerals such as calcium, magnesium and potassium. Interstitial fluid can also hold waste products which result from metabolism." The nutrients in interstitial fluid come from blood capillaries. (https://www.diabetes.co.uk/body/blood-vessels.html).

The sensor is in a fixed position in our body. I assume the interstitial fluid circulates in our body just as blood does. Any idea how long it takes interstitial fluid to circulate? Any idea how long it takes actual blood to circulate?

My question, after all this verbiage, “Are the components of Interstitial fluid uniform?” If not, is it possible that our CGM can give us a different readings simply because the fluid passing the sensor is not uniform?

Could that be a reason why the CGM can go way up or down 5 minutes later and then return? Could that be a reason why a CGM reading can be different from mater reading other than the 10-20 minute lag?

Anyone have any thoughts on this?

If you check a manual BG in one hand vs another you may get different readings. There is some of that at play.

But, unless you have real crap circulation, I wouldn’t expect that this is playing a noticeable/detectable role in your BG.

You have about 6L of blood. So, your cardiac output will give you an estimate of how fast the blood turns over. This source suggests that about every 70 seconds all the blood should circulate. “A normal adult has a cardiac output of 4.7 liters (5 quarts) of blood per minute.”

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Good answer. Thanks.

I see this as noise caused by uneven glucose/sensor enzyme action. The sensor noise is dampened by the algorithm as it tries to make some rational sense of this non-metabolic uneven movement.

I don’t believe it is driven by actual blood glucose or interstitial fluid events. Blood glucose metabolism does not move in a jumpy up and down motion. It can move rapidly but doesn’t move, for example, in a five minutes series 80-95-80-65-80.

An academic paper (linked below) puts the delay to the “raw signal on the wire” at 9.5 minutes.

There are further delays - I think the Dexcom G6 poll rate is once every five minutes, and then the Dexcom does some kind of filtering on the data that smooths and trends data over several polls. So it’s easy to add 5 minutes to 9.5 minutes than then a couple extra poll cycles for smoothing to catch up with a sudden change to end up with the 20 minutes you suggest, or even more.

The paper I link below mentions interstitial fluid as the delay, but I will continue clicking links until I find some sort of hydrodynamic model of how interstitial fluid works.

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For me, “interstial fluid event” is “I rolled over on my sensor while sleeping” :slight_smile:

Yes there is averaging and trending going on inside the Dexcom system (I’m actually unclear how much smarts is in the transmitter and how much is in the receiver) that would make your example of 80-95-80-65-80 average out into a smoother curve that might show up as 87-83-80-77-74 in the averaged and smoothed data.

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Thanks all. Good articles. Was most interested in your thoughts about the interstitial fluid being uniform. If it is not then your sensor reading could be wrong for that sensor reading and then back to correct on the next reading.

A lot of us are familiar with “pressure hypos” where we roll over onto the sensor and it produces a falsely low reading. Roll back over to the other side and it reads fine. Maybe the pressure “dilutes” the interstitial fluid or something. Again, I am forming mental models for the interstitial fluid dynamics but I have no idea if they’re based on any actual reality :slight_smile:

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I found several articles about interstitial fluid dynamics that I understood absolutely none of and perhaps they were not directly relevant to CGM’s.

But I also found this nice article, that I understand at least sections of. It starts with the difference between fingerstick and plasma glucose, then goes deep into interstitial fluid research specifically on glucose measurements, and even talks some about how CGM sensor technology: : https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2903977/

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Thanks. The article was interesting but I understood very little of it. It indicated that capillary blood readings (from finger sticks) can be off from arterial blood (from your arm) and both can all be off from interstitial fluid CGMs. I was surprised at the error rate of CGMs. Thought they were less. "Overall percentage of error for the CGM runs around 15%. As mentioned before, accuracy depends on multiple factors like current glucose concentration and rate of change of glucose values, with poor correlation during hypoglycemia and times of rapid change. Percentage of error for individual sensors have been reported as 17% for the Guardian® REAL-Time (Minimed, Northridge, CA),11–16% for DexCom STS (DexCom, San Diego, CA), and 12–14% for the Navigator (Abbott Diabetes Care, Alameda, CA).

The article is older then dirt - 2009

I have been cross-checking calibration between my G6 and fingersticks at least once a day since I got the G6 over 2 months ago. Because of the interstitial fluid delay, I try to choose a time of day where my bg is calm and flat.

Often they agree within +/-10% but sometimes they are off by 20% or more.

Of course my fingerstick measurement has some uncertainties as well so I can’t put all the difference in measurement, onto the G6.

If you use xDrip, it allows you to view the data without smoothing, so that would seem to confirm this. I think xDrip has its own smoothing algorithm, which would indicate that the “smarts” are in the app/receiver, rather than the transmitter. Which would make sense, since the less processing the transmitter has to do, the less battery gets consumed.

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I should have check the date. Dexcom today claims its CGM is off an average 8-9%. I checked over 500 readings and it was off from my Contour Next by <7% so I am quite happy with my Dexcom G5. Was just wondering why all of a sudden the CGM reading can drop or increase 10 points and then go back to what it was before. Could only come up with blood and interstitial fluid is not uniform.

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I do know I like to calibrate it at the same levels. So I always calibrate it when I am steady at 95-105. That is the level I want the most accuracy. I have found when I calibrate it at a higher number then it is off more at the lower numbers or visa versa. Mine usually stays within 5 points, although sometimes 10 points in that range. And I will wait days to check it again as it stays on track so well when I have checked it at the same levels. Of course this is one reason I like restarts, they seem to stay very even keel for me.

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