How come the sensor takes forever to give accurate readings once you have a low or a high even though your meter states you are normal?
Hi Karen,
From what I’ve been taught this is because it takes a while for the changes in actual blood glucose to diffuse through your tissue and show up in the interstitial fluid. I have definitely noticed I have more of a “lag time” when I have the sensor in fattier tissue like my butt or stomach than when I have it in my arm, which makes sense based on what I’ve heard. It’s annoying, but hopefully they will come up with some software tricks to help with it!
I notice that too. Basically, anytime there is a very rapid drop or rise in my blood sugars (like after a low or during exercise) it just can’t keep up. But, most of the time I find it is within 20 points. Most of the time even 10 points. So I don’t complain. And it has really helped me in catching nighttime lows and afternoon lows (which I seemed to have quite a few of).
What the others said is correct. The CGMS isn’t measuring your blood glucose, but rather the glucose concentration in your body’s interstitial fluid. Quite simply, it takes longer for changes in blood glucose to be reflected in the interstitial fluid.
Ahhh that makes sense guys, but still it seems like a lifetime and all the bells and whistles keep ringing and I end up meter testing more than I want too.
I sometimes feel I test more with the meter on the sensor than before. 
It take my DexCom 7 about 5 to 10 min after a low or high to read… It’s about 5 to 10 points off though
The concept of "the intersticial fluid carrying glucose that is measured is not quite correct. It is an older concept that needs to be replaced. As I understand from one of the Dexcom people, the insertion of the sensor probe creates a small wound that provokes an immune response from the body. Blood is the body’s line of defense for tissue breaks. Blood flows into the area and sets up an immune defense about the probe. It takes the body several hours to setup the defense and stabilize in the area. That is why there is a 3 hour start up for the sensor before calibration. While the sensor remains under the skin the body continually delivers small amounts of blood to the “wound” and this blood is the sample that the probe measures. The sensor probe is a micro electrical cell (similar to a battery) that measures the oxidation of glucose by glucose oxidase. For more information see:
http://en.wikipedia.org/wiki/Glucose_oxidase
This information will not be material to DexCom users in general, however, conclusions drawn from the “intersticial fluid model” are not really valid.
Aaron
I’m not sure where this information came from. Since blood, during the healing process creates a barrier between the blood and the wound area, the explanation is not making sense to me. The delay in glucose measured has to do with the transfer of fluid from blood to muscle.
Here is a description of how the measurement of how interstitial glucose measurement works.
Performance Metrics for Continuous Interstitial Glucose Monitoring; Approved Guideline
David Klonoff, MD, FACP Patricia Bernhardt, MT(ASCP) Barry H. Ginsberg, MD, PhD Jeff Joseph, DO
John Mastrototaro, PhD Donald R. Parker, PhD, DABCC, FACB Hubert Vesper, PhD Robert Vigersky, MD
The intended users of this guideline are health care professionals, in vitro diagnostic (IVD) and medical device manufacturers, and regulatory agencies.
The use of self-management of blood glucose (SMBG) devices or glucose meters has led to more normal glucose levels and lower risk of cardiovascular and long-term complications in both type 1 and type 2 diabetes. Self-management of blood glucose devices are typically used to test patients’ blood glucose levels one to four or more times a day to manage diabetes medications, including insulin dosages and correct abnormal blood glucose values. Despite being easier to use than in the past, many patients with diabetes are not compliant with SMBG testing at the frequency recommended by their physician, because of the cost of testing supplies, the pain of repeated fingersticks, and the overall inconvenience of monitoring.
Continuous interstitial glucose monitors (CGM) are medical devices that measure glucose in the interstitial fluid (ISF) just under the skin. Continuous glucose monitor devices are typically attached to the skin by an adhesive patch and can be worn for up to several days. Continuous glucose monitors offer the patients the potential of monitoring their glucose and managing insulin levels without repeated fingersticks. Unlike SMBG devices that measure glucose levels in blood (capillary), CGM samples are from the ISF under the skin. Circulating blood glucose distributes into ISF where it is absorbed by cells. Interstitial glucose levels, therefore, lag behind blood glucose by the amount of time that is required for glucose to diffuse from the circulatory system into ISF, on the order of 3 to 10 minutes or more; additionally, the interstitial glucose may have a bias offset in addition to the lag. However, ISF may be more reflective of the amount of glucose available for cellular metabolism. Although CGM is called continuous, CGM devices actually only sample ISF glucose intermittently, varying from every few seconds to several minutes between measurements. Software within the CGM devices can combine current levels with previous results to predict a future direction of glucose change. Continuous glucose monitors can thus display not only a single glucose result, but also the direction of glucose change (up, down, or stable), as well as the magnitude of change (amount of glucose change per minute). Continuous glucose monitors thus offer the potential to predict hypoglycemic events before they occur, monitor for glucose variations that may not be detectable with SMBG monitoring only a few times a day, and predict future glucose values for determining therapy adjustments.
Hi Karen. This has been a frustration for me too. I find that I still do my regular meter checks, like i did before CGM, and rely on real blood glucose and less on the cGM values for treatment. What I do use it for is overall daily patterns to identify rough spots and improve basal rates. Hope you find a good balance!
My balance was to give it up
I am waiting for something that gives me real time readings.
CGM was just too much work, and I test all the time so that is what I am doing for now.
I need something that gives me real time readings and insulin that reacts instantly to highs, brings me down instantly and keeps me steady. 
A cure??!!
I don’t want to be argumentative, but the info came from DexCom tech service. If the sensor worked on intersticial fluid alone the sensor would work immediately. As I reported before the body senses the probe as an intruder and begins setting up an immune devense. It takes three hours or more before that immune reaction develops. That is why the sensor does not function for the first 3 hours minimum and often takes more time. The article that you posted does not relate to the DexCom CGM system. Yes there is interstitial fluid but the DexCom sensor also needs minute amounts of blood which flows into the wound area.
Aaron, the DexCom sensor does not need blood. You might enjoy reading
Please discuss this issue with DexCom. I have no vested interest in pushing this issue. Believe anything you wish or read. I discussed this matter with DexCom people several times. They gave me the information I requested. For what it was worth, I shared that information here. Contraversy in matters of scientific theory is par for the course, a natural state of affairs. Above all our opinions are not truly material in the real world as neither of us is actively working in this field. If you and I hold different opinions so be it. AaronM
Sorry to point out the science you seem to be siting is not how a CGM glucose sensor works. The reason for the 3 hour wait is to achieve a stable interstitial fluid environment around the sensor. Blood will disrupt the chem/electric function of the sensor.
You are correct about the article. It does not talk about the DexCom, just the science and technology of how the sensor they use works.
Actually that is how an artificial pancreas works. It cuts off the basal and/or bolus when BG is low or dropping and introduces insulin when BG is rising, for what ever reason. Sound so simple, but the challenge is the glucose monitoring system. In the testing of the AP they use new sensors not on the market yet. The math and support technology behind it all works great!
Not a cure, but better than anything we have now.
By the way…I do work in a related field and have discussed sensor technology with some of the inventors of the sensor technology in use currently.
I’ll throw this into this discussion so you can evaluate.
From everything I’ve researched which is extensive, the statements made here by users about how a CGM works relative to reading interstitial fluids is correct. However, I’m sure each CGM manufacturer likely has a different methodology about how it’s done.
My experience comes from using a Dexcom Plus system that’s been in use for 7 months. Also, I take Novolog about fifteen minutes before meals and Lantus each night. My CGM readings are usually always above my glucometer reading; i.e., from 0 to 20 points. Whenever I experience a rapid rise or low in my glucose due to food intake or not eating food after taking insulin, the CGM will get out of sync with my glucometer readings and sometimes very much out of sync. My experience is that it will eventually catch up, but it can take far too long. The trending indicator is usually reason enough to perform a BG check if you have eaten something that’s likely to cause a rapid rise or if you have not eaten causing a rapid low.
The trick I use is to perform a BG test when I suspect something is off. If the result is within 100 points of my CGM reading, I enter the result and then wait about 15-20 minutes and enter another BG test reading which usually calibrates the CGM quite nicely. If it’s over 100 points I do not enter the result. I already know that something unusual has occurred and I don’t really care what the CGM reading is. If I enter a large difference it will usually freak out the CGM. Instead, I wait about 30 minutes and then make another BG test and in between react accordingly for food or insulin intake. If it’s still way off, I’ll wait another 30 minutes and then perform another test. That’s about the same technique I would use if I didn’t have a CGM. By the time I take the second test after about an hour, the CGM is closer to what my actual BG is, but may still require another calibration.
The thing to remember is that a BG test is a reading at a point in time. It doesn’t give you a clue as to whether your glucose is trending up or down. My CGM will tell me that even if it’s off in its BG reading and more importantly it indicates how fast it is rising or falling.
If you’re frequently experiencing rapid rises or lows causing your CGM to get out of sync, the CGM is not much use to you for BG readings. It is definitely not a perfect instrument. Your diet is most likely the culprit. I also know there are exceptions to this theory. The very time I eat something that I do not think will cause a rapid rise in BG is when it will happen and for seemingly inexplicable reasons. Note: there’s always a reason, but it takes an analysis of everything you’ve done or eaten within the last 12 to 24 hours. Most of us don’t track and record our events closely so we immediately think the cause is what we’ve recently done. It’s usually never just that event.
As long as I control my diet reasonably well, I only test about three times a day. This is one more than what’s required for daily calibration and the CGM will remain very close to my actual BG test results.
If you’ve made the investment in getting a CGM and experiencing problems, I would not give up on it. Work with your dietician and/or Endo to manipulate either your diet or insulin intake or a combination of both. After a relatively short time, I believe you find it an invaluable tool and you not require frequent BG tests.
Thank you for your post, and I do understand what you are saying and it was very informative.
You are so right about diet that is the real issue and for me not bolusing 15 minutes before I eat, I am a sloppy diabetic and it frustrates me as I know there are some things I can do to fix some of my issues.
Still for me I always felt like I was testing my bloodsugar to see if my CGM was giving me accurate information and that frustrated me. The CGM was wonderful just like meter testing if I do not eat or move, I flatline