How To Get Better Agreement With Dexcom and Giucometer

Not all of the following items are in the Dexcom manual. They were recommended by very experienced Dexcom users who discovered these items on their own:

1. After putting on a new sensor, wait at least 6 hours before using the receiver to actually start the sensor. If I start immediately after putting my sensor in place, I find crazy numbers for many hours. The sensor needs time to become adjusted to your body. ( I actually wait 12 hours so I can have better results from the start.)

2. Start your sensor when you are in your comfort range, and when you are not stressed or exercising.

3. Actually test two times when Dex requests two BG's. Entering the first test twice is not a good idea. The fingersticks on two different fingers, or hands, might differ by 10 or more points. The variability of the glucometer needs to become involved in regulating the accuracy of the Dexcom.

4. One experienced user enters BG's only when she is less than 10 points different from the meter number. Another user called Dexcom on the phone. He was told to enter a few high and low meter numbers, so that the Dexcom will be more accurate when we have highs and lows. I agree with the phone reply, but I do choose the majority of the numbers that I enter when I am at least reasonably near my comfortable range. You do not have to enter a BG when it is first requested, you can delay an hour or more.

5. There is some disagreement about where the sensor should be placed on our bodies. I place mine near the center of my abdomen, but not close to my navel. I sleep on my sides and I do not want my sensor to be squished when I am in that position. Several users have had crazy numbers when placing their sensor on their sides or buttocks and then having
it squished against the mattress at night. I also am careful to place my sensor so my belt will not press against it when I am dressed. (Some light weight people have not had the problem when sleeping. It seems to more of a problem with heavier people.)

6. Keep the receiver out of damp places and never immerse it in water. It is not waterproof! I know one user whose receiver fell into the dishwater while she was washing dishes. A man dropped his receiver in the toilet bowl. The receivers were ruined. Replacements are very expensive!!! You can call the Dexcom number and order replacements on the phone.
(I leave my Dex in a safe dry place before going to the bath room. I carry it in my pocket if I am outdoors and it starts to rain.)

7. If you start having crazy numbers for no apparent reason, be sure the transmitter has not popped out of place. If it has, then snap it back in place very carefully. Check this frequently. If the transmitter is not firmly in place it might slip loose and fall off your body. It is small and would be easy to lose.

8. I use strips of tape to hold the sensor in place, but I do not apply the tape until after the first few days. The tape initially on the sensor seems to do its job for a few days, bur after a few showers/baths it starts to come losse at the edges. That is when I tape it down. Do not put tape on the transmitter. There is a diagram in the Dex manual illustrating the use of tape. If you are allergic to the tape and develop a rash, you can use Skin Prep. I use a Skin Prep pad and wipe the area where the tape is to be applied. After it dries the tape can be applied and should not cause any allergic reaction.

Let me know if there are any questions on these eight items. Please add any of your own suggestions. you may learn from what I have written, but I want to learn from you too!

[quote user="Richard Vaughn"]

4. One experienced user enters BG's only when she is less than 10 points different from the meter number.


Hi Richard,

Why would you only want to calibrate when the numbers are so close? I realize that the ultimate goal of the CGM is accuracy, but if the numbers differ greatly (or even just more than 10), then wouldn't the error be with the sensor data, and not the fingerstick?

Ashley, I don't calibrate only when the numbers are close. I was referring to "one experienced user" whom I know. I do not agree with her at all. I make sure I calibrate a few times when I am high, and when I am low. That will make the Dexcom more accurate at those levels. If you call Dexcom on the phone, they will tell you to calibrate with a few lows and highs, for that reason.

Unfortunately there is not a lot of science to many of the recommendations for calibrating or use. The recommendations provided by the Dexcom recommendations stem from the approaches taken in studies used to obtain FDA approval for the device. Unfortunately, there is also quite a bit of folklore that develops among the Dexcom educators and technical support representatives that somehow becomes ingrained in the standard responses.

I have quite a bit of experience with this as I worked as a technical support representative and taught other physicians about the system for 18 months following graduation.

Here are some things that I have learned from my own experience & can be really helpful - AGAIN, this information is based on my experience/opionions and do not necessarily reflect those of Dexcom (liability disclaimer):

1. The sensor is waterproof but many times the adhesive is the length-limiting factor. The STS shower covers can preserve the adhesive and sensors can last for 2-3 weeks if in a "sweet spot."

2. The system is only FDA-approved for the abdomen and so Dexcom reps are not supposed to give you information about inserting it anywhere else. However, this does not mean that it is necessarily most effective when placed in the abdomen - again, it was the only site studied.

3. Qualities of an ideal site include: adequate amount of subcutaneous tissue (fat), no scar tissue (like fibrous surgical scars), and ideally on the same side of your body as the receiver

4. The dexcom and sensor communicate using radio waves - not a very strong signal (and for good reason - we don't want any sort of radiation going through us). For this reason, the receiver and transmitter do not communicate through your body. You can have a sensor on the left and receiver on the right and provided they are both on your front or back, they'll communicate find. However, if you have a sensor on the front and receiver in your back pocket, you may intermittently receiver the Y symbol (for no signal)

5. The Dexcom receiver cannot calibrate when there are ??? in the status box. I think the algorithm makes it think there is no number so it can't compare and effect change when a number is used. It doesn't know if 124 is below or above ??? if that makes sense. It will also not calibrate with numbers <40 or >400 mg/dl

6. Some people swear by calibrating it frequently. However, more frequent calibrations do not necessarily make a huge difference in the accuracy. This is one fact that I cannot confirm or deny because I've never really closely examined it but the engineers state that 2 calibrations a day are all it needs, no more (unless prompted) and no less.

7. If you fail to calibrate, the receiver will still provide readings but the accuracy starts to decline -- this seems counterintuitive to the above and so I still am not sure which to believe.

8. The reason that two calibrations are advised initially to give the Dexcom information about how your glucometer interprets the blood glucose at that time. It uses this with an algorithm to equate what the sensor is seeing to the blood glucose at that time. I don't, nor have I EVER, been a big supporter of taking two fingersticks from two different hands (ouch!). The point of it is just to get a basis for what your glucometer interprets. If you were to test repeatedly within 1 minute, the glucometer will read the glucose level differently each time. Some glucometers are more consistent than others but there have been several studies published recently that examine the accuracy of the glucometers and there is definitely a margin of error.

9. Though not FDA-approved for use in children, it is a great device for kids because the sensors last longer, the sensors are smaller, the lag-time is 5 minutes (as opposed to up to 20-30 minutes for Minimed), and the blood glucose level does not have to be stable (unchanging) to calibrate. The one caveat with all of the CGMS products is that kids have much less fat so finding a good site is more difficult! However, a lot of parents use the buttocks.

10. sensor insertion hurts (I think). Though going faster doesn't necessarily ease this pain, it can increase the likelihood that the sensor will function. The longer it takes to go in, the more opportunity to get "caught" on parts of your skin.

11. if your sensor fails before day 7, has greater than 30% discrepancy with your glucometer or is consistently ??? before day 7, call Dexcom for a replacement. They are expensive little buggars!

12. Finally - if 2 sensors in a box fail, it does not mean the whole box is bad. Each sensor is made individually and the success of one sensor is independent of the others. If you have sequential sensor failures from the same box - there could be a problem with insertion or a problem with your transmitter (as transmitters get older, I've found that they start to have more frequent failures. This might be from the transmitter battery.

Remember, for liability purposes, the information provided by Dexcom representatives and technical support are limited by the studies and FDA approval.


hope this helps - I think its a great idea and would love to have more people share their experiences.


Hayley, thank so much for this! As a technician who uses the Dexcom, you have provided us with very valuable information.

Something else a dexcom rep told me was if it seemed off, calibrate every 15 minutes, three times, and that *sometimes* helps me...

Hi Richard,

My daughter started her Dexcom yesterday afternoon. The numbers have been meaningless. Before bed she was showing a steady 156 and we had her test anyway and she was 69. Earlier the receiver had shown she was 290 to 320. We are wondering if this thing is going to work for her at all. It worries me that she was low and it showed she was high. Everything was set up by the Dexcom rep just before dinner time.

I never get good agreement between my Dex and my meter until the 3'rd or 4'th day after I start a new sensor. There are many tips that the Dex manual does not show. To find out about these tips you can go to  a very good Dexcom Users group on another diabetes site. Here is the link:

There are more than 600 members in that group. You should find good help there. I suggest that you read messages that have been posted, and look at some back pages. To be able to post your own questions you will have to join the main site. Here is the link for the site:

If you continue having problems and the group cannot help you, please let me know. Post your messages here, or send me messages on my profile page.


Well her first sensor failed tonight after only 3 days. Not a good start with this thing. We only will get 4 sensors per month so this is disappointing.

It is frustrating to have sensor failures, but call dexcom tech support and they'll send a new one to replace it free of charge. I have heard of people restarting failed sensors, but I've never been successful with it. it usually takes me about 36 hours to have accurate readings- less if I let the sensor marinate before starting it with the receiver. Once it is accurate, it is (almost) a life saver. It comes with frustration, for sure, but for me it is totally worth it.

6. Some people swear by calibrating it frequently. However, more frequent calibrations do not necessarily make a huge difference in the accuracy. This is one fact that I cannot confirm or deny because I've never really closely examined it but the engineers state that 2 calibrations a day are all it needs, no more (unless prompted) and no less.

- I have tried both frequent calibrations (about every 6 hours, or when there was a 20% or greater discrepancy), and more recently, only every 12 hours when prompted by the dexcom or if there is a 20% discrepancy. I can tell you that, undoubtedly, calibrating only every 12 hours or during large discrepancies when my BG is stable leads to much greater accuracy than frequent calibrations

Terry, I typically find it takes approximately 12-24 hours for the dexcom to become accurate. As a result, I continue using my glucometer to make all decisions prior to this point


p.s. make sure to calibrate only when the dexcom is reading stable