2 days with Dexcom g5 & I am perplexed

I started talking with my doctor last fall about getting a CGM and due to my insurance deductible, I decided to wait until the new year. So this month, I took the plunge. My new Dexcom g5 arrived on Thursday. I read the Getting Started Guide and watched YouTube videos (over & over) until I was comfortable with inserting the sensor (into my abdomen). I was so excited to get started. I had read on this site, and many others, how having a CGM was life-changing. I was hopeful that my poor little fingers were going to get a much needed break from 6-8 finger sticks per day and that knowing my numbers every 5 minutes would give me the necessary information to go about my day confidently.

Day One: I calibrated twice after the 2-hour start up period. My numbers on my CGM were off by 30-40 all afternoon. So I calibrated a few more times. Then I read somewhere that “over calibrating” can cause problems with the monitor and two times a day should be sufficient. I ended up sticking my finger just as many times yesterday as I did before I was hooked up to my CGM.

Day Two (today): I calibrated once in the morning. Numbers have been off every time I’ve checked 30-50. Now I am sticking my finger to make sure my CGM is working correctly. This is counterproductive and not what I was hoping for. I’ve also read that the numbers on the CGM will be delayed from the numbers on my Meter. I’m thinking this is defeating the purpose of my getting this device.

I am really wanting this to work for me and wondering if there is something I am doing wrong or if my expectations are too high. Any expert advice would be greatly appreciated.

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I use the G5. I am usually within 10 points or so. Are you only calibrating when the arrow is level? And what is your sugar running? Within 20% is considered acceptable. Also, sometimes the first day or two is not as accurate. Don’t give up, give it some time until you learn the system. I’m sure there will be others here who can give you better advice than I have.


Thank you, Dave. My sugars have been all over the place - which is why my doctor recommended I get one. And I haven’t even looked at the arrow when calibrating (thank you for bringing that up!). I’m definitely not ready to give up…I’m hoping time and experience will help :slight_smile:

CGM 108 Meter 139
CGM 169 Meter 225

@Suzanne17, as @KCsHubby_Dave states (which is the same as Dexcom statements), the accuracy of the Dexcom improves after the first day assuming proper calibration techniques. For most people.

At this point, the extra calibrations may be causing you more trouble then it is worth.

My suggestion is to start over. Start with recommended calibration techniques. See how it works. Once you get a bit more familiar with the system then certainly experiment with what calibration approach works best for you.

To clear all calibrations to date, you can simply STOP SENSOR then START SENSOR. This will clear all calibrations and let you start fresh. (No need to remove the sensor or transmitter.) Do this when you see that you have a flat line cgm in good range and you expect the same for the next two hours. If you can avoid carbs and bolus insulin to maintain the flat line - that is great and may be helpful. When the two hours are up, you perform two back-to-back fingersticks (no waiting between - back-to-back) then input both those meter readings in as your initial double calibration.
At this point, do not over obsess about how close the Dexcom is. Let it be. If close - great. If not - it is what it is. At least give it a chance. In about 12 hours (plus or minus a few hours is no difference) the Dex will ask for another calibration. If you are not level, wait longer until you are level. Calibrations with UP or DOWN arrow are generally not helpful. Diagnonal (cgm) arrow is not the end of the world but certainly level arrow is the best. Put the calibration in. Don’t worry about how close it is or is not. Now wait for the next 12 hour calibration. Do the same.
At this point, you have the initial double calibration, then the first 12-hour then the second 12-hour. Your Dexcom should now be getting into the time of its more accurate reading particular when you get the next two calibrations in the next 12 hours and the following 12 hours.
At THIS point, see what the accuracy is.

Hopefully it is accurate and works.

If still not accurate (by Day #3) then at least you tried the recommended approach. If this approach does not work then further troubleshooting might be appropriate.


This is soooo helpful, Tim. Thank you so much for taking the time with explicit directions. (I am quite obsessed so that statement alone is worth gold!). I am going to try exactly what you said. I probably have not calibrated correctly because I just wanted to plug it in and - voila! I will let you know :slight_smile:

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Check you current medications list. Acetaminophen and ascorbic acid will affect the accuracy.


If bg’s are all over the place, as you stated, it’s nigh impossible to make ANY CGM give accurate numbers. I’ve had a G5 since Sept, as has my wife, and we are blown away by the extreme accuracy of it nearly all of the time. When it is not matching the meter reading, usually waiting 15 minutes and checking the G5 again, it will generally correlate well with the meter reading from 15 minutes prior.

Never calibrate when it’s got anything other than a level arrow. Don’t over calibrate. the first day might be “iffy” for the first part of the day. Mine usually is accurate from the get-go. Since September, I’ve had just one sensor that didn’t work well by day 2 so it was replaced by Dexcom.


@Suzanne17, congrats on the new G5! My son’s CGM was a life changer.

You are absolutely right in thinking that calibrations are the key to getting a good CGM reading. We spent probably 6 months figuring out what it takes to do it right, and we had to read many different people’s input to figure it out. So I understand what you are going through!

If you are interested in learning further about calibration, an in-depth discussion of calibration procedures is presented in detail here:

We have found that, for calibration, no shortcuts are worth the loss in accuracy: we tend to go strictly by the book every time. In particular, unless we are truly far off, we wait until the curve is as flat as can be to calibrate, without either a recent meal or snack, or a lot of IOB. Good luck!


Some sensors/insertions are way off the first day. It can take 24 h or longer to settle down. Remember, the sensor will stop after 7 days but you can leave it in and simply re-start the old one as if it were a new sensor. Given the day-1 problems, it will actually be more accurate that way. People tend to get anywhere from 10 days to 3 weeks out of each sensor.


I just want to say thank you and let you all know I REALLY appreciate your expertise and feedback. I took Tim’s advice and last evening, when I was leveled out, I started over. I input new numbers after the initial 2-hour start up, then I ignored my CGM until it asked me to calibrate this morning, 12 hours later. While the numbers weren’t right on, I already feel so much better about this whole process. I’m sure I totally messed up on Day 1 by over calibrating. Dexcom sends the “Getting Started Guide,” but there is very little about HOW to correctly calibrate. It makes sense with the arrows - but the guide says nothing about where they should be during calibration. I will also read the article sent above from WestofPecos, Thank You! I’m going to calibrate again tonight (after 12 more hours) and hopefully see better numbers tomorrow.


How will I know when the sensor needs to be replaced if I leave it in for more than 7 days?

Short answer - it will be obvious.

Duration can be substantially different for different people.

For us, when calibrating, the meter and Dexcom are usually within single digits of each other. The graph is a smooth curve/line. It is a system that is obviously working.
Then we get to the day where the difference between the meter and the Dexcom is larger than it has been. The graph starts to get “jumpy”. We start getting more dropouts and missed readings. Sometimes we replace the sensor and sometimes let it sit. If we let it sit, almost always the next day the graph is even worse. Starts to look like a shotgun pattern. The Dexcom moves further and further away from the meter reading.

That being said - easiest way is to just watch yourself.
Chances are - it will be obvious.

Some people report being able to use the same sensor for six weeks.
We usually get about 12 good days.


Thank you, Tim.

@Suzanne17, I could have written @Tim35’s post :slight_smile: But my boy has a very lean body, a body type that is often associated with short sensor durations. Some people get multiple weeks with restarts.


Another hint in terms of restarting the same sensor.

You can restart anytime you like. As you know as you mentioned you just did a restart.

A single sensor session can not last more than exactly 7 days. Or 168 hours.

You can look on the receiver under settings to see when your session started (in case you forget - like me - I can’t remember exactly when) so you can see exactly when the session will end.

If the session is going to end at an inconvenient time (sleep, meal, etc…) you can always restart the session early by hours or maybe the day before at a time of convenience when you expect to have a level 2 hour BG while you wait for the “warm up”. At the session restart, the 7 day counter resets regardless of how much time is left or not on the previous session.


That is awesome, thank you! I have already learned so much here & appreciate your help :smiley:


Besides all the great advice above (I also often have issues on the first day), you might also think about how confident you are in your meter readings. When I first started with the G5 while using my old OneTouch Ultra, I’d see general agreement, but still off by 10-20 pts or more on a regular basis. Since changing to the Contour Next, it’s seldom more than a few points different (unless obvious rise/drops are occurring). Obviously, if the meter isn’t that accurate/repeatable, the G5 calibrations may get off as well.
(btw, I’m currently on day 9 of a session that was terrible for the first 12 hrs)


If you are in the second or third week, replace the sensor after you start seeing a jumpy graph or get “???” that won’t resolve. If those things happen in the FIRST seven days, Dexcom will replace the sensor, but they want the “???” to be present for at least 3 hours. They also want you to calibrate every 15 minutes, 3X if you get readings far off from your meter. I usually get 9-12 days out of each sensor. One time I went 19. My wife goes over 20 days, all the time.

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Thank you, vanc. I also use the One Touch Ultra and never thought my meter could be wrong. I will definitely keep that in mind.

I had a similar experience as Vanc. Starting out with Dexcom I was using OneTouch meter for calibrations. After reading the meter comparison studies linked on this site I switched to Contour Next. Dexcom accuracy improved after the switch, likely because I was calibrating based on a meter that was not accurate.

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