My first CGM: Is this normal?

Hey everyone,
I just got my first CGM yesterday (a Dexcom G5). However, my experience so far has been pretty poor.

Once I got booted up and running, around 4 pm, I had a small snack and bolus. Shortly after, it showed my BG jumping well over the 200 range. I waited it out for it to drop then had dinner a few hours later and it again shot above the 200 range, sitting there an hour or so and then dropping to sit around 130. This was pretty unusual as my last reading over 200 was almost 2 weeks ago and it was an anomaly.

Before I went to bed, I tested with my regular glucose monitor when my CGM was still sitting at 130 and found out that I was actually at 68.

So, I went to bed, interested to see my overnight pattern, but several hours of data were lost due to a signal loss, even though my phone was sitting on the night stand right next to me all night (I am using my phone to track, btw).

This morning I recalibrated again and while it wasn’t off by as large of a margin as last night, it was still reading about 20 below what I was really at. Less than an hour after I recalibrated and ate breakfast, it set off an alarm for low glucose, reading 68. I tested and found that I was really at 85.

Is this what it’s supposed to be like or is something going wrong here?

You have to remember that there is a margin of error for your meter as well as the CGM, so 20 point difference isn’t too abnormal–especially in the first 48 hours of use. Getting great accuracy is somewhat of an art.

Some key points to remember for using the CGM:

  1. Your nighttime signal loss could be issues with your device, but likely just that your body covered the sensor. You could consider a different location.

  2. Not sure if you have had this issue yet, but if you put pressure on the sensor (laying on it) it will give false lows or even highs.

  3. Only calibrate when your BG is stable.

  4. Do not over calibrate. Only 1 time every 12 hrs unless greater than 20% difference when BG is stable.
    Official Dexcom statement:

"…and whenever the sensor is inaccurate. Inaccuracy is defined as when the difference between your sensor glucose reading and blood glucose value is greater than 20% of the blood glucose reading and blood glucose value is greater than 20% of the blood glucose value for sensor readings > 80mg/dL or greater than 20 points for sensor readings < 80 mg/dL. "

  1. The first 48 hours are the least accurate. You can restart the sensor (while keeping the same on) after your week is up, but the total lifespan varies among different people.

Hope this helps…


Sounds pretty normal to me. Normal in the sense that I have seen such behavior many times. I also had sensors which were spot-on from day one. Each sensor is different. I always start out not trusting the CGM at all. I use its readout to predict my BG result. As predictions get better, I increase the time between finger sticks. If predictions get worse, I decrease the time between finger sticks.


Hi, Josh! Congrats on your first CGM!

If what you describe is activity that happened within the first two days of a sensor start, I would not be discouraged at all. I agree with what both @Bradford1 and @Helmut have stated. For any new sensor my son, Caleb, starts up, we are very careful for its first day or so. If we see a number or trend we think is off, he checks. Each sensor needs to earn trust, and almost all of them get there pretty quickly.

Nighttime - we don’t have issues at night - knock wood. Lots of people talk about false pressure issues. Maybe we have them, and I’m just not paying close attention, or I wait it out and all is well. Caleb does tend to get ??? more at night (early morning hours) when a sensor gets old. But otherwise nighttime is good. We get occasional “signal loss” messages, even when his phone is right with him, but that’s rare as well.

What are you thinking about the rises after you eat? Do you think they are not accurate? One of the first things we found very eye opening and useful when Caleb started using a CGM eight years ago, was seeing exactly how his body reacting to eating, and how it varied based upon what we was eating. We changed our insulin strategy based upon this information - using super bolus techniques for certain meals, for example. I also find that when there are rapid rises and falls, dexcom can take a little time to catch up, or its margin or error is off a little more than when bgs are more stable. This is particularly true for highs.

I think you’ll get into a groove and begin trusting it with more time. We find it’s really quite reliable, and we do dose off of its readings once we are comfortable with each sensor.

Good luck and please report back!

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My first CGM was NOT a Dexcom, but the experience of the other CGM required me to do all the things that helped make my Dexcom as accurate as it can be.

Preparation: Preparing my site for sensor placement. Not only do I wipe it down with an alcohol pad, I put a hydrocolloid bandage on the site before that, so I have a barrier between me and the sensor. The bandage helps keep sweat away from my sensor. This helps my sensor to be more accurate and my site to be less itchy. This becomes important when you want to wear your sensor for longer than a week.

Placement: Somewhere that I don’t sleep on it for comfort and good readings.

Coverage: I use Opsite Flex-Fix to protect my sensor from water coming into it. My receiver shows a LOT more ??? when I get water on it. It just does.

Conscious Forgetfulness: This refers to the ability to both remember that I have my sensor (and transmitter) on my body–and where it is-- and at the same time not be annoyed by having to remind myself. You’ll get used to protecting your site without even thinking about eventually.

I would also say that the CGM is really for trends. I can take a BG meter reading if I want to know what my blood glucose is at a certain moment in time. The CGM can tell me where I’m going. It takes a few readings from the meter to do that.

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I like this, Helmut. @Josh_H, I realize that a lot has been written about the G5’s ability to serve as a basis for treatment without a confirming fingerstick. But the manual also says that if you have any reason to doubt the G5 number, confirm with a fingerstick before proceeding.

The first day of a sensor is often less dependable than later.

Don’t be afraid to stop and check with a fingerstick, especially in the first 24 hours. When your CGM indicated a jump to over 200 (11.1 mmol/L), did you check with a fingerstick? Did you do something to help drive down the glucose level, like exercise or take a correction bolus? These types of decisions, especially on sensor day one, are best made with a confirming fingerstick. Once a sensor earns your trust you’ll be able to make decisions on sensor data alone.

CGM technology has been a boon to most people who try it but you learn pretty quickly that it can’t be depended upon 100% of the time. You will learn this quickly as you live with the sensor 24/7.

Welcome to the CGM world. It’s been one of the best tools I use to manage my diabetes. Stick with it and act on what it’s telling you and you will be rewarded with better, less variable glucose levels.


Something that might be found interesting - some people soak their sensors prior to starting them. When they know they are ready for a sensor change, they insert a new sensor with an old transmitter a day or so before shutting down their current sensor. This gives the sensor a good amount of time to soak and they find accuracy more reliable when they go live w it.

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Be patient. Although Dexcom usage is mostly straightforward, there is still a learning curve to being successful with a CGM. You’ll find that a few months from now, you’ll be an expert and of course you’ll have lots of good results before then. I have used CGM’s for 9 years and am still learning.

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Hey Lorraine,

The rises that I saw yesterday just seemed to stay too high for too long compared to what I’d seen with normal finger stick readings. I can understand it peaking high shortly after eating, but after 2 hours, I hadn’t seen a 200+ number in a very long time and the meal I had didn’t make sense for that type of reaction. However, seeing a 130 on the CGM and 68 on my regular meter when I went to sync it up was what really threw me off.

That being said, I guess it is just due to being within the first 48 hours as others have said.

So far today, my readings have been a lot closer in line with what I would expect.


Josh, when you went to sync was the CGM showing flat and straight or was it showing up/down ?

It’s worth mentioning that this sounds like what I’ve experienced when I’ve hit a capillary on initial insertion. It can really throw things off in the first day, much more than the 48-hour variability we’re told to expect. The needle is very fine so it doesn’t necessarily hurt the way it does when you hit one with a syringe or infusion set, but when I see really unexpected readings on the first day (e.g., a strenuous long bike ride where my CGM stayed pegged at 170 and when I got home and did a finger stick it was heading into hypo territory) I tend to suspect this may be the case. Sometimes you can confirm it by looking for the tell-tale discoloration of blood under the sticky patch, but the transmitter holder is big enough that a small bleed can be hidden under it. There have been times when I suspected this problem but only had it confirmed when pulling the patch off for the next sensor. The first time it happened to me (the aforesaid biking example) I could see it had bled and pulled the sensor, thinking I had to replace it, but in fact that was a waste of a good sensor–I later learned that the erratic readings will usually settle out after a day or two.

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When I synced it for the first time, it was just starting to straighten out to the 130-120 range after it had trended downwards from from 200+.

I have off numbers like this all the time with Dex. It’s still something I can’t live without but you have to expect it will be very off at times and never bolus or do anything based on Dex unless you have also done a finger stick or two. I have learned that for myself anyway. It has been FDA approved now for accuracy without a finger stick but I would not rely on it alone due to my experience with it.

I am a relatively new user (6 months) and think it is fantastic.

I have had occasional signal loss at night and think it might be my phone. Now I restart it before I go to bed and have had only one signal loss since.

I found that the advice I had to inject 15 or 20 minutes before a meal was not correct for my insulin - humalog. It needs at least 45 minutes before the blood sugar starts to dip noticeably.

You also need to remember that the fluid just under the skin is 10 minutes behind the actual blood sugar reading and as the G5 transmits every 5 minutes reading is actually 15 minutes behind the actual blood sugar and I guess this may vary when the sensor is placed on different parts of the body as some people report they do.

As someone above said you go on learning about how it works for you.

Incidentally I find my current sensors lasting longer going over 2 weeks. I wonder if Dexcom are slipping out the G6 ones early to avoid a gap in supply before there introduction in 2018?

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Ditto. Seeing is believing. CGMs are awesome !!!

The G6 sensors are incompatible with the G4/G5 transmitters.


I found this post by @rgcainmd to be very helpful for new (and old) Dexcom users, the advice worked for me anyway!
What method do you recommend for calibration?
There are “good” and “not-so-good” ways to calibrate. The following is what has worked well for my daughter:
After starting a new sensor (or re-starting one that’s still working at the 7-day mark), wait at least 15 minutes between the first two calibrations.
Although it can be very tempting, do not calibrate more than twice daily when requested to do so.
*Exception: I throw in one to two extra calibrations during the first 12 hours of a new sensor. Kind of like yelling, “Are you finally understanding what I’m telling you?”
**Sometimes I get a little calibration-happy when my daughter’s sensor is getting a little “ripe”, i.e. over 15 plus days old, if the Dex readings seem significantly wonky. At which point, she’d be better off if I just bit the bullet and put in a new sensor.
CALIBRATE ONLY WHEN THE DIRECTIONAL ARROW IS HORIZONTAL. I repeat: CALIBRATE ONLY WHEN THE DIRECTIONAL ARROW IS HORIZONTAL. Trust me on this! Your sensor continues to function after the big red blood drop screams at you to calibrate right now, dammit. Tell it to hold its horses and you’ll calibrate when you’re good and ready to (when the directional arrow mellows out and becomes horizontal.) No ones Dexcom has stopped working or imploded, creating a black hole that engulfed the universe, because they didn’t calibrate the instant the Red Drop of Dexcom appeared.
Try to avoid calibrating when BG is below 80 or over 250. Correct and get your BG within this range and then calibrate ONCE THE DIRECTIONAL ARROW IS HORIZONTAL.
Learn to question the accuracy of your BG meter as much as, if not more than, your Dexcom. Personally, I trust the Dexcom more often than I trust the BG meter. But my daughter’s Type 1 vibes seem to resonate exceptionally well with those of her Dexcom. I often think that my daughter and her Dexcom are as one.
Stay well hydrated. I believe it helps produce enough interstitial fluid that more accurately reflects BG levels.
Good luck. In short order, you will wonder how you lived without this miracle of diabetes technology.

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One other thing that I found personally, my nighttime readings will read low if I haven’t stayed well hydrated during the day. I’m guessing its because it reads the interstitial fluid and not the blood, when I stop moving much the readings seem to drift falsely low. Problem solved if I drink enough water.

I do see this continually being brought up from the same OP. Be aware this is absolutely not what the Dexcom manual (nor Dexcom technical support) states to do.

In my opinion, this does not make any sense.

We have ~ 5 years good experience with Dexcom.


I agree with most of the advice given there, except the 15 min at the beginning calibration bit. It doesn’t make sense IMO either, since the point seems to be just to avoid a single meter reading
(and its inevitable error) from overly swaying the initial calibration. It’s different when you are correcting an errant sensor later on and trying to steer it back on course (that’s when Dex tells you to do that 15 min thing, because one time point may not be enough to change the Dex’s course if it’s way off).


Taking into account the 15-minute lag with the CGM, wouldn’t that mean that your blood sugar in reality starts dropping 30 minutes after injection, not 45?