Wildly Inaccurate Medtronic Guardian CGM

Looking for who else was maybe in this predicament and what you did.
My insurance recently dropped coverage for CGMs other than Medtronic’s Guardian. I got into a 30 day trial with them, and am now 3.5 weeks in. Ever since starting with the system, my SG has been wildly inaccurate, sometimes up to 75%. This is all while my BG trend is stable, between 70 and 250, and I’m comparing my BG to the SG 10 mins later to account for the delay. On top of this, any time I am below 100 (quite often as this is my target BG), it automatically brings me down to a SG of “below 40” even if I remain steady at whatever my BG is. I know people say don’t get hung up on the numbers, just use the trend line, but at this point, I can’t even trust the trend line. Even if I could, what’s the point of the CGM if my readings are wildly off? I even have some left over Dexcom sensors that I’ve kept using to try and compare and it continues to read my BGs perfectly. So it’s clearly not that I have a bad placement on my body or something else either.
My trainer can’t give me any answers as to why this is happening at this point and the rest of the Medtronic customer service reps I talk to say that margins of error up to 40% are perfectly reasonable. WHAT?! How is that reasonable?
I’m just looking for anyone else who had a similar issue and what you did about it? Did you just give up? Was there something you were doing wrong?

I think that this could be a dangerous situation. You simply have to take manual glucose readings, now. Do me that favor because it could save a lot of heartache and we have to be especially careful during this period of time.

You did a 30 trial. You gained valuable information by doing your due diligence.

I don’t think that you should be wearing the MT sensor without wearing a Dexcom sensor at the same time. That’s the ideal way to test a new sensor while protecting your own safety, I think.

You might try to gain insurance approval for the Dex by having the Doc write a note that the MT sensor just doesn’t work for you. Sometimes they can claim ‘medical necessity,’ and override those rules.

Who’s your insurer? Maybe someone on here has worked with them before.

1 Like

Don’t worry, I’ve been going off manual BG readings and my Dex ever since I started because I haven’t trusted MT’s.

I’m certainly going to try getting insurance to cover Dexcom out of medical necessity, but I’ve been told it’s unlikely to happen. I have BCBS of MN, so they’re pretty stuck on Medtronic since it’s based here.

I just can’t understand the reasoning behind why it’s soo off. I can’t seem to find anyone else who’s had this level of accuracy issues.

1 Like

I’ve had mixed results with MT. When I started Dexcom 3 weeks ago, I kept my MT in to compare and they were pretty much dead on the same.
However I know the MT does go weird often.
I wouldn’t trust it to make corrections on my pump. That’s why I switched to tandem and dexcom.


@Rphil2, any suggestions ?

Hi MM, I am sorry this is happening.

I have one suggestion and three observations. My suggestion is to place the transmitter on your arm. It will improve the accuracy several times over. Several also place it on their leg . I find it tends to rip off when walking or dressing or undressing. But those who use it in this way have found it to be very good. In almost 5 years i have only had to place it on my abdomen twice and I found it to be unsuccessful.

Now my three observations. The sensors are far more accurate below 200. Anything above 200 is iffy. To some degree this occurs with Dexcom as well. All sensors (cars, water, diabetes, electrical are designed to operate best in a rather tight tolerance. Going above 200 magnifies any inaccuracy. It makes sense. Suppose the standard error is 10% at 100 that means anything between 90 and 100 is acceptable. at 200 it spreads to 20 above and below (180 to 220), at 300 the spread is 270 to 330. The higher your blood sugar the more off it can be and still be within tolerance .

Generally i work hard to keep my blood sugar between 80 and 150 and the sensor works well for me.

Second observation - as you likely have heard the first 24 hours can be a bit funky. This happens with the dexcom and Libre as well. In my wearing both of the G5 and the Libre I have found few differences. But definitely the first 12 hours can be more iffy, the data suggests the first 24 hours, but i generally find it is not an issue after 12 hours.

My final observation is less a rule instead of just a feeling. The more secure I get the sensor affixed the better it operates. This is especially important when it is affixed to the arm.

I hope something int hat might help out.


Now I have to do a disclaimer. Sorry for this

I am a Medtronic ambassador. My opinions are my own. They did not pay me to say nice things. OK, they sent me a shirt and a cup but even I am more expensive than that.


Its a common complaint - about the error.

Ahhhh, sure. I’m in MN, too. Yep. Everybody here is married to Medtronic. Its an unhealthy marriage.

We all just unhooked United Health and Medtronic. Since they are both based here, they were kissing cousins.


My Guardian 3 sensors often register wacky readings during the first 6 hours. Are your sensor numbers well off even after 12 hours?

I have read that some users wait 6 to 8 hours after the warm up period is over before calibrating which seems to help.

1 Like

Rphil makes a good point point. Instead of just dumping on MT (which some of us really enjoy doing), I suppose we should troubleshoot like we do with every other system.

Do you have any data where sensor compares to manual sugar that we could look at?
It might help steer us.

BTW, Dr Farah Khan, over at Minnetonka Medical, knows that sensor error exists and takes it pretty serious. She knows how to write a letter of medical necessity. She’s the president of ADA in MN. If your insurance works with her and you need a new Doc, she might be someone to try. Khan likes Tressler.

Dr Tressler is also very good - he’s at the U of MN, last I heard. People :hearts: Tressler - patients, staff, everyone. Tressler says that there’s a Anders Carlson over on the East Side of St Paul who is very good with tech. I tried to schedule with him the other week, but it was a 6 month wait.

Can somebody explain a little deeper about Letters of Med Necessity? I’ve gotten lots of them. Is this difficulty on the insurance side (we need a Doc who will follow through and do the work required for basic patient advocacy?) or, what? What’s the brick wall here?

@svlask, I would do detailed documentation of the device failure in your BG records so the Doc has strong evidence to support them in the Letter.

1 Like

Thank you for your response! Regarding your 3 observations/suggestion-
Placement suggestion- Yes, I’ve always used my arm for site placement. Abdomen is fine accuracy wise but I can’t keep it adhered well. I can’t use my leg as I play rugby and my position has people grabbing thru and holding onto my legs… essentially a recipe for extreme disaster. I also don’t want to ruin my legs with sites, and I shouldn’t have to since every other CGM works fine on my arm.

  1. Range- the most wildly inaccurate SG readings I’ve had are actually under 200 and above 60. Additionally, their user guides actually provide the study results for the MOE to expect, and tops it goes to ~17%. Either way, a 75% MOE is not acceptable, even if I’m 300. I have never had a MOE beyond 20% on the Libre or Dexcom.
  2. First 24 hrs- yes, I’m aware, but honestly, besides the first 24 hrs of a new transmitter, neither Dexcom or Libre have these issues. That all being said, I know MT’s CGM absolutely sucks the first day, so these MOEs I speak of don’t actually include anything from the first 24 hrs. If I did, the max MOE would be 200%.
  3. Secure-ness- I’m aware, and I don’t think it’s a problem. Playing rugby, I’ve come to know exactly how to make these suckers stay put, and for long lengths of time.

Please know I really appreciate your detailed reply and I know you didn’t know I knew of the observations already, but it’s been gone over ten fold with MT already, and they can’t come up with anything else.
I completely love Medtronic and know they have quality products, but I honestly just think their system doesn’t work for me. I have heard of others with the same issue, but they just gave up, which is why I wanted to see if anyone had kept trying and found a solution. I think at this point, I just have to accept they won’t work and ask my doc to send my insurance a medical necessity form and pray it gets approved for the Dexcom or Libre 2.
Thank you again for the detailed and thought out reply!


Good notes-

I’m not looking for a doctor, and I know mine will provide a great medical necessity letter. She made up a very good letter for my work from home request to my boss via ADA, and she’s great at blending the proof, scientific reasoning/suggestions, and persuasion. So There’s no worry of getting a good form/letter into insurance- it’s just a matter of whether or not insurance approves it. A doc can write a letter and you can prove with records all you want, but insurance can still deny. (America’s great, ain’t it?! Haha)

Also, don’t worry, I’ve been saving my manual Bg readings in the app this whole time for the very reason of proof- and making notes of the MOE and what my Dexcom has shown (to prove what does work for me) as I’ve kept the Dex on to help with the transition.


My trainer did suggest putting the sensor in 12-24 hrs prior to officially attaching the transmitter. It helped one time, in that I had a little less MOE, but it was still over 20% most of the time. The other time it didn’t seem to make a bit of difference.

Fingers crossed for ya’. I don’t think that many are able to overcome that level of sensor error…not with this particular device. I hope your letter gets approved. I hope, I hope, I hope.

1 Like

That’s my feeling too unfortunately :pensive: Thanks to everyone for replying and trying to think of things- I really appreciate it!

1 Like

All the reason to go to DexCom.

@Jay6 Thanks for that super helpful insight, but if you read my first two sentences, you’d know I don’t have coverage of anything but MT.

@svlasak, Sarah, my comment was TOO brief. Your situation, which deserves the very best the world has to offer, may involve some uphill climbs. If MT is not working for YOU, the assistance of your other team members may be necessary.

  1. Will your MD write a letter of medical necessity stating MT is not working & the benefits of DexCom over MT in terms of down the road costs of diabetes complications, etc.? See#3.

  2. Are there any jurisdictional rules your state insurance commissioner’s office can sway?

  3. If your insurance uses a PBM that is making these rules, try these same steps with the insurance company itself as a potential cost saving for them & you. PBM=pharmacy benefit manager.

  4. Involve elected legislative officials at state & Federal levels. Communication from an elected official or #2 regulator may shake the insurance company tree enough to have DexCom coverage moved to a lower branch, easily within your reach.

  5. Involve an investigative journalist. Is MT bribing, now possibly illegally doing kickbacks, their way into your insurance company’s product selection process?

My brain storming is just that. Opening Pandora’s box enough to get YOU what you need to care for yourself, safely, effectively, & economically in what should be an open market. I gleaned these ideas from other posts, here & jdrf.org.

Keep us informed. Your sharing is how we all learn.

I too had seriously wild readings with MT, which is why I switched to Dexcom years ago. A CGM that is not reliably accurate is worse than worthless.

As others suggested, please exhaustively document your observations – MT vs Dex vs BG meter. More is better, including BG graphs.

When fighting my insurance, my doctor added a paragraph in his letter to the insurance that if they reject his observation that the MT doesn’t work for you while the Dexcom does, he will deem that to be an indication that they are taking over medical decisions for your care, and are accepting full responsibility and liability for any related medical consequences.


I won’t use Medtronic anymore because of these issues. I had them for years with Medtronic before switching to Dexcom and multiple other pumps. I have heard people say that they have really improved over the last few years but sadly your post shows that they have not. I switched to a Tandem pump about a year ago. Initially, I had to pay out of pocket to “rent” it. See if your insurance will cover the supplies if you rent one ($1000 to rent and the length of time does not matter.)

This applies to people with a current non-Tandem pump under warranty. The $999 allows you to use it until your non-Tandem pump is out of warranty, then your “rental” ends. The program was supposed to end, but looks like it is still available.