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Medtronic 780G: Guardian 4 sensor still problematic

Diabetes Educator Ariela Nielson reviews the Guardian 4 CGM from Medtronic and how well it works for the newest insulin delivery device the Medtronic 780G.

Review of the Medtronic 780G automated insulin delivery system part 3 of 6.

Spoiler alert: the Guardian 4 CGM is still not up to snuff when compared with other available CGMs. As Shania would say, "That don't impress me much."

Medtronic 780G insulin delivery device

Medtronic has had a stand alone CGM for years but it has been a difficult one to market. Ask any diabetes educator how they would convince a patient to calibrate the CGM multiple times a day when there are 2 other options in the American market that do not require any fingersticks or calibrations. It’s hard to sell something in the diabetes community that requires more work than its competitors.

The Medtronic Guardian 4 which connects with the Medtronic 780G insulin pump system is an upgrade from the earlier Medtronic sensors in that it does not require calibrations and it is more accurate. Otherwise, it has not evolved much. The company continues to target a 7 day wear for sensors which they market as an adjunct to their 7 day wear insets, but this causes some issues for users.

If Medtronic would allow users to pick their CGM, it would make for a better user experience and improved time-in-range.

I have been saying for years that I wish Medtronic would play better with others. They are a proprietary company and do not collaborate with other companies, but at this day and age, this is their downfall. If they would allow users to pick their CGM (say Dexcom or Libre), it would make for a better user experience and improved time-in-range. Tandem is the first pump on the market to integrate with both Dexcom and Libre which should make using their pump easier and more accessible. These integrations also allow for patients to pick the most cost-effective, most accurate, smallest and best option for their body.

Sensor Evolution

If you follow the trajectory of available CGMs in the American market, you notice that there is a drive to improve accuracy, reduce size, simplify the application of the sensor and reduce the warm-up period. With each new generation of the Freestyle Libre and the Dexcom, you see a drop in the MARD (Mean Absolute Relative Difference) or an increase in accuracy. It is fascinating then that the Medtronic Guardian 4 has reduced accuracy with 10.2% MARD compared to the generation before it, the Guardian Sensor 3 which has a MARD of 8.7%. It is also the exact same size as its first generation, requires a complex sensor application and continues to have a 2 hour warm-up period.

The biggest change with the Guardian 4 is that calibrations are no longer required (we can all rejoice in that!). In the same amount of time that Dexcom increased wear time from 7 to 10 days, reduced warm-up time to 30 minutes, drastically reduced size and modified the application to 3 simple steps, Medtronic has managed to get rid of calibrations. If their sensors cannot keep up with the competition, why is Medtronic so focused on continuing to use them?

Sensor Comparison American Market 2024

2024 CGM Comparison chart

Using the Guardian 4 Sensor

When I was wearing the Medtronic 670G insulin pump with the Guardian sensor, I found that I could manage my blood sugar better than the system especially because I was getting kicked out of automated mode. I was getting up to 8 alarms in a day to re-calibrate the sensor even during the night when I was trying to sleep. I don't do well without quality sleep so I ended up switching to using the pump in manual mode and relying on the Dexcom for sensor readings. I did this for 3 years prior to starting on the Omnipod 5 which connects with Dexcom.

So, when I started wearing the Guardian 4 connecting to the Medtronic 780G insulin pump, it took me a while to re-learn how to insert the device and how to use the over-tape. There is a sensor and a reusable, rechargeable transmitter that are separate as well as 2 pieces of adhesive to apply during the application (I still don’t understand how I am supposed to use the over-tape!). My trainer brought Griff Grips for me to try using over the whole device as there are not many people who actually like using the supplied over-tape.

The Over-Tape

I had issues with the over-tape when I tried using it. After swimming, the adhesive came up a bit and I asked for help in fixing it. My friend helped me to fix it but ended up ripping the whole sensor out. The over-tape is too complicated, not great when it gets wet and needs to go!

This will all change with the newest Medtronic CGM called Simplera which is with the FDA waiting for approval in the US but is available in Europe. This device will be all-in-one similar to G7 and the Libres (no over-tape required). However, the Simplera is also designed to last for up to 7 days. I am not sure why Medtronic insists on the 7 day wear of the sensor as it would be so much more beneficial to patients to have a longer wear time. I hadn’t really thought much about this until I actually wore the sensor, but because of the design and wear-time of the sensor, you lack a sensor reading for a longer period of time compared to other models which can lead to less time in range.


The accuracy of the Gaurdian 4 is not a problem even though it is technically not as accurate as Dexcom G7 or Libre 2 or 3. When I first applied the Guardian 4, I still have a few days of my Dexcom G6 so I thought it would be fun to compare them. I had the Gaurdian 4 on my arm (only approved place for wear and supposedly the most accurate place to wear a sensor) and the Dexcom on my leg. I was surprised to see that there was a big difference between the two (100 points sometimes!) and that when I checked with a fingerstick, the Guardian 4 was much closer to the BG reading. I ended up ending my Dexcom sensor session early because it was so inaccurate.

I was pleasantly surprised to see the accuracy given that the research showed worse accuracy with the 4 compared to the 3. I also didn’t notice any issues with compression lows and did not have alarms alerting me that I needed calibrations or that I was getting kicked out of Smartguard (automated-mode). In other words, the overall technology is much improved over the 670G.

The Biggest Flaw: Length of Wear

The biggest issue to me is that the sensor lasts for UP TO 7 days but on average tends to last around 6 days. Medtronic supplies you with 5 sensors each month because of this. You might think that I am just complaining about having to inject something more often using 4-5 sensors each month compared to 3 for Dexcom and 2 for Libre, but it is so much more involved than that.

Each time the sensor is at its end, you first get a message that the sensor is updating and not to change it. The sensor has to fail for a certain time period before you get the message that it is dead and needs to be changed. Then you have to remove the sensor and charge the transmitter - usually for up to 20 minutes. Next you would place a new sensor and the pump automatically picks up the radiofrequency transmission that the transmitter is back on board and starts the 2 hour warm-up period. This would process could take 2.5-5 hours leaving me without Smartgaurd during a good portion of the day.

The biggest issue to me is that the sensor lasts for UP TO 7 days but on average tends to last around 6 days.

You also don’t know when it will fail so this would happen right after I had eaten something and I would end up going into the 300s. I understand that there has to be some time that we won’t be connected to the best technology but the fact that I have to go there maybe 5 times per month is really off-putting. The last sensor I had for my trial died on a day that I was busy working and was super frustrating. I kept having to respond to alarms that the CGM was not connected and then that I needed to change it. The number of interruptions during this time made it so hard to focus on any of my work especially because I was thinking about when I would be able to get the transmitter charged and then sensor placed again and kept hearing those alarms! I had to check my BG at lunch and dose my meal knowing that if I didn’t get the carb count right, my BG would be high soon. I am also someone whose BG increases with stress and frustration so I expected my BG to rise and I was correct.

Why does the sensor require a warm-up period?

The warm up period for CGM sensors has to do with the accuracy of the product. When a foreign object is injected into the body it causes an immune reaction. Our bodies sense that there is damage in the form of a prick and white blood cells rush to the scene to help fix the issue. With the influx of white blood cells, the pool of interstitial fluid gets dispersed meaning that the sensor reading will be lower than expected. In other words, the concentration of sugar under the skin changes as white blood cells rush around the sensor to repair the tissue that has been injured from the injection. With improved automated insertion and smaller filaments needed for reading interstitial sugar concentration, less damage to the body occurs and less white blood cells are required to fix the damage. Thus, warm-up periods can drop

Final Thoughts

Overall, the lack of an evolved sensor makes the use of the device really clunky. I realize how much I worried about the sensor dyeing at a bad time or getting ripped off early because I am not in the habit of taking my glucometer, extra sensor, transmitter charger and over-tape everywhere with me. It’s too many things for me to have to think about when trying to get out of the house, get my son to school and then get to work. The Medtronic algorithm really takes into the account the daily burden of life with diabetes and works to decrease that but the sensor takes us back to the starting line because it is needlessly difficult to use.

If Medtronic would agree to use Dexcom or Libre for the Medtronic 780G, I would definitely jump at the opportunity to order it. If Medtronic came out with a tubeless pump AND allowed you to use Dexcom or Libre sensors, I would be in diabetes heaven! But, as it stands, the sensor stops me from making the 4 year warranty commitment.

As I mentioned, the Simplera CGM from Medtronic is being used in Europe and is waiting for approval with the FDA. This will change the ease of use for the sensor, but the up to 7 day wear will remain. For now, we watch and wait to see if this new design can improve the user experience with the Medtronic 78G pump.

Let me know what your experience with CGMs has been. What frustrates you the most about CGM technology? What do you hope manufacturers are working towards with CGM?

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I’ve had T1 for about 45 years and have been using a Medtronic pump for about 30 years. I recently was using the Dexcom G7 CGM, moving up from the G5 & G6. I found the G7 to be easy to use and accurate. I have been using the Medtronic Guardian 4 with the 780G  for more than 6 months now. Like the author of this article, I used both the Medtronic G4 and the G7 for a few weeks to check on the accuracy of the Medtronic sensor. And, like the author, I found the Medtronic G4 to be more accurate than the G7, especially at levels over 200. I read, or heard, that Medtronic focusses on accuracy of…

Replying to

I'm so happy to hear that you are having a great experience with the Medtronic closed-loop system! Tech should be easy, so I'm looking forward to being able to try out the Simplera. I do still think that Medtronic should target a longer wear for their CGMs as both of their competitors have been able to accomplish this and, decreased breaks in CGM readings means increased time using the algorithm to its full potential.

Let us know when you get to use the Simplera!


Thank You Ariela for a well thought, composed, and informative article. Sorry that I didn't catch this when it came out. It's June 2024 for me. I have been using Medtronic CGMs for a few years. I do use a Guardian 4 now with my 780 pump. I agree with your thoughts on the unit. My main concern is is location. I lose 2-3 out of 5 sensors because when I insert then I hit blood and sometimes i can get the unit to work through it but not to often so they get scrapped. Medtronic doesn't replace them. They call them mis-inserted sensors. The other issue is being only allowed to use them on the arm. I have ri…

Replying to

Hi Paul. Thanks for your comment! So frustrating to not be able to use the sensor because of bleeding!

The arm has been found to be the most accurate place to wear the Gaurdian sensor. If your doctor is okay with you trying other places, you could try wearing the CGM on your abdomen. I'm not sure what the point difference is between the two locations but I will see what I can find out.

Also, you might try putting an ice pack on your arm for 5 minutes before you place a new sensor. This moves some of the blood away from the spot where you are about to inject and can help prevent or decrease the amount of…

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