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Post: Blog2_Post
  • Writer's pictureMichelle Law

Everything you need to know about the Medtronic Guardian 4 sensor

I've been using the Medtronic Guardian 4 continuous glucose monitor (CGM) for a year now as part of the Medtronic 780G system and this post is a run down of EVERYTHING you need to know about it ๐Ÿ‘‡.

๐Ÿ“ƒWhat is covered in this post:

  1. Overview of the kit

  2. Setting up a new sensor

  3. Troubleshooting

  4. Pros and cons

  5. The future

1. Overview of the kit

Here's what you get and what it all looks like:

Collage photo of the components of the Guardian 4 sensor

In the photo: 1. Disposable sensor, 2. Rechargeable transmitter, 3. Transmitter charger, 4. Insertion device, 5. Tape

This is very different from the Freestyle Libre where everything comes in a box ready to go and you just have to stick it on. It's easy to get the hang of though once you know how. Here are the components that make up the Guardian 4:

The CGM is made of two parts - the disposable sensor and the rechargeable transmitter.

Disposable sensor is inserted with the insertion device. The sensor is meant to last for 7 days.

Rechargeable transmitter which needs to be charged before a new sensor is inserted. When you first get the 780G you need to pair the transmitter with the pump which is really easy.

The transmitter charger takes a AAA battery which I've found only needs to be replaced 2 or 3 times a year. You just slide the back off to replace the battery, it's easy.

Transmitter charger with the back taken off to show a AAA battery inside

The insertion device is easy to use but quite big and bulky.

The tape is transparent and is used to secure the sensor to the skin. It's also recommended to apply another layer of tape over the top of the transmitter and sensor (so you use two pieces of tape at a time).

2. Changing a sensor

I made an Instagram reel earlier this week showing how I change my sensor with a step by step guide - check it out here:

Screenshot of instagram reel

The steps in the reel are:

Step 1: Take off the sensor and clean the transmitter

Because there's a lot of tape I find it helps to use an adhesive remover to help loosen the tape. I've used a few different types over the years and they all do a good job. I loosen the tape all around the edges and then pull the sensor away from my skin,

To clean the transmitter you are supposed to attach it to the green attachment so the moisture doesn't get inside the transmitter. I use the same adhesive remover as I use on my skin to get rid of the sticky residue. See this post for more about adhesive removers and cleaning the transmitter.

Photo of transmitter with sticky residue and green attachment

Step 2: Charge the transmitter for an hour

The transmitter needs to be charged before you can attach it for another week so you attach it to the transmitter charger and wait for the green lights to stop flashing. Once they have stopped flashing it means the transmitter is fully charged. I leave mine for an hour and this is always long enough, but I have also heard that some people only charge theirs for 20 minutes and that's enough to get a full week out of it. Experiment I guess?

Step 3: Attach the sensor to the body

The sensor is attached using the insertion device. It's something that is quite tricky to do one handed (i.e. if you are attaching it to your upper arm). You press the insertion device onto the sensor, then pull it away from the "feet", then hold it against your skin and fire it in. Then you pull the adhesive strip to stick it onto your skin. Next you get the transparent tape and apply this over the sensor, keeping the area where the transmitter attaches uncovered. (It's so much easier to watch someone do this than try to understand by reading it).

Step 4: Attach the transmitter, tape it and connect it

When the transmitter is charged, you can attach it to the sensor and add another layer of transparent tape over the top. If all is well you will get a green light flashing on the transmitter and then you will get a message on your pump asking if you want to connect the new sensor. (Yes you do).

Step 5: Wait for the two hour warm up and finger prick

Then we get to the boring part...the two hour warm up period. You will have to wait for two hours to get the message telling you that you need to enter your blood sugar reading. At this point you have to do a finger prick and the sensor will move to Smartguard mode. No more finger pricks after that unless you particularly want to, or if you have low blood sugar and want to verify it. [**Update - Medtronic have clarified that this finger prick step is to enter Smartguard, it's not actually to calibrate the sensor as it is factory calibrated**]


Like all technology, the Guardian 4 isn't without its quirks. In this section, I'll cover common issues like 'Sensor Updating' messages, what to do when sensors fail prematurely and calibration problems.

Problem 1: Sensor updating messages

It's pretty common to get "sensor updating" messages from time to time that look like this:

Photo collage of sensor updating messages

There's not much you can do when this happens other than keep an eye on it. In my experience, sometimes they rectify themselves and you don't need to do anything, but more often than not it means the sensor is giving up...

Problem 2: Sensors fail early

If you've had a sensor updating message, be prepared to get this message next:

Photo collage of change sensor messages

The sensors are supposed to last for 7 days but it's quite a regular occurrence that they don't. It's just an inconvenience really given the more involved process of changing the sensor than other CGMs.

Problem 3: Ordering replacement sensors

Thankfully Medtronic are really good at replacing sensors that don't go the distance. It's a very easy process to order replacement sensors through the Medtronic Penny app. There is a Self Service Request form on the app where you can enter the details of the failed sensor and request a replacement.

The only problem I've had with this in the past is that can take a long time for individual replacement sensors to be sent out (I assume due to supply issues). If at all possible I would always recommend that you have plenty of back up sensors in stock just in case.

Problem 4: Calibration not accepted

There are two reasons this can be a problem - one is that the calibration isn't accepted because the finger prick reading is too far away from the sensor reading. The second is if your blood sugars are particularly high when calibrating (over 20 from memory), then the calibration won't be accepted and you have to try again when your sugars have come down.

If the first calibration isn't accepted you have to wait 15 minutes until you calibrate again (so annoying if it's late at night ๐Ÿ˜ด). If the second calibration isn't accepted, you might get a sensor failed message and have to replace it. Here's a couple of examples of the messages you can get:

Photo collage of calibration not accepted messages

Pros and cons of the Guardian 4 CGM

So that wraps up some of the practical "how to" stuff and despite the occasional hiccup, the Guardian 4 has a solid list of pros:


  1. Doesn't hurt to insert

  2. Comfortable to wear (on my arm)

  3. Works seamlessly with Medtronic pump (obvs!)

  4. Accuracy - I'm happy to trust it

  5. All CGM and pump data is viewable on the Carelink platform

  6. Easy to order replacement sensors

It also comes with some annoying (but, it has to be said, manageable) downsides:


  1. Fiddly to insert

  2. The tape - it's large and looks messy by the end of the week

  3. Not the smallest CGM out there

  4. Uncomfortable to wear on tummy (for me)

  5. Set up takes hours - not practical if you need to change at night

  6. Early failure rate

  7. Sometimes there are supply delays

Overall, it's basically FINE. It does the job, it's accurate, I trust it and it is a critical part of the Medtronic 780G system which has sorted out my blood sugars and quality of life in a major way. Do I wish it was smaller, tidier, cuter and less faffy - YES. But am I happy with the system overall? Double YES!

The future

There's always exciting stuff on the horizon when it comes to diabetes technology. Medtronic are working on a new generation CGM which will address a lot of the things I've moaned about above with the Guardian 4. I am excited to hear when it will be ready for us to use but currently I have no idea on the timeline. Watch this space ๐Ÿ‘€. In the meantime, I'll keep on with the Guardian 4.

If you have any questions or comments, I'd love to hear from you.

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Thanks for reading. ๐Ÿ˜Š

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