How CGM marinating made my sensor more accurate on Day 1

What is "sensor marinating" and how it really helped in my case.

cgm sensor accuracy marinading
How to make your CGM sensor more accurate on day one.
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My son Gregory is wearing a Medtronic 640G insulin pump, along with Guardian Sensor 3 CGM (Continuous Glucose Monitoring). Before switching to an insulin pump he was using Freestyle Libre 1 and Freestyle Libre 2 sensors. It is very frustrating to see that sensors are not accurate enough on the first day of the sensor installation. You are actually losing a whole day until the readings are close to your finger prick readings.

CGM sensor inaccuracy on Day 1

Most CGM sensors are pretty inaccurate on Day 1, especially right after the installation. From my experience this applies to Freestyle Libre 1 and 2 but also now that he is using Guardian Sensor 3 from Medtronic.

Exercise your patience and wait

Most times things will be much better the day after. But isn’t it unfair to wait for a whole day to achieve an acceptable accuracy?

Our example:

Gregory goes to school on Monday, so we have to change his sensor early on Sunday in order to achieve accurate readings on Monday at school and get a clear view of his BG on Nightscout.

CGM Sensor marinating

It is a common tip around the T1D communities that if you “marinate” the sensor the day before, or at least some hours before, it will work better. TRUE STORY

Accuracy Test #1 – Marinating the sensor for 24h

I have installed the Guardian Sensor 3 on Gregory’s thigh 24h before the active one expired. I haven’t activated the new marinating GCM sensor until 24h passed.


  1. I have installed the new sensor on his thigh.
  2. I have waterproofed the sensor.
    Note: The sensor is not water resistant anymore as the Guardian Link 3 transmitter is still connected to the active one. So there is no gasket to seal the sensor metal contacts. How to do it?
    a) Apply a small cotton patch over the exposed sensor and the first oval adhesive patch.
    b) Apply a large Tegaderm over the sensor in order to protect it from water spills or even water during a shower.
  3. The day after, I have disconnected and removed the old sensor and transmitter.
  4. Charged the transmitter.
  5. Removed the Tegarderm and cotton patch very carefully in order not to remove the oval protective patch.
  6. Connected the fully charged transmitter to the new sensor.
  7. Activated the sensor from Medtronic 640G pump’s menu (Sensor Settings – Sensor Connections – Start New Sensor)
  8. I waited less than two hours for sensor warm up and pump alarmed for a calibration (Calibration needed).

What was different?

Usually you have another calibration 4h after and the next one is 12h later. In this case the next calibration was requested 12h after! I have also checked 2 times Gregory’s BG with a finger prick and it was really close to the sensor’s readings.

Sensor total duration: 7 full days

Accuracy Test #2 – Marinating for 12h

I have tried the same procedure but I have installed the Guardian Sensor 3, around 12h before the old one expired. The results were much different. Most calibrations were needed and the sensor was quite inaccurate. But not more inaccurate than installing the new sensor right away.

Sensor total duration: 7 full days

How was it before CGM sensor marinating?

Before starting to marinate the CGM sensors, their duration was from 4-6 days on average for Guardian Sensor 3 and around 10-13 days for Freestyle Libre 1 and 2. The accuracy remained almost the same on Days 2-7 but the sensors were really inaccurate on Day 1.

Disclaimer: This is just my personal opinion according to my experience with Gregory. This is not a medical advice. Any medical advice shall be taken only in coordination with your doctor.

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