The ultimate guide to Continuous Glucose Monitors (CGM)

By Henrik, FounderJune 5, 2020

Whether you have diabetes, are struggling to improve your diet, or are just curious to understand how your blood sugar changes over the course of a day, a continuous glucose monitor (CGM) can be a really valuable tool. In this article, I’ll explain what a CGM is, how it works, and what you can learn from using one. I’ll also review three different models that are currently available in the US, and review their functionality, price, and more.

What is a Continuous Glucose Monitor?

A CGM is a small wearable device that you attach to your body to measure the concentration of sugar in your blood. Monitoring blood sugar levels is critical for anyone living with diabetes, but can also give insight into how things like food, exercise, and medication affect your body.

Without a CGM, measuring your blood is both painful and cumbersome. The most common way to do this is via the finger prick method — you prick your finger, place a drop a blood on a test strip, and insert it into a meter to get a single reading. With a CGM, you automatically get an updated reading every 5 minutes. This makes it far easier to keep track of your levels throughout the day, even while sleeping or exercising.

There are three main manufacturers of CGMs on the market today: Dexcom, Medtronic, and Abbott. I’ll be comparing their latest models below.

How do CGMs work?

All CGMs on the market today use roughly the same technology. They are invasive, which means they puncture your skin to measure glucose through interstitial fluid found just a few millimeters below.

Source: Abbott

The sensor’s filament is coated with glucose oxidase that converts glucose into hydrogen peroxide, which reacts with a platinum receiver. This generates an electric signal that is then sent to a chip located inside the outer casing.

These signals are then sent to an external transmitter (either a custom device or your phone) where an algorithm is used to predict the actual glucose level in your blood. This algorithm is not 100% accurate, but has high precision and is approved by the FDA for use by both patients and doctors. In fact, it is shown to be more accurate than some finger prick meters.

What can you learn from a CGM?

At any given moment, 42 different factors can impact your blood sugar level. This means there are many things we can learn from gathering and analyzing more data. The three most impactful categories are diet, exercise, and medication, closely followed by sleep and stress. Some examples of things you can learn include:

  • Which meals spike your levels, leading to fatigue and hunger?
  • What time should you eat dinner to ensure stable overnight glucose levels?
  • Which type of exercise has the least impact on your levels?
  • What happens to your levels on six vs eight hours of sleep?

Which CGM is the best? Let’s compare the Freestyle Libre, Dexcom G6, and Medtronic Guardian.

In this section, I’ll compare the three major brands in the US today. I’ve personally worn a CGM every single day for the past three years, have tested several different models, and have used over 100 individual sensors. I am also in regular contact with all three manufacturers through my work at Steady Health.

Abbott Freestyle Libre

The Freestyle Libre from Abbott

Let’s start with the most popular model, the Freestyle Libre from Abbott. It’s the most common device due to its sleek design and low cost, but it’s not delivering data continuously, as it requires you to actively scan the sensor with your phone to download the latest data. (It’s actually considered a flash glucose monitor, or FGM.) The sensor itself stores up to 8 hours of history, so as long as you scan regularly you won’t miss any data.

The LibreLink App

Functionality: The sensor is low profile, easy to insert, and stays on pretty well. Towards the end of each session, you might want to put on some tape to ensure it stays on. It has a 14-day wear time and a 1-hour warm-up period before it’s ready to use.

The app experience is basic but works well. It shows you daily patterns, average glucose levels, and other standard charts. It also allows you to log events like meals and exercise, but does not support pictures which makes it harder to use for food journaling.

Cost: The price is its largest benefit. Every sensor can be worn for up to 14 days and the cash price is around $60 per sensor. If you have commercial insurance, many pharmacies guarantee a max out of pocket amount of $75 per month. This means you can each sensor for $37.50.

In summary: Great wear experience, 14 day wear time and affordable, but only has basic app functionality. I consider Freestyle Libre to be a great choice for anyone that is curious about CGM and doesn’t need all the bells and whistles.

Dexcom G6

Dexcom G6 (source)

Next up is the G6 from Dexcom. This device is the most advanced CGM on the market as it sends data directly to your phone via Bluetooth. The data it provides is also available in an API so you can easily access it in raw form.

The Dexcom G6 app

Functionality: The G6 sensor is both larger in size and higher profile than the Libre, but still small enough to not be in the way. It has more of a medical look and the adhesive around it is bigger too. It has a 10-day wear time and a 2-hour warm-up period before it can be used.

The app experience is similar to the Libre: basic, but works well. You can log events, but pictures are also unsupported.

The biggest advantage is that it does deliver data continuously directly to your phone. This means you can easily set up automated alarms and see your latest numbers via a connected watch.

Cost: The Dexcom G6 is significantly more expensive. Every sensor is about $115 if you pay out of pocket, and unlike the Libre, you also need a separate transmitter (which sends data from the sensor to your phone) that’s attached to it. Transmitters cost about $240 and can be worn for three months before running out of charge. So all in all, you are looking at $425 per month for a complete sensor and transmitter set.

In summary: Dexcom has all the features you want, including a Bluetooth connection and open API, but this is reflected in its hefty price tag. If monitoring your levels closely is important to you, it’s a great choice.

Medtronic Guardian Connect

Medtronic Guardian Connect

The Guardian Connect from Medtronic is the least popular CGM of the three. It’s lagging behind both in overall functionality and experience, but has a few advantages as well.

Medtronic Guardian Connect

Functionality: The Guardian sensor lies somewhere between the Libre and the G6 in terms of size, and has a relatively low profile. But, it only has a 7-day wear time and a longer 2-hour warm-up period. The Guardian also requires calibration via a finger prick measurement done twice a day; this is a significant drawback as it is both painful and cumbersome.

One big advantage of the Guardian Connect over Dexcom is that the transmitter is rechargeable and reusable.

The mobile experience is on par with the others and supports automated alarms, smartwatch support, and more. You can similarly log events in the app, but pictures are again unsupported.

Cost: Due to the rechargeable transmitter, the monthly cost for the Guardian Connect is somewhat hard to predict. Every transmitter costs $775 and a box of 5 sensors is $553. If we assume the life of a transmitter is 24 months, the monthly cost of this system is $474.

In summary: Since the Guardian Connect system requires both daily calibrations and has a significantly shorter wear-time, it ends up being both more expensive and less usable than the other models.

How to get a CGM?

To get your hands on a CGM in the US you need a prescription. If you have any type of diabetes it should be relatively straightforward — just ask your doctor during your next visit or sign up for Steady and we will prescribe one for you. If you don’t have diabetes the cheapest option is to ask your Primary Care Provider for a prescription. You can also use one of the relatively new CGM consumer services on the market, but be aware that these may cost you up to five times more per sensor than just going directly to your PCP.

I hope this overview is helpful for anyone looking to learn about CGMs, the different models, and which one to use. If you already use a CGM today and struggle with blood sugar control, check out Steady Health, as we specialize in CGM-based care for diabetes.

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