What is a CGM (Continuous Glucose Monitor) and How Do I Choose One?

Let’s face it, people love acronyms. And chances are that if you’re even remotely connected to the diabetes community, “CGM” is a term you hear and see tossed around often.

As a reminder, CGM stands for “continuous glucose monitor.” Even those familiar with the term can find it challenging to figure out what a CGM does exactly, how to choose the right one, how to determine cost and insurance coverage, and where to buy one.

Enter this DiabetesMine primer on continuous glucose monitoring. We’ll start with the basics and seek to answer all the nuts-and-bolts questions we can.

Whether you’re a newly diagnosed adult with any type of diabetes, a parent or guardian of a child with type 1 diabetes (T1D), or a caregiver, we hope this overview will provide the answers you seek.

What is continuous glucose monitoring (CGM)?

A CGM is a compact medical system that continuously monitors your glucose levels in more or less real time (there’s normally a 5-minute interval between readings).

To use a CGM, you insert a small sensor onto your abdomen (or arm) with a tiny plastic tube known as a cannula penetrating the top layer of skin. An adhesive patch holds the sensor in place, allowing it to take glucose readings in interstitial fluid (the fluid that surrounds cells in the body) throughout the day and night. Generally, the sensors have to be replaced every 7 to 14 days.

A small, reusable transmitter connected to the sensor allows the system to send real-time readings wirelessly to a monitor device that displays your blood glucose data. Some systems come with a dedicated monitor, and some now display the information via a smartphone app, so you don’t even need to carry an extra device around with you.

Aside from the constant stream of data, most CGMs can send alerts telling you when your blood sugar levels are rising too high or dropping too low. You can also set the alert parameters and customize how you’re notified.

It’s not an understatement to say that CGMs have revolutionized diabetes care. Unlike a traditional fingerstick (blood glucose meters), which provide just a single glucose reading, CGMs provide continuous, dynamic glucose information every 5 minutes. That equates to roughly 288 readings in a day.

How CGMs change life with diabetes

The idea here is empowerment, as these devices provide some serious medical and lifestyle benefits.

First off, you can literally see in real time the effects of food and exercise on your blood glucose levels, and catch cases of hyperglycemia (blood sugar too high) and hypoglycemia (blood sugar too low) as they happen, avoiding the potentially dangerous consequences. This is a huge advantage over historic “static” blood glucose monitoring, which only provides a single glucose reading at a time.

There’s also the convenience factor. CGMs can essentially eliminate the need for regular fingerstick tests, the long-standing only way to check blood sugar levels. Though fingersticks are sometimes needed to calibrate CGM systems and can still serve as a backup data source, they are no longer a constant, nagging, unpleasant to-do.

Furthermore, studies (here and here, for instance) have shown CGMs to be among the best outpatient blood sugar management options for reducing A1C — the traditional “gold standard” test of blood glucose management.

Other studies (here and here) show that CGMs can help increase the user’s time- in-range (TIR), the overall percentage of time spent in a healthy glucose range. TIR is quickly becoming the new gold standard measure of positive diabetes outcomes.

CGMs can be especially beneficial for active children (and adults too), for ensuring safety during physical activity and during nighttime glucose fluctuations.

It’s also a potentially life-saving tool for people with diabetes who experience hypoglycemia unawareness, alerting them to impending low blood sugars when their own bodies fail to recognize the warning signs.

Four leading CGM systems

Currently, there are four Food and Drug Administration (FDA)-approved CGM systems on the market in the United States.

Two require no fingersticks at all for calibration, while the other two now only require two fingerstick calibrations per day versus past requirements for as many as eight daily. The price and where they are sold varies.

Dexcom G6

San Diego-based Dexcom was the pioneer in this field, introducing the first- ever real-time CGM in 2006. The latest version is the Dexcom G6 CGM, which comes “factory calibrated,” eliminating the need for users to set baseline levels with a fingerstick test. It’s FDA approved to be safely used in children as young as 2 years old.

The sensor transmitter uses Bluetooth connectivity to talk with the Dexcom G6 mobile app for both iOS and Android devices, as well as Apple Watch and other devices, including insulin pumps like the Tandem t: slim X2.

The system offers customizable alerts, compatibility with Dexcom’s Clarity software and smartphone app for reviewing data, and the ability to easily share device data with up to 10 followers (which can include your doctor, diabetes educator, caregiver, or family members). It also includes voice integration via Apple’s Siri technology.

Rough retail costs per year without insurance:

• a total of $1,200 for Dexcom G6 transmitters (each lasts 90 days, so 4 transmitters per year)
• a total of $4,800 for a box of 3 Dexcom G6 sensors (each sensor is supposed to last up to 10 days)
• estimated total: $6,000 per year, or $500 a month

Medtronic Minimed Guardian Connect

This system includes a small sensor that can be worn on the upper arm or abdomen for up to 7 days, and a Bluetooth transmitter that sends glucose readings to a cell phone app every 5 minutes.

Guardian Connect’s big selling point — in keeping with the product name — is protection. The company bills its product as “the only CGM system that helps patients on multiple daily injections outsmart highs and lows.”

That’s because Guardian’s smart technology not only predicts where glucose levels are headed, but it also alerts users from 10 to 60 minutes before a “glucose excursion,” allowing them to take appropriate actions in advance to avoid high and low episodes.

The stand-alone Guardian Connect is FDA approved for users ages 14 to 75 years old, although the Guardian version connected with Medtronic’s Minimed 670G and 770G pumps are approved for use in younger children in the context of those combo systems.

Learn more

Medtronic does not sell its CGM sensors and supplies at pharmacies, but you can find these at mail-order distribution companies and directly through Medtronic’s online shopping hub.

Rough retail costs per year without insurance:

• one transmitter (with a 12-month warranty): $620
• Medtronic Minimed Guardian Connect CGM sensors (5-pack, each approved for 7-day wear): $345
• estimated total: approximately $4,760 per year, or $397 per month

Abbott FreeStyle Libre

Abbott has been a longtime diabetes technology manufacturer, but the company got into the CGM game in just the past decade with its unique FreeStyle Libre Flash glucose monitor. It’s been available overseas since 2014 and received FDA approval in 2017.

What’s different about a “flash” system is that users wear a small round sensor inserted on the upper arm, but it does not automatically send readings. Instead, users must manually swipe the handheld receiver or smartphone app over the sensor to get glucose readings. The latest version is the FreeStyle Libre 2, approved in 2020 for the United States.

The sensor is quite small — about the size of two stacked quarters — and can be scanned through clothing. It’s also water-resistant, allowing users to wear it when swimming or bathing. The sensor also comes factory calibrated, so it needs no calibration, and is approved for 14-day wear.

Data can be read and analyzed on the receiver or a smartphone using Abbott’s LibreLink app, which offers remote data sharing with up to 20 people. The Libre is FDA approved for ages 4 years and older.

Approximate prices in pharmacies:

• list price of $54 per 14-day sensor, according to Abbott — generally $58 to $69 at retail pharmacies like Costco and Walgreens
• with commercial insurance, most people pay between $10 and $75 per month for the Abbott Libre 14-day sensors at participating pharmacies, according to the company
• hand-held reader: $70 (not required if you use a smartphone app to scan the Libre 2 sensor)
• total: $1,582 to $1,868 for a year’s worth, or as much as $160 per month

Eversense implantable CGM

The newest system is the Eversense system, the world’s first long-term implantable CGM made by Senseonics. It consists of a tiny sensor the size of a small twig that is implanted underneath the skin in the upper arm. Currently approved for 90 days of wear in the United States and 180 days of wear in Europe, it is by far the longest-lasting sensor.

The company initially offered a $99 start price, but that was discontinued in 2020 due to the impact of the COVID-19 pandemic. Also, that didn’t include the doctor’s office visits necessary to insert and remove implanted sensors.

Approximate pricing without insurance:

• a healthcare provider usually determines the cost of sensor insertion and removal, which typically run between $200 to $300 for insertion and $300 to $400 for removal and reinsertion
• estimated total: $6,400 per year, or $533 a month.

How do I get a CGM?

As with most medical technology, you need a prescription to get a CGM. Your doctor can write a prescription for any of the above CGM devices.

At this point, most major insurers (and Medicare) require a PA before extending coverage for CGM. Here’s how to go about securing a PA, according to JDRF, a leading T1D research and support organization:

1. Check your insurance plan’s policy documents and formulary to see if any of your treatments require a PA. You may find these on the plan’s website. If you have Medicare coverage, check the Medicare & Youhandbook for more information.
2. If a PA is needed, locate your insurance provider’s process for submitting and obtain any required forms. This information is typically found on the plan’s website, or you may call the member services number found on the back of your insurance card.
3. Your doctor’s office is responsible for submitting PAs, so it will be important to work with your doctor or the staff member in the doctor’s office designated to handle the paperwork.
4. Ensure that the PA request is submitted according to the plan’s guidelines and double-check that you meet all requirements before they’re submitted.
5. Once your request is submitted, the insurance company may approve or deny it. If approved, be aware that the approval letter may include rules about how you obtain the care. If so, you will need to abide by those terms to be covered.
6. If the request is denied, you should plan to appeal the decision.

You may be wondering what constitutes medical necessity? Here are some general PA criteria used to see if a patient meets requirements to get a CGM:

• Type 1 diabetes diagnosis
• completion of a comprehensive diabetes education program
• requires multiple, daily insulin injections or insulin pump therapy with frequent dosage adjustments
• documented average frequency of glucose self- testing greater than four times per day during the previous 2 months
• intention to use monitoring device as an adjunct to standard care
• frequent unexplained, hypoglycemic episodes or frequent nocturnal hypoglycemia

How much does a CGM cost?

As you can see from the prices listed above, CGM systems are not cheap, especially if you pay out of pocket.

While the base hardware for CGMs (transmitters and receivers) do not need to be replaced often, the costs of the disposable sensors and implant procedures can quickly add up.

Without insurance factored in, CGM systems supplies can range from roughly $160 per month all the way up to $500 per month (based on retail prices provided by manufacturers online).

Don’t be afraid to speak up or ask for deals or discounts from the manufacturers. Most medtech companies have patient assistance programs in place for those who qualify for financial help.

Once approved, you can buy the system and supplies for some CGMs directly from the manufacturer or a third-party medical equipment supplier like Edgepark, and some are also now available in U.S. retail pharmacies like Walgreens, Rite Aid, and Costco.

What about health insurance coverage?

Historically, people with type 1 diabetes (T1D) have had a much easier time qualifying for a CGM through insurance than those with type 2 diabetes (T2D) — because it was easier to establish medical necessity.

Still, if you have T2D and your doctor can make a case for medical necessity, you may qualify. But your insurance company may still want to confirm whether you actually need a CGM.

If you can prove you’re experiencing both high blood sugars and low blood sugars regularly, you will likely qualify. This can be proven by providing blood glucose logs uploaded from your fingerstick meter. Generally, hyperglycemia is defined as a blood glucose level of 250 mg/dL or over, while hypoglycemia is a blood sugar 70 mg/dL or below.

If your insurance company denies your coverage for a CGM, talk with your doctor’s office about peer-to-peer reviews and appeals. Pushing your doctor to fight for you matters.

Keep in mind that Medicare coverage is always evolving for diabetes technology, and details can vary depending on what particular coverage or supplemental plans you may have.

Pros and cons of using a CGM

CGMs are the most advanced tools currently available for diabetes care, but like everything else, they have pros and cons in terms of lifestyle impact.

Choosing the right CGM for you

Here are some details that might help you determine which CGM is best for you:

Medtronic’s Guardian Connect is the most similar to Dexcom’s line, and the costs are on par. The Guardian Connect is reportedly close in accuracy (within 10 percent of lab values), although many patients claim that Dexcom seems more accurate in real-world settings.

The Guardian Connect offers increased connectivity, with data automatically uploaded into the company’s Carelink app. That means your doctors can get your numbers without you having to do a thing. The predictive alerts are an interesting feature as well.

If you’re looking for a CGM that tries to predict your body’s future, and minimizes the amount of work you need to do in retrieving your data, the Guardian Connect might be for you. This one isn’t cheap either, and the sensors (at $80 each) only last 7 days compared to Dexcom’s 7-day wear.

Abbott’s FreeStyle Libre brings cost effectiveness and streamlined design to the forefront. It’s relatively cheap (about $60 for the receiver/scanner and $40 per sensor) and accurate (10 percent deviations between lab values and sensor values).

However, it lacks some of the features of traditional CGMs, mainly true continuous monitoring and programmable high-low alerts. The Libre requires you to wave a scanner over the sensor to get your levels. While this can prevent data overload, it can also expose you to unknown highs and lows while sleeping, driving, or exercising (anytime you may not be scanning).

The implantable Eversense offers some cutting-edge features but has some potential drawbacks. On the upside, the system is very accurate, has the longest sensor life, and offers customizable alerts.

But while it is implantable, you still have to wear a small black unit attached to your upper arm at all times to get readings. And you have to navigate the process of having it inserted, which currently means a visit to your doctor’s office every 3 months to have the sensor removed and a new sensor inserted. Some people have reported small scars on their shoulders as a result.

Alternatives to CGM?

DiabetesMine has covered the dream of noninvasive glucose monitoring technology for over a decade, with its promise of a gadget that doesn’t penetrate the skin in any way to get a glucose reading.

Many companies have promoted medtech concepts that they believe could someday revolutionize the way we manage this condition. Most have never materialized, and the small number that have ultimately made it to market have been the opposite of successful. Today, a few companies that look promising to make a breakthrough in noninvasive glucose monitoring include Know Labs, LifePlus, and Waveform.

Bottom line

CGM technology has been a game-changer for people with diabetes, as it offers a hugely improved ability to keep track of glucose levels on a constant basis. Unlike traditional fingerstick meters that only provide a snapshot of blood sugars in the moment, CGM systems provide a more complete picture of how one’s blood sugars are trending both in the moment and over time.

CGMs are being connected to insulin delivery devices these days, to create combo systems that can automatically adjust insulin doses based on the glucose data being streamed via Bluetooth. As these “closed loop” systems become more discreet and lower cost, they’re bringing much more lifestyle flexibility and better health outcomes for people with diabetes.

This content is created for Diabetes Mine, a leading consumer health blog focused on the diabetes community that joined ishonest Media in 2015. The Diabetes Mine team is made up of informed patient advocates who are also trained journalists. We focus on providing content that informs and inspires people affected by diabetes.

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