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.

For those who don’t know, CGM is short for continuous glucose monitor. But even for those familiar with the term, it can be hard to figure out what a CGM does exactly, how to choose the right one, how to determine costs 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 type 1 or type 2 diabetes, a parent or guardian of a child with type 1, or a diabetes caregiver, we hope this overview will provide the answers you seek.

A continuous glucose monitoring system, or CGM for short, is a compact medical system that continuously monitors your glucose levels in more or less real time (there’s normally a five-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 10 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 meter (BGM), which provides just a single glucose reading, CGM systems provide continuous, dynamic glucose information every five minutes. That equates to roughly 288 readings in a day.

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 can catch cases of hyperglycemia (too-high blood sugar) and hypoglycemia (too-low blood sugar) as they happen, avoiding the potentially dangerous consequences. This is a huge advantage over historic “static” blood glucose monitoring, which only provided a single glucose reading at a time.

There’s also the convenience factor. CGMs can essentially eliminate the need for those 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 backup health data sources, 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 “gold standard” test of blood glucose management.

Other studies (here and here) show CGMs can decrease a patient’s time spent in hypoglycemia, and help to increase their time in a healthy BG range.

With active monitoring and alert settings, the devices can be especially beneficial for children (and adults, really) dealing with nighttime fluctuations, bringing safer care and more peace of mind to parents and caregivers.

It’s also a very helpful tool for patients who have hypoglycemia unawareness, alerting them to impending low blood sugars.

Currently, there are four FDA-approved CGM systems on the market. Two require no fingersticks at all for calibration, while the two others reduce fingerstick requirements from as many as eight to only two a day (for calibration purposes). The price and where they are sold varies.

Dexcom, based in San Diego, was the pioneer in CGM and continues to lead the market. Their most recent system, the Dexcom G6 CGM, offers zero-fingerstick technology, as it 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 two years old.

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). The newest feature is voice integration via Apple’s Siri technology.

Medtronic, the leader in insulin pump technology, also makes a CGM device called the Guardian, which was originally only sold in a combo system with its insulin pumps. But in March 2018, the FDA approved Medtronic’s latest model Guardian Connect, the company’s first stand-alone CGM.

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

Guardian Connect’s big selling point (not surprisingly considering the product’s 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 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 Guardian Connect is FDA-approved for users ages 17 to 75.

Abbott Diabetes Care got into the CGM game in 2017, with FDA approval of its unique FreeStyle Libre Flash Glucose Monitor.

What’s different about a “flash” system is that users wear a small round sensor inserted on the upper arm, but it doesn’t automatically send readings. Instead, users have to manually swipe the handheld receiver (or now also a smartphone) over the sensor to get glucose readings.

The sensor is quite small — about the size of two stacked quarters, according to the company — and can be scanned through clothing and is 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, that offers remote data sharing with up to 20 people. The Abbott FreeStyle Libre is FDA-approved for users 18 years of age and older.

Senseonics rounds out the big four with its Eversense system, the world’s first long-term implantable CGM. It consists of a tiny sensor the size of a small twig that is implanted underneath the skin in the upper arm. Approved for 90 days of wear in the U.S. and 180 days of wear in Europe, it is by far the longest-lasting sensor.

A flat oval black transmitter is worn over the insertion site and held in place with an adhesive. The transmitter must be taken off and charged daily. The system is viewed and controlled by a smartphone app that offers several data reports that can be easily sent to a doctor (or anyone) with the click of a button. More specs are available here. The company was forced into “hybernation mode” in the wake of the pandemic, but resumed operations in September 2020 after getting a cash infusion from Ascensia Diabetes Care.

As with most medical technology, you need a prescription to get a CGM. Your doctor can write a prescription for any of the major CGM devices. Most are available through traditional pharmacies at this point, though online ordering directly through the manufacturer or many third-party vendors is an option too.

But getting a prescription for a CGM is often not as simple as just walking in to your doctor’s office and asking for one. Instead, you’ll likely need a Prior Authorization (PA) to get a CGM through your health insurance. A PA is form/process that your doctor has to go through to prove “medical necessity” to obtain approval from your insurance plan to cover the costs associated with the prescribed treatment (CGM, in our case here).

At this point, most major insurers (and Medicare) require a Prior Authorization 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 your Medicare & You
    for more information.
  2. If a PA is needed, locate the process for submitting and obtain
    any required prior authorization 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. Because your doctor’s office is responsible for submitting PAs, it
    will be important to work with your doctor, or the staff member in the doctor’s office designated to handle PAs, to ensure they have all the necessary information.
  4. Ensure that the PA request is submitted according to the plan’s
    guidelines, and double-check that you meet all requirements before they are submitted.
  5. Once your request is submitted, the insurance company may approve
    or deny it. If it is approved, you will be able to receive the requested
    treatment. 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 two months
  • Intention to use monitoring device as an adjunct to standard care
  • Frequent unexplained, hypoglycemic episodes or frequent nocturnal

CGM systems aren’t cheap, especially if you pay out-of-pocket. In a moment we’ll talk about insurance coverage. First, here’s a basic rundown of the approximate costs for the major CGM systems:

  • Dexcom G5/G6: $300 for the transmitter, $600 for the receiver, and $80 per sensor (G5 sensor life: 7 to 14 days; G6 sensor life: 10 days).
  • Abbott FreeStyle Libre: $60 for the sensor scanner, $40 per sensor (10-day life).
  • Medtronic Guardian Connect: $1,000 for the transmitter, $80 per sensor (7-day life).
  • Senseonics Eversense (while still available): $1,400 for the initial
    package (sensor, transmitter, adhesives, start guide), $200 to $300 for
    insertion (90-day life), and $300 to $400 for removal and reinsertion.

While the base hardware for CGMs (transmitters and receivers) are generally quite durable, the cost of the disposable sensors and implant procedures can quickly add up.

If there are costs that you cannot afford or are not covered by your insurance, CGM companies will generally work with patients to assist with some sort of payment plan. Eversense, for instance, offers a patient access program called Eversense Bridge designed to help patients who face challenging out-of-pocket costs.

While you’ll still need to pay for sensor implantation, the bridge program offers qualified patients a flat rate of $99 for all the up-front supplies (sensor, transmitter, adhesives, start guide), instead of the normal cost of roughly $1,400.

Don’t be afraid to speak up or ask for deals, discounts, and help. After all, these medtech companies are businesses: they want people to use their products.

The above costs add up. It’s helpful to know that the Dexcom G5 and Abbott FreeStyle Libre are covered by most private and public health insurance plans, including Medicare (more on that below). Medtronic’s Guardian Connect, the Dexcom G6, and Senseonics’ Eversense should be covered by most private plans, but not yet by Medicare.

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

Still, if you have type 2 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, 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 to your doctor’s office about peer-to-peer reviews and appeals. Pushing your doctor to fight for you matters. Manufacturers can have your back here, too. Dexcom, for instance, has an easy-to-complete patient information form you can submit along with your insurance information. The company will then contact your insurer to work on getting your device authorized and covered. Medtronic offers a similar service here.

Medicare coverage of CGMs is a relatively new development and still limited, sadly. In January 2017, CMS signed off on the Dexcom G5 as a “therapeutic” tool because it’s now been FDA-cleared to be accurate enough to make insulin dosing and treatment decisions — and that meant it was eligible for Medicare coverage.

More recently, Abbott Libre managed to secure coverage under Medicare Part D as well, and the hope is that this will pave the way for more expanded coverage going forward.

Among the major benefits of CGM systems are their ability to eliminate fingerstick tests, the constant data stream, and the fact that users continually report that their CGMs help them learn more about their diabetes and its relationship to their bodies.

If those criteria appeal to you, then any of the four major CGMs may be right for you.

If you’re very concerned about real-time safety alerts, you may want to skip the Abbott Libre system, at least until a new model comes out that includes alarms.

As for drawbacks, CGM is wearable tech. It’s always with you, and even though the wires are gone now, you’ll still have to deal with having a device affixed to your body. That constant data stream can be a drawback, too, flooding you with information and potentially creating an obsessive loop where you feel constantly pressured to react to the BG number you see on the screen. Lastly, the costs are a major concern for many people.

All that being said, CGMs are the most advanced tools available for diabetes care, and potentially the only future path for insulin-dependent diabetes care.

Here are some details about which CGM might be the best fit for which users:

If you’re looking for long-standing reputation and the authority of market share, Dexcom fits that bill. The longest player in the CGM game, Dexcom’s CGMs generally boast the most accurate readings, within 9% of lab-measured BG readings.

With its G6 model, the San Diego based company now also offers fingerstick-free calibration and slightly longer sensor life (10 days). Dexcom offers high and low alerts, customizable alarms, data sharing, and a water-resistant device. It’s not cheap, though.

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% of lab values), although many patients claim that Dexcom seems more accurate in real-world settings.

The Guardian Connect offers increased connectivity, with data being 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. Again, this one isn’t cheap either, and the sensors (at $80 a pop, remember) only last seven days.

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

However, it lacks some of the features of traditional CGMs, mainly 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 (any time you may not be scanning).

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 three months to have the sensor removed and a new sensor inserted.

Still hungry for more information to help you choose a CGM system? See these detailed reviews:

Greg Brown is a freelance writer living in western Maine. He has written for Consumer Reports Magazine, Consumer Reports Online, The New York Times, and the Chicago Tribune, among other publications. He can be found online at www.yellowbarncreative.com.

This article has been medically reviewed by Maria S. Prelipcean, MD.