UAB Medicine News


All About Continuous Glucose Monitors for People with Diabetes

Diabetes is a serious and growing problem in the United States. According to the Centers for Disease Control & Prevention, approximately 30.3 million Americans have diabetes, or about one in 10 people. That’s up from about 26 million people in 2010.

Diabetes is a broad term for a group of conditions in which the body’s blood glucose (sugar) levels to rise higher than normal, known as hyperglycemia. In many cases, it can be effectively managed through diet, exercise, medication, and blood glucose monitoring. As a supplement to the traditional finger-prick technology that people with diabetes have relied upon for many years to check their blood glucose levels, continuous glucose monitors (CGMs) offer a more convenient and less painful way for people to track their blood sugar and make positive lifestyle choices.

November is national Diabetes Awareness Month, so UAB Medicine prepared this article to explain CGMs, including information on how they work, their benefits, where to obtain one, and where to get help on using them.

What Are CGMs?

In 1991, the U.S. Food & Drug Administration approved the first continuous glucose monitoring system for home use by adults with diabetes. This technology offers an alternative to taking blood from the fingertips, where the sample is taken from tiny blood vessels called capillaries. According to Alison Wharton, MPH, RDN, CDE, LD, a diabetes and nutrition education specialist with UAB Medicine, CGMs track blood sugar levels 24 hours per day by measuring the amount of glucose in interstitial fluid, which is the fluid that surrounds blood vessels.

Continuous glucose monitoring has proven to be effective for people with type 1 diabetes who use an insulin pump to manage blood sugar changes. Type 1 diabetes means that the person’s body does not produce insulin, which is necessary for regulating blood sugar levels. However, some doctors and researchers believe that CGMs also may be helpful for people who have type 2 diabetes, meaning that their bodies don’t respond properly to insulin. These monitors could even be useful in helping individuals diagnosed with pre-diabetes improve their lifestyle habits based on CGM readings.

How CGMs Work

Instead of requiring users to prick their fingertips as many as 7-10 times per day, CGMs utilize a tiny sensor wire that the user inserts below the surface of the skin and secures with an adhesive patch. The sensor usually is worn on the abdomen or the back of the arm, and it monitors the person’s glucose levels throughout the day and night. The readings taken by the sensor are transmitted to a handheld electronic receiver (a “reader”) or smart device, where real-time data is provided to users about their glucose levels. Current models can be worn for 7-14 days, and some are only recommended for use by people age 18 and older.


Since CGMs continually collect blood sugar data, they can help identify issues and patterns that traditional finger-prick technology cannot. For example, CGMs can record low blood sugar levels while a person is sleeping, track spikes between meals and in the early morning, evaluate diet changes, and provide insight about how medications and exercise regimens are impacting the body. The devices also make it easier for a patient’s health care provider to adjust treatment plans.

“CGMs, when used appropriately, provide daily, continuous guidance on how to adjust therapy to more consistently keep blood sugars within the target range and avoid prolonged, severe highs and lows,” Wharton says.


The accuracy of CGMs varies and is measured by mean absolute relative difference (MARD). This accuracy measurement is not a precise value but more of an indication and reference point, since MARD does not take into account the frequency of measurements. However, CGMs with a MARD of less than 10 is considered to be good, and all new FDA-approved CGMs achieve this level of accuracy. The results generated by CGMs are interpreted differently than those produced by traditional finger-prick technology, because CGMs measure the glucose levels in interstitial fluid rather than glucose levels in the capillaries within the fingertips. Also, the lag time between actual blood sugar levels and CGM values may contribute to minor differences in readings.

“The lag time continues to improve with advances in technology,” Wharton says. “Individuals should be reminded to look more at glucose trending than snapshots of blood sugars.” According to  Dexcom, the G6 model’s lag time is now down to about 4 minutes.

As promising as CGM technology is, it doesn’t entirely replace finger-prick technology, Wharton says. “Sometimes the CGM device requires you to check with your traditional meter, too, but it greatly reduces the need for frequent finger stick checks,” she says.

While some CGMs still require calibration using a reading obtained with finger-prick technology, finger sticks generally shouldn’t be required more than twice a day if the CGM is calibrated correctly, Wharton adds.

Where to Buy CGMs and Differences among Models

There are differences between the various brands and models of CGMs, especially as new and improved devices hit the market, but all require a physician’s prescription. Some CGMs even use implantable long-term sensors. A few common CGMs are listed below, though new products are continually being developed:

  • Freestyle Libre: This CGM is FDA-approved for people age 18 and up, does not require calibration, has a sensor life of 14 days, utilizes a reader and/or a compatible smart device, and is among the most affordable options currently. The sensor/transmitter must be “scanned” to view results. It doesn’t notify the user of a result unless the reader or smart device is swiped over the sensor. Dexcom G5 and G6: These models are FDA-approved for people age two and up, have sensor lives of 7-14 days (G5) and 10 days (G6), have customizable high/low alerts, and are a more expensive option. Results can be viewed on a reader and/or smart device, including some smart watches.
  • The Dexcom devices transmit new results every 5 minutes and can be programmed to alert the user of trending high or low glucose levels. The user and caregiver(s) can be notified of blood glucose problems even when not actively viewing results. The Dexcom G6 can integrate with a Tandem insulin pump, meaning that the pump can receive and react to the results transmitted to the pump via a Bluetooth signal using what Tandem calls Basal IQ technology.
  • Medtronic Guardian Connect: Among the newest models on the market, this CGM is FDA-approved for those ages 14-75, requires calibration 3-4 times daily, has a sensor life of seven days, communicates with a smart device or smart watch, and tends to be more expensive, with more variable coverage by insurance. It can be used as a standalone CGM, but most often it is used as part of the Medtronic 670G insulin pump system.

Cost and Insurance Coverage

Despite the promising benefits of CGMs, many patients face obstacles in obtaining them because of their relative high cost. Since 2017 Medicare decided to cover CGMs for qualifying individuals, and some commercial health insurance plans cover them, too. But at this time, insurance coverage of CGMs varies greatly.

CGMs are more expensive than traditional finger-prick technology. Wharton says CGM technology can cost users $100 or more per month without insurance coverage. People who have a compatible smart device may forego having to purchase a receiver, saving some cost.

When the devices are covered by patients’ health insurance plans, they usually are covered at a rate of 80% as durable medical equipment rather than being treated as a pharmaceutical item. A few plans are offering coverage as part of pharmacy benefits. The manufacturers offer support for where to buy the supplies.

Those with commercial types of insurance can purchase (with a prescription) The Freestyle Libre 14 day at retail pharmacies such as Walgreens, CVS, and Walmart. The reader device costs about $80 without insurance, and the sensors cost roughly $40-$60 each. Two sensors are needed per month. Online purchase options may be provided by the patient’s insurance plan. Some pharmacies are starting to order Dexcom supplies for their customers, but Medtronic CGMs cannot currently be purchased at retail pharmacies, Wharton says.

Wharton says she hopes more commercial insurance providers soon will expand their coverage of CGMs. At this time, the plans that do cover the devices generally only do so for patients with type 1 diabetes who are pregnant or who have significant problems with severely low blood sugar levels (usually under 50 mg). Guidelines for coverage under Medicare Part B are more inclusive, in that individuals could have type 1 or type 2 diabetes as long as they are: 1) on three or more insulin injections per day; 2) testing their blood sugar at least four times a day to help adjust insulin dosing; and 3) seeing their treating physician at least twice a year.


Certified diabetes educators at The Kirklin Clinic of UAB Hospital are available to train people with diabetes in how to operate CGMs and use the results in their daily self-care as prescribed by their UAB Medicine physicians.

Click here to learn more about diabetes care at UAB Medicine, or call 205-801-8711 to inquire about a referral to our Diabetes & Nutrition Education Clinic.

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