What is CGM?

Continuous glucose monitors (CGM) provide real-time feedback on your blood sugar. Here's a guide to why they're great, how they work and where to get one.

My role in metabolic health begins from behind-the-counter. I am a pharmacist and much of my day consists of counseling patients in an attempt to improve health outcomes. I came across the statistic that “only 12% of the population are considered metabolically healthy,” which is alarming, but unfortunately I can’t say I was surprised. Every day I see a similar scenario: A patient goes to their doctor feeling a little “off,” finds out that they have elevated blood sugar, cholesterol, or BMI, and eventually ends up at my pharmacy counter with prescriptions. 

The questions roll in as the patient begins to process the diagnosis: What could I have done better? Do you believe this is genetic and I was always going to get this diagnosis? Could I have done anything earlier, or taken measures to prevent this?

As a pharmacist, I strive to give practical lifestyle changes and insight. In addition to counseling on medications, I look at my patient’s health as an overall picture. This includes helping my patients understand the cause-and-effect relationship our choices have on our health, which I believe should be discussed on a continual basis. Knowledge is power, and when it comes to your metabolic health, the future is bright. With tools like continuous glucose monitors (CGM) giving us unparalleled real-time clarity into our metabolic health, these can allow us to start making personalized decisions to live our best healthy life. 

Why you might want a CGM

CGM opens a new window into our physical body that we’ve never had the chance to explore. By wearing a CGM, we can see in real-time how our bodies are functioning and we can unlock the possibility of changing our future metabolic health. These devices provide instant metabolic data so we can make changes in our lifestyle and reduce the likelihood of a surprise diagnosis later in life. Often, patients didn’t see their diagnosis coming. Often, they knew they “should” be exercising and making healthier food choices, but let’s be honest: with marketing and advertising, it can make it extremely confusing to decipher what exactly is healthy. 

Since I’ve started using CGM myself, I can attest to buying what appeared to be a “healthy” product only to find out a couple of hours later it spiked my blood sugar and left me feeling tired and fatigued. 

I’m a science-minded individual, and I thrive on learning the research and data behind products. Understanding the history and research behind CGM has further deepened my appreciation for the importance of this technology. Interestingly, the use of continuous glucose monitoring has only been mainstream for less than 20 years, and in that short period of time, it has revolutionized the way patients with diabetes manage their blood sugar. Prior to CGM, the gold standard of care for diabetic patients was a blood glucose finger stick which essentially provided a single time-point snapshot of glucose levels. Now, with CGM, we have essentially a continuous video of blood sugar activity, with dozens of data points throughout the day.

The first CGM was approved in 1999 and had very basic functionality. It collected data for 3 days and forwarded it to the provider for interpretation. It’s hard to imagine waiting 3 days for results, considering that at this very moment I could scan my CGM sensor on my arm and have a reading in less than a second. The initial indication for CGM was for the management of Type 1 diabetes. In 2017, the indication was expanded, and now it is recommended for all people with diabetes. 

Slowly, CGM use is expanding into people without diabetes, often paired with software that gives us a new understanding of how our bodies are responding to different foods, and in doing so helps us control our blood sugar and keep it flat and stable. I believe that learning to control blood sugar when still in an “optimal” range of glucose levels may reduce the cases of Type-2 diabetes down the line. Knowing your blood glucose at any given time creates the opportunity to take preventive actions before managing glucose levels get out of hand. Can you imagine identifying a problem in your metabolic health, making changes proactively, and completely dodging a diabetes diagnosis in the future? This is now possible. 

What to expect when wearing a CGM 

Within the first week, you can expect to learn your body’s response to certain foods and lifestyle activities, like when and what caused a glucose spike. I’ve been dispensing CGM at my pharmacy for years, and the first week of wearing one I probably checked my blood sugar 50 times a day. I was completely intrigued by how easy it was to use, and the variations and my trends. 

 What exactly is a blood sugar spike? A sugar spike will show on the CGM as an event in which the blood sugar was raised in response to eating a certain food. The goal is to keep glucose readings fairly level and steady, as large spikes indicate higher blood glucose, and in response, the body needs a more substantial insulin release to bring the glucose levels back down. When this process occurs repeatedly, it can generate the pathologic process of insulin resistance, whereby cells become “numb” to insulin. Insulin resistance happens over time, and if not corrected, could eventually lead to Type-2 diabetes. As a society our “metabolic goal” should be to reduce glucose spikes, lower insulin production, thus decreasing the possibility for insulin resistance. In the short term, when we keep glucose and insulin more stable, we can improve cellular function, balance hormones, and improve our energy. 

How a CGM works

The CGM consists of 3 components: the sensor, transmitter, and receiver. The sensor is placed under the skin and it detects the interstitial fluid. This differs from finger-stick glucose reading, which measures glucose directly in the blood. Interstitial fluid is the fluid that surrounds your cells below your skin, and a reading from this fluid will be slightly delayed 5-10 minutes as compared to a blood reading. The delay can be explained by the way that blood glucose travels, as it starts in the blood vessels and disperses into the capillaries, and finally reaches the interstitial fluid. The sensor uses the enzyme glucose oxidase to convert glucose to hydrogen peroxide, which it measures and converts into a glucose reading. The reading then transmits to the receiver by Bluetooth or near field communication (NFC) and provides a reading. This mechanism is shared among all CGM devices.  

There are slight variations between different CGM on the market, but generally, that is the mechanism of action for each. The most common CGM on the market are the Abbott Freestyle Libre, Dexcom G6, and one from Medtronic, and they all have slightly different features. 

Related article: The future of wearable and implantable bio-monitoring

What are the types of CGM?

Freestyle Libre is the most accessible CGM on the market, and most community pharmacies stock them or can easily order them through a secondary distributor. For this reason, most of the patients I consult with use this type CGM. Freestyle is a well-recognized brand, and is also known for producing self-monitoring glucose meters (SMGM) and test strips and lancets. The Libre works by applying a sensor that contains a wired enzyme glucose-sensing technology into the interstitial fluid on the back of the arm. The sensor filament that goes under the skin is very flexible and is 5mm long and 0.4mm wide. This technology measures glucose every minute and stores readings in 15-minute intervals. The sensor needs to be scanned by either the reader or a smartphone and this will transmit the prior 8 hours of data to the app. Since there is no continuous flow of data, there are settings for alerts and alarms.

One of the major benefits of the Libre is that there is no calibration necessary. It’s also relatively affordable. Most insurance companies will cover this device, but the out-of-pocket expense is approximately $65 for a 14-day sensor. Patients tell me the Libre is easy to use, affordable, and that they appreciate that it can be easily configured with a smartphone, so they don’t have to buy a separate reader device.  

Dexcom CGMs have the advantage of being FDA-permitted to make treatment decisions without the use of confirmatory finger-stick. The Dexcom CGMs have also been proven to help patients lower their HbA1c (average glucose) and reduce hypoglycemic incidents. Dexcom is also more convenient because it doesn’t require scanning, and instead sends glucose data by Bluetooth. It is approved for ages 2 and older, which covers a larger percentage of the diabetic population. The Dexcom device has the ability to obtain more accurate readings, and has a calibration option that is not necessary, but doing so will improve accuracy. The Dexcom takes blood-sugar readings at five-minute intervals and provides up to 288 readings per day. The sensor is placed just under the skin on the abdomen or the lower back, and lasts 10 days. 

Medtronic is the third company that produces CGM hardware. Medtronic offers smart technology including predictive alerts and notifications up to one hour before blood sugar highs and lows. Its featured product is the Guardian Connect, which utilizes a wireless rechargeable transmitter and is approved for ages 14–75. The Guardian has a Bluetooth-enabled transmitter allowing for a continuous flow of data every five minutes. Medtronic is the only brand that has the ability for integrated pump therapy, which streamlines the process for diabetic insulin-dependent patients, eliminating the need to purchase a separate insulin pump. The sensor has the shortest life, lasting only seven days, and is applied to the stomach. The sensor filament that is inserted into the skin is 8.9mm. It also has Carelink software which creates reports thus allowing physicians to better understand how a patient is managing their diabetes. Medtronic is the most expensive CGM on the market; the transmitter costs $699 and the sensors range from $50–$75. 

Conclusion

Science has come a long way, which is good news because we as a population need our health to come a long way. With only 12% of the population considered metabolically healthy, there is no other option besides changing our lifestyle and becoming proactive.