What is a continuous glucose monitor?

A continuous glucose monitor (CGM) can help you track your blood sugar in real-time. So how does a CGM work, and who should use it?

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A continuous glucose monitor (CGM) is a wearable biosensor that can give you real-time information about your blood sugar (glucose) level. This information is vital to people with Type 1 or Type 2 diabetes, for whom high or low blood sugar can be life-threatening. But CGMs are also valuable for people without metabolic impairment, serving as a window onto how diet and lifestyle decisions affect their health. Consistently high glucose levels or frequent glucose spikes and crashes can lead to short-term health effects like fatigue, brain fog, and depression and are associated with several chronic diseases, including cancer, cognitive decline, stroke, and cardiovascular disease. CGM can help you maintain stable glucose levels before metabolic dysfunction like insulin resistance (prediabetes) sets in and sets the stage for more severe disease.

Here’s a deeper look at this biosensing technology. 

What is a Continuous Glucose Monitor?

A continuous glucose monitor is a plastic disc (about the size of two quarters stacked together) that sticks to the skin, typically on the stomach or upper arm. It sends glucose data continuously (at least every 5 minutes) to a smartphone or handheld device, where the user can see it on a graph or chart. People who have Type 1 diabetes sometimes have their CGM connected to an insulin pump that can automatically deliver insulin if their blood sugar gets too high. Most CGMs last anywhere from 7-14 days before they need to be removed and replaced. 

There are three primary manufacturers of CGMs in the US: Abbott, which makes the FreeStyle Libre monitor; Dexcom, which makes the G6; and Medtronic, which makes the Guardian system

How Does a Continuous Glucose Monitor Work?

On the underside of the plastic disc is a flexible filament around 5mm long and 0.4mm wide (about as thick as dental floss). When you apply the CGM with the included applicator, a small spring-loaded needle guides the filament through your skin and retreats (it happens so quickly you typically don’t even feel the needle), leaving the end of the filament in the interstitial fluid just below the skin.

So a CGM doesn’t actually measure blood sugar but rather the glucose that continuously seeps from blood vessels and capillaries into this fluid. The amount of glucose in the interstitial fluid is a close proxy of what’s in the blood, but about 5-15 minutes delayed (the time it takes the glucose to leak into the interstitial space). Many people also calibrate their CGMs with a finger-stick test to improve the device’s accuracy.

Image courtesy Abbott

Coating the filament is an enzyme called glucose oxidase, which converts glucose to hydrogen peroxide. The hydrogen peroxide reacts with platinum to break down into hydrogen, oxygen, and electrons, which generate current. The device wirelessly transmits this signal to a handheld reader or smartphone, which uses an algorithm to convert the electrical signal to a glucose reading. CGMs have a lifespan of 7-14 days because eventually the chemistry on the sensor degrades, and the reading becomes less accurate. 

Future monitors will likely be smaller and last longer, and may measure additional metabolites. They may also incorporate new sensing technology that uses aptamers, or strands of DNA, RNA or amino acids that bind to specific molecules. Aptamers can sense much smaller quantities of a biomarker than current enzymatic sensors.

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

Why Use a Continuous Glucose Monitor?

The benefit of a continuous glucose monitor is that it provides a more granular, real-time look at your blood sugar than alternative methods such as using a glucometer, which reads glucose from a drop of blood, or glucose tests at your doctor, such as fasting plasma glucose or HbA1c. These other methods provide only a single measurement, a snapshot in time. 

This can fail to show you how much your blood sugar is spiking and crashing (known as glycemic variability [more on that below]). They also fail to reveal trends over time or reactions to particular foods. Even if you tested your blood with a glucometer after a meal, you can’t see the entire rise and fall pattern, so your reading may be misleading. A continuous glucose monitor is like listening to the whole symphony instead of just individual notes.

Who Should Use a Continuous Glucose Monitor?

People with Type 1 diabetes can’t make insulin to help maintain stable blood sugar, so they need to monitor their glucose levels closely to keep blood sugar from getting dangerously high or low. Without a CGM, the only way to do that is to use a finger-stick glucometer dozens of times a day. 

But CGMs can also provide several valuable insights that can benefit people with or without diabetes:

  • Glycemic variability: This describes the “spikiness” of your glucose curve. Ideally, we want glucose to remain stable and rise only slightly after meals. If you have high glycemic variability, your blood sugar is frequently spiking and crashing. Not only can this impact your mood and energy, but it can also have long-term impacts as well. Studies show that these big spikes and dips in glucose can damage tissues more than elevated but stable glucose levels. Extreme glucose variability has been linked to heart disease, diabetes, and metabolic dysfunction. High levels of variability can also indicate that you’re becoming insulin resistant—your body is unable to use insulin efficiently to control glucose.     
  • Reaction to individual foods: Our blood-sugar reactions to foods are highly individual. While your blood sugar may spike from a cookie but not from a banana, someone else may have the opposite reaction. Our glucose reactions are also affected by our microbiome, genes, overall fitness, sleep status, stress, and what else we’ve eaten recently. Because of all these factors, simply reading carb content on a label or going by glycemic index is not sufficient to help us find the diet and lifestyle that will allow us to maintain stable glucose. 
  • Train our bodies to be metabolically flexible: Our body has two primary sources of fuel: glucose and fat. Burning fat produces ketones, which have many health benefits. If our body is burning glucose, it’s not burning fat (in fact, insulin, which is produced when our blood sugar is high, blocks fat burning). Metabolic flexibility refers to how efficiently your body can switch between these fuel sources. Without that flexibility, people may struggle with weight loss, fatigue, or excessive hunger because they cannot efficiently tap into and metabolize their fat stores for energy since their bodies are conditioned to rely on glucose. By using a CGM to help eat for stable blood sugar, we train our body to switch to fat seamlessly when it can.
  • Catch metabolic dysfunction early: We can begin to develop insulin resistance more than 10 years before it shows up in our fasting blood sugar test at the doctor. A CGM can help us see our total postprandial blood sugar response, giving us more insight into how our body handles a glucose load. And the earlier we catch metabolic impairment, the easier it is to arrest its development before it reaches prediabetes or diabetes. 

Related article: A pharmacist’s perspective on CGM use in people without diabetes

How do you get a continuous glucose monitor?

CGMs are Class II medical devices in the United States, and a doctor must prescribe them. As of 2017, the American Diabetes Association recommends CGMs for people with Type 1 or Type 2 diabetes who need to monitor their blood sugar, although people with Type 1 diabetes still make up the vast majority of CGM users. Unfortunately, many people with diabetes still struggle to get CGMs approved and covered by their insurance plans. 

In addition, doctors are increasingly prescribing CGMs to people without diabetes to help provide them feedback to optimize their health. (So-called “off-label prescriptions” like this make up about 20% of all prescriptions written in the US each year.) In Europe and Canada, CGMs are available over the counter. 

Conclusion

Continuous glucose monitoring can improve the quality of life for any individual, whether they have diabetes or not. Access to consistent and accurate data about our blood sugar can help us make informed and healthy diet and lifestyle choices to help support our longevity and wellbeing.

Read more Metabolic Basics:

What is glucose?

What is postprandial blood sugar and why does it matter?

What is the metabolic health crisis?