Continuous glucose monitors (CGMs) for type 2 diabetes: When and for whom are they useful?

Continuous glucose monitors, or CGMs, offer valuable and accurate insights into blood sugar levels, allowing some individuals with diabetes to make better-informed decisions about medications, diet, exercise, and overall diabetes management. As their popularity increases, it's important to understand who CGMs are most appropriate for.

What are continuous glucose monitors (CGMs)?

A CGM is a device that can monitor blood sugar levels continuously throughout the day and night.

Unlike traditional fingerstick tests, which provide periodic, infrequent snapshots of blood sugar levels, CGMs measure glucose levels every five minutes, allowing a person with diabetes to know if their blood sugar is trending high or low. The devices offer greater convenience, accuracy, and potential for control for individuals with this condition.

According to Dr. David Nathan, director of the Diabetes Center and Clinical Research Center at Massachusetts General Hospital and professor at Harvard Medical School, continuous glucose monitoring has been revolutionary for some patients with diabetes.

How do continuous glucose monitors work?

CGMs need three parts to work: a sensor, a transmitter, and a receiver. In older models, the sensor and transmitter are separate devices. But newer models combine the sensor and transmitter in one device.

The sensor is a tiny catheter that is inserted under the skin and held to the skin by an adhesive. It must be changed every 10 to 14 days. According to Dr. Nathan, the sensor is painless; most people forget they are wearing one. Newer CGMs are about the size of three quarters stacked together.

The sensor measures glucose levels every one to five minutes in the interstitial fluid between your cells. It then communicates those levels to the transmitter, which sends the glucose readings to a receiver or smartphone app.

Using advanced algorithms, CGMs analyze the data to provide users with insights into their blood sugar trends and patterns throughout the day and night. "CGMs show you where your blood sugar levels are and where they are going," says Dr. Nathan. "It shows you the graph of whether you're on the up slope or the down slope."

However, users should be aware that there is a delay of about 20 minutes between the levels the CGM displays and levels of glucose in the blood.

Benefits of continuous glucose monitoring

With a more detailed picture of glucose levels and patterns, an individual with diabetes can make informed decisions regarding diet, exercise, medication, and overall diabetes management.

CGMs not only offer continuous monitoring, but can also sound alarms if glucose levels deviate from target ranges, enabling intervention to prevent dangerous highs or lows. It's important for a person with diabetes to maintain blood glucose levels within a target range to reduce the risk of short-term complications like dangerously low blood sugar (hypoglycemia), and long-term complications such as cardiovascular disease, kidney damage, nerve damage, and vision problems.

Who can benefit from using CGMs?

As continuous glucose monitoring provides a continuous stream of information regarding glucose levels, it enables patients with fluctuating glucose levels — which means patients with type 1 diabetes, for the most part — to either choose the right doses of insulin or for insulin pumps to choose the right doses.

"They really do improve blood sugar control in type 1 diabetes, and more safely than we were able to do before. And that's been well shown," says Dr. Nathan.

The benefit of CGMs is less clear in patients with type 2 diabetes, which is far more common than type 1 diabetes. Of the 38 million people in the United States with diabetes, 90% to 95% have type 2 diabetes. Of people with type 2 diabetes, according to Dr. Nathan, a small fraction need complex insulin regimens that involve multiple daily insulin injections.

"And for them, it's been suggested that CGM may also be helpful because they are also vulnerable to low blood sugars," explains Dr. Nathan. "In insulin-treated persons with type 2 diabetes, CGM can provide them with an early warning sign if their blood sugars are dropping."

"But people with type 2 diabetes, even those treated with insulin, actually have much less frequent, severe hypoglycemia compared to people with type 1 diabetes," says Dr. Nathan. "So as a safety measure, the vast majority of people with type 2 diabetes really don't need it. They're not even on a medication that will cause low blood sugar."

However, limited, early evidence hints that there might be a role for the devices in the future to help a wider range of patients, including those with non-insulin-treated type 2 diabetes.

Are there any downsides to CGMs?

CGMs are generally safe and not difficult to wear, though they are usually worn 24 hours a day and for 365 days a year. The sensors should be replaced at least every 14 days. A small fraction of people may develop irritation or allergic reaction to the adhesive that secures it to the skin. Local skin infections are "extremely rare," says Dr. Nathan. "And there really is no other complication."

One of the biggest concerns for users of CGMs is cost. CGMs can cost thousands of dollars a year. Medicare covers the devices for eligible patients — those who are on insulin therapy and performing four or more blood sugar checks per day. Private insurance coverage can vary.

As CGMs have become more popular, some people have started to use them to monitor blood sugar, even if they do not have diabetes. "I tend to draw the line at type 1 diabetes and for selected patients with type 2," says Dr. Nathan. "For everyone else, there are almost no data to support using these devices on a regular basis."

Dr. Nathan cautions that overuse of CGM devices by people who do not need them could translate into a strain on supply of the devices, making them less available to people with diabetes for whom they have proven benefit.