A continuous glucose monitor (CGM) is a small wearable device that sends blood sugar measurements directly to your smartphone or to a dedicated receiver device every few minutes. It offers people with diabetes, or anyone else concerned with their metabolic health, a real-time view of their glucose levels. This makes it easier to see the effects of diet, exercise, and other lifestyle decisions on blood sugar management.
Most CGMs use a small, hair-like probe to penetrate the skin. The probe connects to a hard plastic sensor, which is held onto the skin with an adhesive patch. This probe is not felt, and it detects the glucose concentration in the interstitial fluid, which is the fluid between the cells in your body. Sampling this fluid allows the CGM system to estimate your blood glucose level.
The CGM sensor sends your blood sugar measurement to your smart device by a Bluetooth signal, updating a smartphone app as often as every minute. The sensors are disposable and remain attached to the body for up to 15 days.
On the user’s smartphone, the CGM’s app displays your glucose level along with other essential data like time-in-range and estimated A1C levels.
There are also CGM models that work differently, using a sensor that is surgically implanted under the skin, where it can remain for up to a year. Users attach a removable transmitter to the skin on top of the implanted sensor, which transmits blood sugar readings to their smartphone.
CGM technology is primarily intended for people who take insulin to manage type 1 or type 2 diabetes. Insulin users, who tend to have more volatile blood sugar levels, have a greater need for the enhanced monitoring and safety features provided by CGM. If you require insulin to manage your diabetes you may be able to obtain insurance coverage for CGM technology.
CGMs may also be useful for anybody who wants to better understand how their lifestyle — including diet, exercise, and medication choices — impacts their blood sugar levels. Users include people with:
Newer over-the-counter CGM options have made access to this technology more feasible for those without insurance coverage. While over-the-counter CGMs may lack some of the safety features that are included in types that are prescribed, OTC options can still provide in-depth glucose data.
CGMs can be obtained temporarily during in-patient hospital stays or via your doctor’s office to help monitor blood sugar levels in the short-term. Those are known as professional CGM systems, and they can be prescribed by your healthcare team.
Some people without diabetes also use CGM technology to monitor glucose levels — though experts are divided on whether it's truly useful. For some, the data gathered from a CGM may curb habits like monitoring the impact of soda on glucose levels.
For people with diabetes, CGMs can offer significant improvements to monitoring with a traditional glucose meter, which requires the user to draw a drop of blood with a sharp lancing device.
Continuous blood sugar measurements: CGM technology offers an unmatched opportunity to track blood sugar changes around the clock, including after meals and overnight, leading to a greatly enhanced understanding of how lifestyle changes impact your diabetes management.
Time in range analysis: Time in range, a new metric favored by many diabetes experts, measures the percentage of the day your glucose levels are within, above, or below target ranges.
Safety features: Prescription CGMs offer customizable alarms for high, low, and rapidly changing glucose levels, which can help prevent emergencies such as low blood sugar (hypoglycemia) and diabetic ketoacidosis (DKA). These alerts can be lifesaving when they warn people who are asleep, driving, have hypoglycemia unawareness, or are otherwise unaware that their blood sugar is moving in a dangerous direction.
Data sharing: Users can share their CGM data remotely with family members and their healthcare team.
Remote monitoring options also enhance safety by letting loved ones and caretakers monitor blood sugar levels (and sound alarms) from afar. These features are especially valued by parents of children with type 1 diabetes, or the family of older patients who may be less capable of managing their own blood sugar.
There’s a wealth of scientific evidence suggesting that people with type 1 and type 2 diabetes who use CGMs achieve healthier glucose levels. One analysis found that professional CGM use was associated with an average A1C reduction of 0.51 percent. Other research has found that improvements can come relatively quickly, too. People with type 1 or type 2 diabetes usually see improvements to their A1C level in a matter of months of CGM use.
CGMs offer a level of information about your glucose health that traditional blood glucose meters cannot. While glucose meters can provide your blood glucose level at any given moment when you test, CGMs offer ongoing data every few minutes, collected automatically.
Estimated A1C
Because a CGM collects data so frequently, it can predict A1C levels relatively accurately. A1C is a measurement that estimates average blood sugar levels over the past three months, and it’s often used to assess diabetes management success. Your healthcare team likely has given you an A1C range to shoot for, and CGMs can help you stay on top of that. Some models estimate A1C with a feature called a glucose management indicator (GMI).
These CGM estimates do not always precisely match the A1C results you get from a blood test, but they should give users a general idea of their overall management, especially when combined with time-in-range reports.
Time in Range
Time in range is widely considered a very reliable measure of glucose control and overall quality of life with diabetes, and this information can only be tracked with a CGM. Time in range refers to the proportion of time spent in an approved blood sugar range, neither too low nor too high. The most common general range used by diabetes authorities, and the standard setting on CGMs, is 70 to 180 milligrams per deciliter (mg/dL).
When your blood sugar is above 70 mg/dL and below 180 mg/dL for most of the day, it suggests you aren’t experiencing frequent hypo- or hyperglycemia episodes.
Some CGMs have been approved to work in tandem with insulin pumps, combining the two devices to create one system that can partially automate blood sugar management for people with type 1 or type 2 diabetes who use insulin multiple times every day.
In an automated insulin delivery (AID) system, a CGM provides real-time blood glucose measurements to automatically adjust insulin delivery from an insulin pump. An AID system is also sometimes called a closed-loop system or "artificial pancreas.”
While these systems still require daily attention and some management, they aim to significantly ease the hour-by-hour burden of insulin management by taking care of a large number of the insulin dosing decisions that people with diabetes make every day. The benefits include increased time in range, better safety, and improved quality of life, including improved sleep and reduced anxiety.
Today’s CGMs meet federal accuracy standards for use in insulin management decisions and in automated insulin delivery systems. In other words, they are accurate enough to replace the traditional blood sugar meter and to use for all your diabetes decisions.
It is worth noting that there is a slight delay in every CGM measurement. The glucose concentration in the interstitial fluid, which the CGM is sampling, lags for up to 15 minutes behind the glucose concentration of the bloodstream; your CGM is really showing you what your blood sugar level was 5 or 15 minutes ago. Users should take this into consideration, especially when blood glucose levels are rapidly changing. You might feel more comfortable using a “finger stick” to test your blood sugar during such times.
There are also a small number of supplements and medications that can interfere with CGM accuracy, including vitamin C and acetaminophen.
Continuous glucose monitors (CGMs) are wearable devices that provide automatic real-time blood sugar measurements around the clock.
Rapid blood sugar feedback can help people with diabetes improve A1C levels, lower their risk of complications, and improve their quality of life.
Prescription CGM models for people who use insulin offer safety features and data sharing, allowing caretakers to monitor a loved one’s health remotely.
Over-the-counter CGMs can help users pinpoint the impact of lifestyle choices on their blood sugar management, allowing them to pivot toward healthier choices.
