What Is Continuous Glucose Monitoring (CGM)?
Health ConditionsDiabetes
Explore This Topic

What Is a Continuous Glucose Monitor (CGM)?

What Is a Continuous Glucose Monitor (CGM)?
Julia Amaral/Getty Images
By
Published on April 18, 2025
by
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.

How Do CGMs Work?

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.

Health Benefits of Continuous Glucose Monitoring (CGM) Call-outs: Rapid blood sugar feedback Round-the-clock measurements Trend arrows and safety alerts Remote monitoring and data sharing. Everyday Health logo.
Everyday Health

Who Should Use a CGM?

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.

What Are the Benefits of CGMs?

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.

Can CGMs Improve Your Blood Sugar Management?

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.

CGM Data

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.

How CGMs Work With Insulin Pumps

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.

How Accurate Are CGMs?

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.

The Takeaway

  • 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.

Resources We Trust

EDITORIAL SOURCES
Everyday Health follows strict sourcing guidelines to ensure the accuracy of its content, outlined in our editorial policy. We use only trustworthy sources, including peer-reviewed studies, board-certified medical experts, patients with lived experience, and information from top institutions.
Resources
  1. Continuous Glucose Monitors (CGM). American Diabetes Association.
  2. Continuous Glucose Monitoring. National Institute of Diabetes and Digestive and Kidney Diseases. June 2023.
  3. Who Should Use a Continuous Glucose Monitor? Yale Medicine. November 2024.
  4. Use of Continuous Glucose Monitors by People Without Diabetes: An Idea Whose Time Has Come? Journal of Diabetes Science and Technology. July 20, 2022.
  5. Weighing CGM Pros and Cons. Association of Diabetes Care & Education Specialists. February 2025.
  6. Time in Range (TIR) for Diabetes. Cleveland Clinic. July 9, 2024.
  7. Haas NL. Analytical Accuracy of a Continuous Glucose Monitor in Adult Diabetic Ketoacidosis. Chest Critical Care. March 2025.
  8. How Hypoglycemia Unawareness Affects People with Diabetes. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). April 2023.
  9. Continuous Glucose Monitoring (CGM). Cleveland Clinic. May 24, 2024.
  10. Association Between Change in A1C and Use of Professional Continuous Glucose Monitoring in People With Type 1 and Type 2 Diabetes on Noninsulin Therapies: A Real-World Evidence Study. Clinical Diabetes. January 24, 2023.
  11. Manov AE et al. The Effectiveness of Continuous Glucose Monitoring Devices in Managing Uncontrolled Diabetes Mellitus: A Retrospective Study. Cureus. July 27, 2023.
  12. The Discrepancy Between Hemoglobin A1C and Glucose Management Indicators in 26 Patients Treated With Continuous Glucose Monitoring in an Internal Medicine Residency Clinic. National Library of Medicine. March 23, 2024.
  13. Does Time-in-Range Matter? Perspectives From People With Diabetes on the Success of Current Therapies and the Drivers of Improved Outcomes. National Library of Medicine. AprIl 2018.
  14. Aslani S et al. Time In Range is Associated With Less Hypoglycemia Fear and Higher Diabetes Technology Acceptance in Adults With Well-Controlled T1D. Journal of Diabetes and Its Complications. February 2023.
  15. Sherr JL et al. Automated Insulin Delivery: Benefits, Challenges, and Recommendations. Diabetes Care. October 6, 2022.
  16. Accuracy of the Third Generation of a 14-Day Continuous Glucose Monitoring System. Diabetes Therapy. March 6, 2023.
  17. Lag Time. Association of Diabetes Care & Education Specialists (ADCES).
  18. CGM Interfering Substances & Procedures. Association of Diabetes Care & Education Specialists (ADCES). August 2024.
Meet Our Experts
See Our Editorial PolicyMeet Our Health Expert Network
Elise-M-Brett-bio

Elise M. Brett, MD

Medical Reviewer
Castle Connolly Top Doctor
Elise M Brett, MD, is a board-certified adult endocrinologist. She received a bachelor's degree from the University of Michigan and her MD degree from the Icahn School of Medicine at Mount Sinai. She completed her residency training in internal medicine and fellowship in endocrinology and metabolism at The Mount Sinai Hospital. She has been in private practice in Manhattan since 1999.

Dr. Brett practices general endocrinology and diabetes and has additional certification in neck ultrasound and fine-needle aspiration biopsy, which she performs regularly in the office. She is voluntary faculty and associate clinical professor at the Icahn School of Medicine at Mount Sinai. She is a former member of the board of directors of the American Association of Clinical Endocrinology. She has lectured nationally and published book chapters and peer reviewed articles on various topics, including thyroid cancer, neck ultrasound, parathyroid disease, obesity, diabetes, and nutrition support.
See full bio

Ginger Vieira

Author

Ginger Vieira has lived with type 1 diabetes and celiac disease since 1999, and fibromyalgia since 2014. She is the author of Pregnancy with Type 1 Diabetes, Dealing with Diabetes Burnout, Emotional Eating with Diabetes, and Your Diabetes Science Experiment.

Ginger is a freelance writer and editor with a bachelor's degree in professional writing, and a background in cognitive coaching, video blogging, record-setting competitive powerlifting, personal training, Ashtanga yoga, and motivational speaking.

She lives in Vermont with a handsome husband, two daughters, and a loyal dog named Pedro.

See full bio
See Our Editorial PolicyMeet Our Health Expert Network