Can a Continuous Glucose Monitor (CGM) Help Keep Your A1C in Range?

For people with diabetes, a continuous glucose monitor (CGM) can help improve hemoglobin A1C (HbA1C or A1C) levels. 

CGMs work by relaying up-to-the-minute glucose information to a receiving device. This can be a smartphone app, brand-specific monitor, or even an insulin pump. 

The software in the receiving device will help you better understand your unique glucose patterns and trends — and how they respond to everyday activities like exercise. In turn, this information allows you to respond with appropriate actions to keep your glucose within your target range. 

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In other words, CGMs remove some of the guesswork in diabetes management. Let’s look closer at how this works in practice.

CGMs improve your ‘time in range’

Time in range (TIR) is the amount of time you spend in your target glucose range. For most people, that’s between 70 mg/dL and 180 mg/dL. 

Target ranges vary depending on your age and other health conditions. Your diabetes care team will let you know what’s safest for you, and they’ll help you make improvements over time. CGMs track glucose levels minute by minute, and this lets people quickly respond to high or low glucose levels and make changes in real time. This increases their TIR. 

Experts recommend that someone’s TIR should be around 70%. Put another way, people living with diabetes should aim to spend at least 70% of the time (17 out of 24 hours) within their target glucose range. If you can meet this goal, it’s likely that your A1C will be around 7%. 

A CGM also calculates “variability,” which is how much your glucose levels rise or drop when you’re out of range. Both TIR and variability are important for improving diabetes treatment. A1C blood tests can’t provide this level of detailed information (more on this below). 

Newer CGMs can help estimate A1C with a glucose management indicator (GMI)

A glucose management indicator (GMI) is a new metric that is available on some CGMs. It estimates your A1C from your CGM data. While A1C and GMI numbers are similar, they don’t measure the same thing: 

• Hemoglobin A1C test (HbA1C): This measures how much glucose is attached to hemoglobin in your red blood cells over the past 120 days. 

• Glucose management indicator (GMI): Previously called “estimated A1C” (eA1C), GMI predicts what your lab A1C will be from your average CGM data. Most CGMs calculate this number when you upload data (at least 14 days’ worth) to the device’s app.

Why the name change? While an estimated average can be helpful, both healthcare professionals and people living with diabetes were becoming frustrated when “eA1C” wasn’t closely matching their “A1C.” So, the decision was made to leave “estimated” and “A1C” out of the new name for the updated calculation.

Is a GMI measure as accurate as A1C?

In most cases, GMI is an accurate predictor of A1C test results. Some people argue it may be even more useful than A1C because it measures what is really important: how much time you spend in your target glucose range and how high or low your glucose levels go. 

A recent study compared GMI to A1C. It found that while A1C and GMI are close, they aren’t a perfect match. GMI can be lower or higher than your A1C lab result. But most results fell within 0.5 percentage points of the other. 

So, although GMI may not be a spot-on prediction for your A1C, it does give a general idea of what it will be. 

CGM improves A1C, but A1C is not a perfect measure 

Hemoglobin A1C is an average of blood glucose levels over a period of several months. This long time period means that someone could have moments of very high and very low glucose levels but still have a “normal” A1C. That’s because the average over several months can end up hiding the day-to-day changes in blood glucose.

It’s also important to be aware that many different medical conditions and medications can affect the accuracy of A1C test results. 

Research shows that when used alone, A1C doesn’t represent daily glucose changes very well. 

So, in summary, CGM improves TIR, which the GMI captures. This will likely translate to improved A1C levels. But because other factors unrelated to glucose levels can affect A1C, not everyone with a given GMI will have the same A1C lab result. 

Different CGMs have different functionality

Data reports from a CGM provide estimates for expected A1C lab results. You can use the device’s app to upload the suggested days of data and generate reports. This may vary among CGM devices.

So, before you buy a CGM device, it helps to know your options. The “Consumer Guide” from the American Diabetes Association can help you learn about the different CGMs. Simply select those that interest you and look at side-by-side comparisons. 

Keep in mind: Your out-of-pocket costs may vary depending on your health insurance and their preferred devices. 

Abbott

FreeStyle Libre 2 and FreeStyle Libre 14-day system:

• To get glucose results, you must scan the sensor with a handheld reading device

• One-hour warm-up 

• Sensor life of 14 days

• Alerts and alarms

• You don’t need to calibrate (test) this CGM against a fingerprick blood glucose every day like with some other machines

• Shares data via app for estimated A1C

• Stand-alone device (not compatible with an insulin pump — more on this below)

Dexcom

G6 CGM System:

• Provides new results every 5 minutes that are shared with a smartphone or another compatible device 

• Two-hour warm-up 

• Sensor life of 10 days

• Alerts and alarms

• New severe hypoglycemia alert 20 minutes before a glucose of 55 mg/dL is predicted

• You don’t need to calibrate (test) this CGM against a fingerprick blood glucose every day like with some other machines

• Shares data via G6 app for GMI

• Compatible with Omnipod and Tandem automated insulin delivery (AID) devices

G7 CGM System:

• Provides new results every 5 minutes that are shared with a smartphone or another compatible device

• Up to a 30-minute warm-up and 60% smaller than G6

• Sensor life of 10 days with a 12-hour grace period

• You'll need three sensors per month

• All-in-one wearable sensor/transmitter

• Alerts and alarms

• New severe hypoglycemia alert 20 minutes before a glucose of 55 mg/dL is predicted

• You don’t need to calibrate (test) this CGM against a fingerprick blood glucose every day like with some other machines

• Shares data via G7 app for GMI

• Compatible with Tandem and iLet Bionic Pancreas AID systems

Medtronic

Guardian Connect CGM System and Guardian 3 Sensor with Guardian Link 3 Transmitter:

• Provides new results every 5 minutes that are shared with a smart device 

• Two-hour warm-up 

• Sensor life of 7 days

• Alerts and alarms

• You need to calibrate (test) this CGM against a fingerprick blood glucose test from a glucometer every 12 hours

• Shares data via app for GMI

• Guardian 3 CGM is compatible with 630G and 770G series MiniMed insulin pumps; Guardian Connect is a stand-alone device

Senseonics

Eversense E3 CGM System:

• Implantable under-the-skin CGM sensor, with a life of 6 months

• After insertion, a 24-hour warm up with no CGM data available

• Requires a transmitter that sits on top of the skin to share data continuously

• You need to calibrate (test) this CGM against a fingerprick blood glucose test from a glucometer every 12 hours 

• Sensor life of 180 days

• Alerts and alarms

• Shares data via app for GMI

• Stand-alone device 

Automated insulin delivery (AID) devices

An automated insulin delivery (AID) device combines a CGM with an insulin pump. This is a whole level up from a stand-alone CGM. But it’s worth mentioning here because this setup allows automated insulin adjustments based on the CGM readings. AIDs help to improve TIR while reducing the mental burden of diabetes. 

Examples of FDA-approved AIDs include:

• Insulet: Omnipod 5 automated insulin delivery system

• Medtronic: MiniMed 770G hybrid closed-loop system

• Tandem: T:slim x2 insulin pump with Control-IQ technology

• Beta Bionics: iLet Bionic Pancreas

Why trust our experts?

Written by:

Kerry R. McGee, MD, FAAP

Kerry McGee, MD, FAAP, has over a decade of experience caring for babies, children, and teenagers as a primary care pediatrician. She has a special interest in adolescent health, particularly in adolescent mental health.

Edited by:

Alex Eastman, PhD, RN

Alex Eastman, PhD, RN, is a California-based registered nurse and staff medical editor at GoodRx, where he focuses on clinical updates and Latino health.

Reviewed by:

Jewels Doskicz, RN, BA

Jewels Doskicz, RN, BA, is an Arizona-based registered nurse with 22 years of hospital-wide clinical nursing experience and 11 years of medical writing and editing experience. She was a former healthcare content creator for a digital health tool at HealthLoop and director of clinical content operations at GetWellNetwork.

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