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How to Read Your CGM Data

Continuous glucose monitoring (CGM) systems are great for managing Type 1, Type 2, and gestational diabetes. But, like any tool, to get the best results from a CGM, you have to know how to use it correctly. And because CGM systems are collecting ambulatory glucose profile (AGP) data 24/7, the most important thing to know about yours is how to read and understand that information.

Here is a breakdown of CGM system readings and how to interpret them with your health care team.

Important Features

Certain parts of your CGM system may differ depending on the brand and style you own. Some include a one-piece applicator, like these available through TMS:

  • FreeStyle Libre 3

Two-piece systems available through TMS include:

  • Dexcom G7
  • FreeStyle Libre 2

Three-piece systems available from TMS include:

  • Dexcom G6

But regardless of how many pieces your CGM system has, all systems have certain things in common. Here are a few to be aware of.


Your CGM system is designed to alert you to changes in blood glucose levels. Some of those alerts are adjustable, while others are not. Here’s a quick overview of some of those alerts.

⇒ Hypoglycemia: This condition occurs when blood glucose levels are too low. Symptoms can range from irritability and headaches to dizziness and seizures.

Because hypoglycemia is so dangerous, the alerts for hypoglycemia are not adjustable. You’ll start to be alerted when your blood sugar levels dip below 70 mg/dl, and you won’t be able to silence the alerts once they hit 54 mg/dl. You’ll also get a 20-minute warning alarm once your blood sugar levels begin to head toward hypoglycemia. And while you can’t change those alert levels, you can add to them. Some patients, for example, choose to have alerts begin when their glucose levels hit 100 mg/dl so they can take action to prevent hypoglycemia before it happens.

⇒ Hyperglycemia: This is the opposite of hypoglycemia and occurs when your blood glucose levels are too high. Symptoms include dry mouth, frequent urination, tiredness, blurred vision, and unintentional weight loss.

These alerts are fully customizable on your CGM system. You can set them to alert at levels that work for you. Because many patients with diabetes have blood sugar levels on the higher side, they often choose to set their hyperglycemia alerts higher than their normal levels to avoid constant alerts interrupting their daily activities. Hyperglycemia alerts are usually set to begin at 250 mg/dl, but many diabetes patients choose to set their alerts for when theirs hit 300 or 350 mg/dl. After a month or so of wearing your CGM system, your doctor or diabetes educator will ask you what your normal blood sugar readings are and take that into account when setting your hyperglycemia alerts.

This personalization is important for avoiding something called alert fatigue. If your car alarm goes off in your driveway, you probably look outside to see what set it off. But if that alarm went off multiple times a day, you’d probably stop checking. And when someone was actually stealing your car, you might miss it completely. When you personalize your alerts, you avoid hearing them so often that you start to ignore them, reducing your chances of missing something important.


Next to all your readings, you’ll see a series of arrows. The exact number of arrows will differ depending on the model you own. These are designed to show the severity of the low or high. If you’ve ever taken a survey asking you how much you agree or disagree with certain statements on a scale of one to 10, you’re familiar with the concept. Many upward arrows indicate your levels are very high, few upward arrows indicate they are slightly high, few downward arrows show levels that are slightly low, and many downward arrows show levels that are very low.

You’ll wear your CGM system for at least two weeks before visiting a doctor or diabetes educator to review the data. By then, you would have worn it all day, every day for the duration, so you’ll have an idea of what your normal ranges are. You and your health care team can make decisions about where to set your alerts that make sense for you.

Key Metrics

In addition to lows and highs, your CGM system will keep track of several key metrics. These include:

  • Number of Days Worn: This will depend on the kind of device you have. Most CGM systems range from 10 to 14 days of recommended use.
  • Mean Glucose Management Indicator (GMI): This metric uses a formula to estimate your A1C level.
  • Coefficient of Variation (CV): This metric measures how much your glycemic levels vary. The normal range is 36% or lower. Above 36% is considered unstable and requires medical intervention.
  • Time in Range: This is one of the most important metrics to keep an eye on. It’s the percentage of readings and time between 70 and 180 mg/dl, which is considered the normal range. Over 181 is considered high, and below 69 is considered low. Above 250 mg/dl is considered very high and below 54 is considered very low. Very high and very low readings require medical intervention.

Interpreting the Data

Now that you know the features of your CGM system and the metrics to look out for, here’s how you and your care team will read your CGM system data when you come in for a checkup.

Step 1: Have enough data.

Your doctor or diabetes educator needs enough data to look for trends in your glucose levels. The usual recommendation is 70% of the readings over two weeks.

Step 2: Print out the data.

During this step, your care team will ask you questions to help assess trends they may see. They’ll need to know about your diet and exercise habits, your insulin intake and schedule, your sleeping patterns, and other information that could affect your glucose readings. Knowing these details could help them understand spikes and dips in your glucose levels.

Step 3: Look for patterns.

Your care team will probably look for and notify you of patterns in order of importance. Because hypoglycemia is particularly dangerous, they’ll look for that first. Then they’ll look for hyperglycemia and wide glycemic variability. Next, they’ll look at when patterns happen. Are your levels usually low in the morning? Do they spike after dinner? This will give them an idea of what your levels are doing so they can adjust your care plan accordingly.

Step 4: Reflect.

This is where you can provide valuable insight into why your levels may be dipping or spiking at certain times. If, for instance, your levels are high every weekend or in the middle of the night, it’s helpful to know that you usually eat more fast food on Saturdays or get up at 1 a.m. to have a bowl of ice cream. Changes in medication, stress, and sleep can also affect your levels, so it’s important to discuss those with your care team as well. Without these details, they can’t help you adjust your insulin regimen to avoid these fluctuations.

Step 5: Make a Plan.

Once you and your care team have uncovered problems, they’ll work with you to develop a solution. It could be changing the way you eat, increasing or decreasing your insulin, or changing your physical activity. They’ll make sure you have the tools you need to implement the necessary changes and understand the effects they’ll have on your health and wellbeing. You’ll probably also receive a printout of your data and notes about the action plan.

Ready to get a CGM system?
Here are the steps to take to advocate for one:

  1. Print the CGM Fact Sheet.
  2. Take the CGM Fact Sheet to your next doctor's appointment.
  3. Point out which CGM system will work best for your unique needs. 
  4. Ask your doctor for a prescription.

It’s that easy!

A CGM system can be life-changing for patients with diabetes because it’s an easier, more comprehensive, and more convenient way to manage your health and improve your quality of life.

Find out if a CGM system is right for you!

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This information, including, but not limited to, text, graphics, images, and other material contained in this document, is provided for informational purposes only and does not constitute providing medical advice or professional services of any kind. No material on this site is intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health care provider with any questions you may have regarding a medical condition or treatment and before undertaking a new health care regimen, and you should never disregard professional medical advice or delay in seeking it because of something you have read on this document. If you think you may have a medical emergency, call 911 or go to the nearest emergency room immediately. Neither Total Medical Supply nor its employees make any representations, express or implied, with respect to the information provided herein or to its use.

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