How To Make Sense of CGM Data

Using a continuous glucose monitor (CGM) can be a game changer for understanding trends in your blood sugar levels. 

The data that CGM devices collect, combined with smartphone and computer programs that come with them, can help you identify what might be affecting your blood sugar. But making sense of the data – collected as often as every 5 minutes – can feel overwhelming, and figuring out exactly what is causing the changes and what to do about them can be difficult. 

Here are tips on how to interpret your CGM readings and make small changes that can lead to big gains for your day-to-day and long-term health.

Know how CGM data is tracked

CGMs automatically take glucose readings every few minutes, providing a more comprehensive view of your glucose values. After each reading, the device transmits data to a receiver, pump, phone, or watch, eliminating the need to manually record readings.

Unlike glucometers, which measure glucose from the blood, CGMs measure glucose in interstitial fluid, a substance found in the spaces between cells. This may result in a lag time between a glucose reading in the blood, which comes first. A glucose reading in the interstitial fluid can be delayed anywhere from 2-3 minutes for the newest CGMs to 15 minutes for older models.

Melanie Batchelor, chair of the Certification Board for Diabetes Care and Education, recommends wearing your CGM for at least 10 days before trying to draw any insights from graphs. The interval will provide the sensor enough time to collect data that establishes trends, for example, high blood sugar levels at a certain time of day when you haven’t eaten, versus underestimating carbs in a meal or an illness that could cause a short-term blood sugar spike. 

Every CGM also comes with a free app that continuously tracks glucose readings and presents them visually, so it’s helpful to familiarize yourself with that program, too.

Get to know your average values

CGM brands like Abbott (Freestyle Libre), Dexcom, Medtronic (Guardian), and Senseionics (Eversense) typically come with diabetes data management software, such as the Abbott LibreView, Dexcom Clarity, Medtronic Carelink, and Eversense’s CGM app. In addition, there are a number of stand-alone apps that will help you track and analyze your CGM data. Many of these programs will show you a summary glucose profile with average sensor values over a certain time period, such as a week or a month.

“These apps not only show your current sensor glucose value – a new reading every 5-15 minutes – they also show your average sensor values, often shown as a graph,” Batchelor said. “When looking at the average graph, you can quickly see if there are any patterns of highs or lows throughout your day or overnight.”

Just as improving your time in range can help you feel your best and avoid complications related to diabetes, viewing your average values can help you understand how well you are managing your diabetes at the moment.

Learn what factors may affect blood sugar

“Research has shown that over 42 factors affect glucose in your body,” Batchelor said. 

Those 42 factors include everything from dehydration to allergies. If this number is overwhelming, Batchelor suggests starting with several of the best-known variables. 

“The most common in contributing to patterns of highs and lows include under- and overestimating carbs, exercise, illness, and stress,” she said.

Become a trendspotter: Take notes and ask the right questions

Since there are dozens of factors that can affect your blood sugar, the best way to figure out what is happening is to track your numbers over time with notes about what was occurring before you see highs and lows.

Batchelor advised reflecting back on your schedule or routine, something that is made easier if you log any of the 42 factors that may be at work.

“Start by identifying the high or the low and work back from there. What were you doing 15-30 minutes before the glucose started to rise or fall? How long did your glucose stay high or low?” she said.

“If you can use your CGM app or software – or paper and pencil – to log your daily activities, medications, meals, and special events, finding trends related to certain factors that affect glucose will be easier,” she added.

What to do if you’ve identified a CGM pattern

If you’ve spotted a pattern and the potential cause – like a high-carb meal, exercise, or a change in your medicine – that you can control, Batchelor suggested making an adjustment for at least three days and seeing how the change worked out (consult your healthcare provider, of course, before making any significant changes to your care).

“If you notice that your behavior adjustment helped to improve the high or the low pattern, keep up the great work,” she said. 

“Be sure to make notes and share those with your healthcare team. If your behavior change didn’t result in your desired outcome, your healthcare provider can use that information to suggest further changes to your treatment regimen,” Batchelor said.

A last reminder about reviewing CGM data

It’s important to accept that there will be factors out of your hands. No two days are the same, even when you’re doing the same things. 

Not all of the factors that can affect your blood sugar are under your control (like an unexpected stress-inducing family visit) or easy to recognize (like a site on your body that isn’t absorbing insulin correctly).

The main thing to remember, Batchelor said, is to not be too hard on yourself if you don’t get it right when making changes: “Give yourself some grace.”

Get more tips about CGM use:

• Is CGM Right for Everyone?
• 6 Things to Know About Wearing a CGM
• How to Navigate CGM Insurance Coverage