Why Your CGM “Glucose” Level is Not the Same as Your “Blood Sugar” Level 


 2022-04-13

If you’ve ever felt frustrated when the number on your continuous glucose monitor (CGM) doesn’t match the number on your blood glucose meter (BGM), here are some lesser-known facts that you need to know.

Fact #1: Your CGM and your BGM are measuring two different things.

While we often refer to the numbers from a CGM as blood sugar levels, it’s actually not measuring the sugar in your blood at all. Instead, your CGM sensor measures the sugar in the interstitial fluid of your subcutaneous tissue—your body fat. There is no blood in this thin layer of tissue, but there is glucose—which we refer to as sugar in conversational language.

Additionally, you could check your blood sugar with a BGM three times in a row and get a slightly different number each time—like 99, 103, 109 mg/dL. It’s important to understand that every at-home glucose measuring technology has an allowable range of accuracy.

Every FDA-approved BGM and CGM technology in the United States must be within 15% of the true laboratory value at least 95% of the time. It also needs to be within 20% of the true laboratory value at least 99% of the time.

And in rare situations, it may also be as simple as a rare calibration issue if you’re wearing a CGM that allows the user to enter a level from your BGM to improve the accuracy on your CGM. These days, CGMs simply shouldn’t need to be calibrated regularly. Most likely, you’re simply seeing the difference in the sugar in your blood versus the sugar in interstitial fluid.

Fact #2: The food you eat affects your BGM level before your CGM level.

One of the biggest reasons the number on your CGM doesn’t match your BGM is because the glucose from the food you eat actually reaches your bloodstream before it reaches the interstitial fluid where your CGM sensor sits.

There’s about a 15-minute difference between when glucose enters your bloodstream versus when it enters interstitial fluid. That means your BGM and CGM numbers are more likely to be significantly different in the hour or two after eating when your blood sugar is quickly rising or falling.

If you’re making decisions about insulin based on your CGM, it may be wise, in some circumstances, to confirm the number with your BGM to ensure you don’t take more insulin than you need. This should also be kept in mind when you are treating a low blood sugar. Using your BGM to keep an eye on falling blood sugar levels might help you determine if you need to consume extra carbohydrates or not.

Fact #3: You can expect more considerable differences in CGM and BGM levels in certain situations.

If your blood sugar level is changing quickly, you should anticipate that your CGM and BGM levels will be significantly different until the factors affecting your blood sugar settle down.

Factors that can affect how close your CGM and BGM levels are include:

  • After eating
  • After taking a large dose of insulin
  • During and immediately after exercise
  • After taking medications that affect blood sugar

Keep in mind: One of the easiest ways to get yourself in a technology-induced blood sugar pickle is by reacting too quickly to a level on your CGM. When it comes to diabetes—especially when correcting high blood sugars—patience is a critical virtue!

Before you respond to any glucose level, think about the factors affecting the number on your CGM versus your BGM. Sometimes, waiting 15 more minutes can prove that you didn’t need more insulin or more carbohydrates. It can be frustrating, but it’s worth the wait!

The bottom line

With all this in mind, it’s important to remember that every now and then you may encounter an inaccurate sensor. If your CGM sensor is significantly different from your BGM level throughout the entire day—even when the other factors listed above shouldn’t be an issue—then you may want to call the manufacturer of your CGM to report the faulty sensor.

If there is an accuracy issue, customer service will likely advise you to remove your sensor and insert a new one. You can get a replacement sensor shipped to you at no charge, in most cases.

It can be frustrating as a person with diabetes when it feels like the technology you depend on for your day-to-day health and safety isn’t perfect. But it’s also important to understand the science behind how and why your CGM says something different than your BGM—demonstrating that both are likely working as they should!

There is a lot to celebrate about how far we’ve come in diabetes tech, and the future is bound to be even brighter!


Editor’s Note: Educational content related to CGM technology is made possible with support from Dexcom, and editorial control rests solely on Beyond Type 1.

WRITTEN BY Ginger Vieira, POSTED 04/13/22, UPDATED 04/13/22

Ginger Vieira is the Senior Content Manager at Beyond Type 1. She is also an author and writer living with type 1 diabetes, Celiac disease, fibromyalgia, and hypothyroidism. She’s authored a variety of books, including “When I Go Low” (for kids), “Pregnancy with Type 1 Diabetes,” and “Dealing with Diabetes Burnout.” Before joining Beyond Type 1, Ginger spent the last 15 years writing for Diabetes Mine, Healthline, T1D Exchange, Diabetes Strong, and more! In her free time, she is jumping rope, scootering with her daughters, or walking with her handsome fella and their dog.