Why are my CGM readings different than my BGM results?

BY Timothy Balder, MD  |  April 14th, 2021  |  No Comments
Timothy Balder, MD
Dr. Timothy Balder, MD is a Family Medicine Specialist in Bloomington, MN, and has over 36 years of experience in the medical field. He graduated from the University of Minnesota Medical School in 1985.

Continuous Glucose Monitors (CGMs) are a wonderful technology that allows people with diabetes to see an ongoing set of glucose results as a trend line.   Meanwhile, Blood Glucose Monitors (BGMs) are individual glucose results from lancing a finger and measuring the glucose in that blood sample.  So, can you compare these two results?  

People will naturally try to compare these two results as both are indicators of glucose levels. But when a comparison is done, it is important to understand the differences between CGM and BGM.

The primary difference is BGM measures glucose in the actual blood while CGM measures glucose in the interstitial fluid.

Interstitial fluid

Interstitial fluid is found in spaces around the body’s cells. It comes from substances that diffuse out of blood capillaries. One of the substances that comes out of the capillaries is glucose. Because a sensor can survive longer in the interstitial fluid versus blood, CGM sensors only penetrate under skin far enough to be in the area of interstitial fluid.

The glucose in the bloodstream initiates from the digestion of what a person eats. The blood in the capillaries carries the glucose to all parts of the body for nutrition.  The truest number of a person’s glucose level is therefore in the blood. So a BGM reading from a blood sample gives the most accurate reading of glucose.  

However, glucose does over time diffuse out of the capillaries into the interstitial fluid, where a CGM sensor can read it. Because of the process of moving from the bloodstream to the interstitial fluid, the interstitial glucose level (CGM) is delayed, and may not reach the same peak as the blood glucose level (BGM). 

As an example of this difference, consider the graph below that shows the relation of one study to compare BGM and CGM. At 180 minutes after ingesting carbohydrates, the BGM measurement of glucose in the blood peaks at 280 mg/dl, but the CGM result of glucose in the interstitial fluid only shows 220 mg/dl at the same time. And the interstitial glucose never reaches the same peak of the glucose in the blood.  

This delay between BGM and CGM will most often show up when glucose is changing, which is likely:

As another example, the Pops Rebel meter was used while wearing an Abbott FreeStyle Libre to compare results. Accuracy was overall good with two key findings:  

During the 1-2 hours after eating, the Pops Rebel picked up the glucose rise more quickly than the CGM, so Pops was as much as 20% higher in results than the CGM.  

During times of rest and no eating where the CGM trend line was flat indicating glucose was stable, the difference between the Pops Rebel and CGM was less than 4%.

Other factors are also known to affect CGM accuracy and should be considered if you compare to a BGM.  Two key factors: 

  1. The beginning and end of life of a sensor can result in different accuracies
  2. Applying pressure to the area of the body where the CGM sensor is worn can affect the accuracy.

Comparisons of two different methods of checking glucose can be difficult, and it is necessary to understand these differences.      

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Timothy Balder, MD
Dr. Timothy Balder, MD is a Family Medicine Specialist in Bloomington, MN, and has over 36 years of experience in the medical field. He graduated from the University of Minnesota Medical School in 1985.
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