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CGM or BGM? Interstitial Fluid or Blood Glucose? Your Questions Answered

 Level2 members are setting off into a whole new world of possibility as they begin their journey to remission from type 2 diabetes.
They’re also entering a whole new world of vocabulary and acronyms. Let us help define a few important terms and explain why they’re important for good health and well-being.

CGM and Interstitial Fluid

A continuous glucose monitor (CGM) measures glucose levels in the interstitial fluid just beneath your skin. (Most Level2 members use the Dexcom G6 CGM. You can learn more about troubleshooting a Dexcom G6 here.)
Interstitial fluid is a thin layer of fluid which surrounds your body’s cells. This fluid acts as a sort of fueling station for your cells because it contains all sorts of nutrients.Within the fluid there is glucose, salt, fatty acids and minerals like magnesium and potassium. When a CGM is applied to your skin, it inserts a tiny sensor that is able to measure the glucose level in the interstitial fluid.
Since the glucose level in your interstitial fluid is not necessarily the same as the glucose level in your blood, the CGM applies a formula to give you a very strong indication of what your blood glucose level is likely to be at any given time.

BGM and Blood Glucose

Blood glucose refers to the glucose levels within your blood. As the name implies, blood glucose meters (BGMs) measure glucose levels in your blood, and they are typically done by fingerstick.
Because BGMs measure blood glucose levels and CGMs monitor the glucose levels in interstitial fluid, they are analyzing two different types of fluids.  Different body fluids can sometimes give different numbers, so there will be times when the readings on your fingerstick and on your CGM temporarily don’t match.

When Fingerstick Readings Don’t Match CGM

A difference in readings between a CGM and a fingerstick reading of blood glucose is more likely to occur in certain circumstances that are listed below:

  • First 24 hours of a new sensor. Newly-inserted Dexcom G6 sensors can register glucose readings that are less consistent with those of a blood glucose meter. This is because interstitial fluid can change at different rates. The CGM needs to adjust to this. The readings become more aligned as the sensor becomes more accustomed to your body over the first 24 hours of sensor use. Be sure to use finger sticks during this period if your readings don’t match how you feel. Or you are making any medication or dosing decisions.
  • Physical pressure on the sensor. Sometimes we might not even realize when we’re pressing on the sensor, and it can create a difference in your readings. Have you been sleeping on the sensor? Wearing tight clothing? Leaning against a standing desk while working? Think about whether something is pressing against the sensor.
  • Quick changes in glucose levels. If your glucose level quickly changes – blood glucose and interstitial fluid do not change at the same rate so there can be some differentiation between those two values, however as they stabilize, the numbers should get closer and be easier to compare
  • A lag as data is relayed. A lag is typical in relaying CGM data from your body to your app, and it should be expected.

The CGM reports a  glucose value every five minutes, which actually represents the average of readings over the past five minutes.  Note that when blood glucose is rising, the CGM is likely to display values that are temporarily LOWER than the actual blood glucose numbers. When it is falling, the CGM is likely to display values that are temporarily HIGHER than the blood glucose.

Which Reading is More Accurate: BGM or CGM?

Both readings can be accurate, even if they don’t match. Your body’s fluids just behave a little differently.
It’s important to understand that BGM readings will never exactly match the CGM sensor reading. The degree of error in the Dexcom G6 CGM system (9.5%) is equal to the most accurate meters available today (10%). Older and off-brand meters may have considerably lower accuracy than the G6 system.
Dexcom’s instructions advise that when working correctly, your Dexcom G6 device should obey the “20 rule,” which means numbers fall within:

  • 20% of the meter value (blood glucose meter) when the meter value is 80 mg/dL or higher
  • 20 mg/dL of the meter value (blood glucose meter) when the meter value is under 80 mg/dL

If your sensor readings are in fact not lining up, contact Level2 Support at 1-844-302-2821, TTY 711 to see if they can help determine if you should receive a replacement device if necessary. However, if you think your level is rising or falling quickly, a fingerstick blood glucose test will give you a more accurate real-time level.
What is most important is not the blood glucose number itself, but rather, the overall trends you are seeing. Think about how you’re feeling and make a note of any trends or signals your body is telling you. (Do you want a list of ways to get better in tune with what your body is telling you? Read this. Also, learn the signs of hypoglycemia and hyperglycemia.)
If you’re concerned your CGM is not working, please contact Level2 Support:  1-844-302-2821, TTY 711