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Why BGM and CGM readings are sometimes different


Oct. 18, 2024 5 min read

Glucose meter accuracy BGM vs Dexcom CGM
Have you ever compared your blood glucose meter (BGM) and Dexcom G7 Continuous Glucose Monitoring (CGM) System readings and wondered why they’re not the same? One of the main reasons the two may differ is that a BGM measures glucose in blood, while Dexcom G7 measures glucose in interstitial fluid (a fluid just below the surface of the skin).
Watch the video below to learn why your BGM and CGM readings may differ, and how to ensure you get the most accurate readings.

Here are some other common reasons why your BGM and CGM readings could differ:
  • Hand cleanliness: Many inaccurate BGM values are caused by hands not being clean. Wash your hands with soap and water (and dry them) right before testing.
  • Sensor’s first day: The difference between your BGM and CGM readings may be greater when you insert a new sensor. Generally, the readings get closer over the first 24 hours.
  • Pressure on your CGM sensor: When something is pressing on your sensor, it can affect your readings. Relieve the pressure and your BGM and CGM readings should get closer.
  • Rapid glucose change: It can be tricky to compare your BGM and CGM readings when your glucose is changing quickly, since blood glucose changes before interstitial fluid glucose does. The match should get closer when your glucose stabilizes.
  • Test strips: If test strips aren’t stored properly or are expired, they may not work correctly. Make sure your test strips are stored as directed and not expired, and that you use enough blood when testing


Comparing BGM and CGM readings

You can compare your BGM and CGM readings to see how close the numbers are. Follow these two simple steps to find out how your BGM reading compares to your Dexcom G7 reading:
  • In the first column, find the BGM reading closest to yours.
  • In that row, check whether your Dexcom G7 reading fits into the range in the second column.
If it fits, your CGM reading falls within the acceptable range for accuracy and does not need to be calibrated. If it doesn’t fit, consider calibrating your CGM.
This table provides sensor readings that are within ± 20 mg/dL of the BG meter value for BG meter values <70 mg/dL and within ± 20% of the BG meter value for BG meter values ≥70 mg/dL.

Get started on Dexcom G7

If you are living with diabetes and not using CGM, talk to your doctor about Dexcom G7. We can help you get started with a free benefits check. Click the button below to send us some basic information.
  • Get started

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