Calibrating CGMs: What You Need to Know

Continuous glucose monitors (CGMs) have become increasingly popular. They are replacing traditional blood glucose monitoring systems for many people with type 2 diabetes. Continuous glucose monitors are small devices worn on your body that consistently monitor glucose. They reduce or eliminate the need for checking finger sticks.

You may notice a difference between a finger stick and a CGM reading because they do not measure the same thing. A glucometer measures blood glucose, while a CGM measures interstitial fluid.

If you currently wear or are planning to start wearing a CGM, it is important to understand what calibration is. You should also know how and when to calibrate your CGM to help ensure you are getting the most accurate data from it.

How to calibrate a CGM

Calibration is a process that ensures the CGM's sensor provides accurate glucose readings. Calibrations can be done by the manufacturer, the person wearing the CGM, or both. To calibrate accurately, you will need to have a glucose monitor and testing supplies on hand.

Be sure your testing supplies are stored at a cool temperature. Make sure test strips have not expired before use. Wash your hands using soap, water, or alcohol to prevent anything from causing an inaccurate reading. You should be checking blood glucose only on your fingertips.

After checking your blood glucose with your meter, enter this value into your CGM receiver, insulin pump, or phone app.

Calibration tips

Enter the calibration into the CGM receiver or pump as soon as possible. Remember, your glucose is constantly changing, so you do not want to calibrate with old blood glucose numbers.

Because sensor glucose can lag behind blood glucose, the best times to calibrate are when blood glucose levels are stable, or when the trend arrow is lying flat. This time will be first thing in the morning or before meals for most people.

Avoid calibrating when your blood glucose levels are changing quickly, or when the trend arrows point up or down. This may occur after eating, taking insulin, or exercising.

If there is a big difference (more than 20 percent) between blood glucose and sensor glucose, wait to calibrate.

Inaccurate readings

The below medicines may lead to inaccurate CGM readings. Use a finger stick if you are taking:^1-3

• Vitamin C/ascorbic acid – Taking doses higher than 500 mg as a supplement or cold medicine may falsely increase Libre sensor readings.
• Hydroxyurea, acetaminophen, and paracetamol – These may falsely raise sensor glucose readings on the Dexcom system.
• Tetracycline – This may cause inaccurate readings on the Eversense sensor.

Does my CGM need calibration?

Find your CGM model below to see if it and when it needs calibration.

Dexcom G6

The Dexcom G6 CGM does not require daily calibrations as long as you enter the 4-digit sensor code on the sensor.⁴

Libre systems

No calibrations are required for the Libre and Libre 2.0.⁵

Medtronic Guardian

The Medtronic Guardian Connect requires calibrations:⁶

• 2 hours after the sensor is inserted
• 6 hours after your first calibration
• Every 12 hours (2 to 3 times per day)

Eversense

After the initial warm-up of the Eversense CGM, you must complete 4 calibrations that are 2 to 12 hours apart. Then do 2 calibrations daily for the first 21 days of wear. After 21 days, 1 calibration is required daily.³