Calibrating CGMs: What You Need to Know

Continuous glucose monitors (CGMs) have become increasingly popular, replacing traditional blood glucose monitoring systems for many people with type 2 diabetes.

These small devices are worn on the body to consistently track glucose levels, significantly reducing or eliminating the need for daily finger sticks.

You may notice a difference between a finger stick and a CGM reading because they do not measure the exact same fluid. A traditional glucometer measures blood glucose, while a CGM measures glucose in interstitial fluid (the fluid surrounding your body's cells). Because of this natural lag between blood and interstitial fluid, understanding calibration is essential for getting the most accurate data from your device.

How calibration works

Calibration aligns your CGM sensor readings with a standard blood glucose meter. Depending on the design, calibration may be handled entirely by the manufacturer before you open the box, completed by you at specific intervals, or a combination of both.

If your device requires manual calibration, keep a blood glucose monitor and fresh testing supplies on hand.

• Store your supplies according to package instructions and ensure test strips are not expired.
• Wash your hands thoroughly with soap and water to prevent surface contaminants from skewing your test.
• Always use blood from your fingertips for calibration checks, as alternative site testing is less reliable for this purpose.
• Once you take your blood glucose reading, enter the exact value into your CGM receiver, insulin pump, or mobile app immediately.

Calibration tips

• Act quickly: Enter your calibration reading right away. Glucose fluctuates constantly, so calibrating with older blood glucose numbers leads to inaccurate sensor adjustments.
• Wait for stable levels: The best time to calibrate is when your glucose is steady, such as first thing in the morning or before meals when the trend arrow is lying flat.
• Avoid rapid shifts: Do not calibrate when your glucose levels are changing quickly, such as right after eating, taking insulin, or exercising, or while trend arrows point up or down.
• Watch for big gaps: If there is a large difference (generally more than 20 percent) between your finger stick and sensor reading, wait until your glucose stabilizes before trying to calibrate.

Medications and sensor accuracy

Certain medications and supplements can interfere with CGM sensor chemistry and cause falsely high or low readings. Common culprits across various sensor technologies include high-dose Vitamin C (ascorbic acid), acetaminophen, hydroxyurea, and certain antibiotics.^1-3

Because interference risks vary based on the specific sensor technology, always check your device's package insert or user guide for a full list of drug interactions. If you are taking a medication known to interfere with your sensor, rely on traditional finger sticks for diabetes management decisions.

Does your CGM need calibration?

Different CGM models handle calibration in different ways:

• Factory-Calibrated Systems: Many modern CGMs come pre-calibrated from the factory. These models generally do not require routine daily finger sticks, as long as setup instructions are followed correctly and your sensor readings match how you feel.
• User-Calibrated Systems: Other devices require manual finger-stick calibrations—either during an initial warm-up period, at set daily intervals, or periodically throughout the wear period.

Because features and guidelines evolve as new models enter the market, always consult the user manual, app setup instructions, or manufacturer website for your specific CGM model to confirm its exact calibration routine.

Regardless of the device you use, if your sensor readings do not match your physical symptoms, perform a traditional finger stick to verify your blood glucose before making treatment decisions.