What should I do if my sensor reading and blood glucose value don’t match?
Whether you’re new to Dexcom or experienced, review product instructions before using Dexcom CGM. Keep using your blood glucose meter to make treatment decisions until you know how Dexcom works for you. Your meter gives you one number, if you test twice (using the same or a different meter) it gives you another number, and your Dexcom CGM gives you a third. What do you do with all those numbers?
One explanation is that a BG meter measures glucose from the blood while Dexcom CGM measures interstitial fluid, so those numbers may be different. And, if your doctor did a lab test at the same time, the lab result may give a third number. The lab result is considered the most accurate number. Even if you wore two CGM sensors at the same time CGM sensor performance may vary slightly.
Good fingerprick practices
When using your meter, make sure:
When using your meter, make sure:
- Your test strips are stored as directed and not expired.
- Your hands are clean for the fingerprick. Wash your hands thoroughly with soap and water (not hand gel) and dry them. Then test. There are times when the numbers temporarily don’t match but are likely to become closer over time. For example:
- Sensor’s first day. With newly inserted sensors, the differences between your meter value and the Dexcom CGM reading may be greater. Generally, the match gets closer over the first 24 hours.
- Glucose changing quickly. When your glucose is changing quickly, it’s more difficult to compare numbers. The meter and Dexcom CGM measure glucose from different sources (blood and interstitial fluid), and blood glucose changes a little before interstitial fluid glucose. The match gets closer and easier to compare when your glucose stabilises.
- Pressure on sensor. When something is pressing on your sensor it can affect your Dexcom CGM readings. The match should get closer after the pressure is relieved.
Calibrating vs. Comparing
Calibrating is when you take a fingerprick and enter that BG reading into your Dexcom CGM.
Comparing is when you take a fingerprick and do not use it to calibrate, but rather for comparison and awareness. With Dexcom CGM, you can choose to log blood glucose instead of calibrating.
You can log your BG meter value in the Dexcom CGM app's Glucose or History tabs. In the receiver, go to Menu > Event > Blood Glucose.
If your sensor readings are always much higher (or always much lower) than your BG meter values for several hours, consider calibrating.
Comparing
You can compare your BG meter and Dexcom CGM reading to see how closely the numbers match each other.
Use the table to find out how your BG meter compares to your Dexcom G7 reading. Follow these steps:
- In the first column, find the BG meter value closest to yours.
- In that row, check whether your Dexcom CGM reading fits into the range in the second column.
If it fits, treat using the number that best fits your symptoms, whether it’s from your Dexcom CGM or your BG meter. If it doesn’t fit, don’t treat with your Dexcom CGM. Use your BG meter value instead.
BG Meter Value (mmol/L) | Dexcom CGM Reading (mmol/L) |
2.2 | 1.1-3.3 |
2.8 | 1.7-3.9 |
3.3 | 2.2-4.4 |
3.9 | 3.1-4.7 |
4.4 | 3.6-5.3 |
5.0 | 4.0-6.0 |
5.6 | 4.4-6.7 |
6.1 | 4.9-7.3 |
6.7 | 5.3-8.0 |
7.2 | 5.8-8.7 |
7.8 | 6.2-10.0 |
8.9 | 7.1-10.7 |
BG Meter Value (mmol/L) | Dexcom CGM Reading (mmol/L) |
9.4 | 7.6-11.3 |
10.0 | 8.0-12.0 |
10.6 | 8.4-12.7 |
11.1 | 8.9-13.3 |
11.7 | 9.3-14.0 |
12.2 | 9.8-14.0 |
12.8 | 10.2-15.3 |
13.3 | 10.7-16.0 |
14.4 | 11.6-17.3 |
15.0 | 12.0-18.0 |
15.6 | 12.4-18.7 |
16.1 | 12.9-19.3 |
BG Meter Value (mmol/L) | Dexcom CGM Reading (mmol/L) |
16.7 | 13.3-20.0 |
17.2 | 13.8-20.7 |
17.8 | 14.2-21.3 |
18.9 | 15.1-22.7 |
19.4 | 15.6-23.3 |
20.0 | 16.0-24.0 |
20.6 | 16.4-24.7 |
21.1 | 16.9-25.3 |
21.7 | 17.3-26.0 |
22.2 | 17.8-26.7 |
This table provides sensor readings that are within ± 20% of the BG meter value for BG meter values ≥3.9 mmol/L and within ± 1.1 mmol/L of the BG meter value for BG meter values <3.9 mmol/L.
iCGM accuracy is established by comparing to a lab analyzer YSI. Use this table as a reference only.
Calibration
Calibrating your Dexcom CGM is optional. It can make the Dexcom CGM more accurate or less accurate compared to the lab result, but it should bring the Dexcom CGM readings closer to your meter value. When calibrating, make sure to:
- Clean hands: Wash your hands with soap and water and dry them.
- Enter within 5 minutes: Enter the calibration within 5 minutes of taking a finger prick.
- Pressure on sensor: If the sensor reading seems inaccurate because of pressure on the sensor, don't calibrate. The sensor should recover on its own when the pressure is relieved.
- Stable glucose: Calibration works best when your glucose is stable. Consider waiting to calibrate if your glucose is changing rapidly.
In the app, tap + in the Glucose or History tabs.
In the receiver, go to Menu > Event > Blood Glucose.
Select Use as Calibration and follow onscreen instructions to enter the BG meter value as a calibration.
When you calibrate your Dexcom CGM using your BG meter, remember:
- Calibrate in one display device: Calibrate in one display device, even if you use both the app and receiver.
- Meter values: Only calibrate with BG meter values from 2.2 mmol/L to 22.2 mmol/L.
- Calibration Not Used alert: If you get a Calibration Not Used alert, take another fingerstick/finger prick and calibrate again.
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