Why BGM and CGM readings are sometimes different
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
Calibrating vs. comparing
Calibrating your CGM system using your BGM reading is optional, but it can help align your CGM values with your BGM values. Calibrating is when you take a fingerstick and enter that BGM reading into your Dexcom G7 app as a calibration. Comparing is when you take a fingerstick and use the BGM reading for for awareness. The G7 App also allows you to log that BGM reading without using it as a calibration for tracking purposes.
Calibrating your CGM
Calibrating your Dexcom G7 is optional. Because it is the most accurate CGM system available,1 there is no need to constantly calibrate it. It can be helpful to calibrate if your Dexcom G7 reading is outside the ranges shown in the second column below for several hours. Don’t calibrate if the Dexcom G7 reading is off due to added pressure on the sensor; relieve the pressure to allow the reading to return to normal. When calibrating, make sure to:
- Wash your hands with soap and water and dry them
- Enter the calibration within 5 minutes of taking a fingerstick
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.
BG Meter Value (mg/dL) | Dexcom G7 Reading (mg/dL) |
40 | 20-60 |
50 | 30-70 |
60 | 40-80 |
70 | 56-84 |
80 | 64-96 |
90 | 72-108 |
100 | 80-120 |
110 | 88-132 |
120 | 96-144 |
130 | 104-156 |
140 | 112-168 |
150 | 120-180 |
160 | 128-192 |
BG Meter Value (mg/dL) | Dexcom G7 Reading (mg/dL) |
170 | 136-204 |
180 | 144-216 |
190 | 152-228 |
200 | 160-240 |
210 | 168-252 |
220 | 176-264 |
230 | 184-276 |
240 | 192-288 |
250 | 200-300 |
260 | 208-312 |
270 | 216-324 |
280 | 224-336 |
290 | 232-348 |
Bg Meter Value (mg/dL) | Dexcom G7 Reading (mg/dL) |
300 | 240-360 |
310 | 248-372 |
320 | 256-384 |
330 | 264-396 |
340 | 272-408 |
350 | 280-420 |
360 | 288-432 |
370 | 296-444 |
380 | 304-456 |
390 | 312-468 |
400 | 320-480 |
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.
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1 Dexcom, data on file, 2022.
BRIEF SAFETY STATEMENT: Failure to use the Dexcom Continuous Glucose Monitoring System and its components according to the instructions for use provided with your device and available at https://www.dexcom.com/safety-information and to properly consider all indications, contraindications, warnings, precautions, and cautions in those instructions for use may result in you missing a severe hypoglycemia (low blood glucose) or hyperglycemia (high blood glucose) occurrence and/or making a treatment decision that may result in injury. If your glucose alerts and readings from the Dexcom CGM do not match symptoms, use a blood glucose meter to make diabetes treatment decisions. Seek medical advice and attention when appropriate, including for any medical emergency.