Dexcom G7 - Accuracy and Calibration
This page is not giving you medical advice or treatment advice, and that is very much on purpose! I am not a medical professional, and even if I were, I don’t know your personal circumstances. If you need medical advice, consult your physician! If, at any time, you have reason to doubt your G7 readings, use one or more finger-stick measurements to confirm your G7 readings. Base your treatment decisions (bolus, etc.) on the finger stick reading, and in accordance with your physician’s instructions.
For users with an insulin pump controlled by the G7: Until you have reached a full understanding of your G7’s behavior and when, in your particular case, its readings can be trusted, you may need to double-check, as frequently as necessary to give you confidence, with finger stick readings. Read the page on reading comparisons for important information on interpreting the differences.
Pages
- Introduction
- How it works
- Accuracy and Calibration
- Comparison with finger stick readings
- My Sensor Failed
- Known Issues
- Contacting Dexcom
Dexcom G7 - Accuracy and Calibration
Published accuracy figures for the 10-day G7 device (the 15-day version is reported to have similar accuracy characteristics) as follows1:
- MARD: Mean Absolute Relative Difference (lower is better). It indicates that if the glucose level in ISF and blood are equal, you can expect the difference in measurement between blood glucose (using very accurate lab equipment) and ISF glucose levels (G7) to be, on average, within a certain percentage of the lab-determined blood glucose level (depending on placement):
- For arm placement: 8.2%
- For abdomen placement: 9.1%
- Agreement Rates. This indicates how often G7 readings will be within a certain range of the true blood glucose measurement. A “%15/15” agreement means that the reading is correct within 15% for true glucose > 100 mg/dL, or ≤ 15 mg/dL for true levels ≤ 100 mg/dL. The table below illustrates this for the cited agreements, at three different true glucose levels.
| Agreement | Max ∆ ≤ 100 | Max ∆ @ 150 | Max ∆ @ 250 | Arm | Abdomen |
|---|---|---|---|---|---|
| 15%/15 | 15.0 | 22.5 | 37.5 | 89.6% | 85.5% |
| 20%/20 | 20.0 | 30.0 | 50.0 | 95.3% | 93.2% |
| 30%/30 | 30.0 | 45.0 | 75.0 | 98.8% | 98.1% |
Table: Agreement levels, allowable differences (∆), and agreement rates
The table and data show that for arm placement, about 90% of the time readings will be within 15%/15 of true, and for 30%/30 agreement it is almost always the case.
Most of us don’t have lab equipment to compare with; we use finger stick measurements. When considering this, it is important to recognize that, unlike lab equipment, fingerstick measurements are not as accurate. The FDA requires these devices to be accurate within ±15% of laboratory values 95% of the time (ISO 15197 guidelines), and within ±20% 99% of the time. You can read more about using fingerstick measurements for comparison.
Factors affecting G7 accuracy:
- ISF vs. blood glucose levels time-lag. The lag can be largest right after eating or intense exercise.
- Sensor location: The upper arm is the recommended, most accurate, and FDA-cleared site, though some users report success elsewhere. Some users only report success elsewhere. The thickness of the skin, subcutaneous fat content, etc., may all be factors.
- Environment/physical factors can cause inaccurate or false readings:
- Dehydration “thickens” the blood and makes the glucose transfer to ISF more difficult. In addition, it may dilate the blood vessels, leading to low blood pressure.
- Low blood pressure affects the glucose transfer as well. If blood vessels are generally dilated, the permeability of the vessel walls is affected.
- Extreme temperatures can cause blood vessel dilation or contraction
- First day: Generally, the G7 is expected to be accurate from the start, but anecdotal evidence suggests the first 24-hour period may see less accurate and consistent results.
- Pressure lows: If pressure is applied to the sensor, the underlying tissue is squeezed, which may cause temporary evacuation of a portion of ISF, and or hinder the glucose transfer. This can cause artificially low readings. Depending on sensor placement and sleeping habits, this happens to some during sleep. Other “pressure” incidents at other times are usually too brief to make any difference.
- Certain medications in sufficiently high doses
Factors affecting finger stick accuracy:
- Technical issue, such as:
- Insufficient blood on the strip
- Strip coding errors (some older devices need coded strips)
- Expired or improperly stored strips can affect accuracy
- Hands containing sugar residue from food (wash your hands)
- Too much “milking” (squeezing too hard to form a droplet)
- Environmental factors such as extreme temperatures or high altitude
- Steroid use can cause spikes
- Critically ill patients often have sufficiently altered physiological conditions, causing higher error margins
Calibration with finger stick readings
The G7 application allows the user to enter finger-stick readings and indicate that they are to be used to calibrate the sensor results, rather than simply being recorded. Based on all the above, it should be clear that doing this while glucose levels have not been stable for at least 15 minutes is highly prone to problems. Ideally, you would know you are stable both by looking at the G7 data (graph and arrow) and at least two finger stick readings.
Assuming you are stable, you can enter the last finger-stick reading as soon as it is taken and mark it for calibration. The application should respond with “OK,” but it is known that at times it does not accept the calibration.
Dexcom does not say why or how to prevent this, but anecdotal evidence suggests that you have the highest likelihood of success when:
- You have been stable
- The difference between finger stick and G7 is less than 50. If the difference is more than that, enter a value that corresponds to 50, wait, and repeat
- You have not, very recently, already done a calibration. I would suggest at least 1 hr between attempts
In particular, during the first 24 hours after warm-up, many users experience wild fluctuations, making it unlikely you have a stable enough period to calibrate. Some users, like myself, seem to fare better earlier in the 24-hour period and can therefore calibrate sooner.