As much as we wish it wasn’t true, there can be times when the reading you get from your glucose sensor is different from the one on your finger stick. While it could be that your sensor is no longer working, there are several other reasons why there could be a difference.
Sensors and fingersticks measure different things.
The sensor in your continuous glucose or flash monitor is taking readings from what is known as your interstitial fluid. This is the fluid that surrounds the cells of your tissue just below your skin.
A fingerstick is measuring the glucose in your blood. Glucose moves from your blood vessels and capillaries first and then goes into your interstitial fluid. This explains why there can sometimes be a lag between fingerstick results and sensor results.
Medtronic explains it best by asking you to think of it like a rollercoaster. The front car is the blood glucose reading from a fingerstick and the car in the back is the reading from your glucose sensor.
How much of a lag is there between sensors and fingerstick readings?
The amount of lag time between results can depend on a lot of things including
- Location of sensor
- Brand of sensor being used
- If blood glucose is dropping
- If blood glucose is rising
- The individual
Are readings ever the same?
Your sensor and fingerstick will not always be off. For optimal results, try to calibrate your sensor when your blood glucose are more stable. Calibration during rapid swings such as following a meal or insulin dose, can cause accuracy problems.
Why use a sensor?
One of the biggest advantages of a glucose sensor is that they are designed to show you trends. A fingerstick alone cannot tell you if your blood glucose levels are rising or falling without follow up checks. A continuous glucose or flash monitoring systems are built to help you track patterns and visualize rises and drops in blood sugar levels. The algorithms used in these systems attempt to smooth out the differences between the interstitial glucose values and what the expected blood glucose reading would be.
When not to calibrate your sensor
As we have said, there are some times when it is less than optimal to try to calibrate your sensor. You should always follow the guidelines of your diabetes clinic and the manufacturer but you may also want to avoid calibration
- Immediately after dosing insulin
- After eating a meal
- After exercising
You can read more great tips on getting the most out of your sensor in this diatribe article.
When in doubt, use a fingerstick
No matter what system you are using, if you feel that the sensor is not accurate fall back on a fingerstick. If you aren’t 100% sure of the fingerstick, thoroughly clean your hands, make sure that they are dried properly, grab a fresh test strip, lance your finger and try again.
Download our free CGM comparison.
Compare CGM Features
Download the free PDF to compare Continuous Glucose Monitors
Why was my Dexcom6 always 40 points higher than fingersticks? I check my blood sugar every 3 hours. (Before eating and before insulin). Over a 20 day period (2 sensors) approximately 150 tests, only 2 had less than plus 40 discrepancies! I am very slender (22BMI) and I think the sensor canula is going into the muscle because the surrounding muscle gets very sore! Could this be the problem?
I reached out to someone at Dexcom and first, they ask that you report this issue to Dexcom technical support. This allows them to check into things on a deeper level. My contact also noted that “being lean and having the sensor possibly touch/close to the muscle wall can make a difference with results for sure physiology plays a part here. Equally problematic can be the meter as well. 40 points (2.2 mmol/L) is strange, especially with the fact this has been going on for a while.” Again, please contact contact Technical Service and ask for escalations, they may be able to help out as this is quite complicated as there are many variables. I hope that this helps you!