As much as we wish it wasn’t true, there can be times when the sensor reading from your continuous glucose monitor can be different from your finger stick. While it could be that your sensor is no longer working, there are also several other reasons why there could be a difference. Here are just a few other possibilities.
Sensors and fingersticks measure different things.
It is important to remember that your sensor 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?
Yes, your fingerstick and glucose sensor can be the same or exceptionally close in readings.
For optimal results, if your sensor requires calibration, make sure to do this when your blood glucose are relatively stable. Calibration during rapid swings such as following a meal or an 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 multiple follow up checks.
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.
Ideally this means that you can plan ahead to avoid blood sugars spiking or plummeting and therefore spend more time in range.
Learn more about time in range >>>
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.
If you are finding that your sensor is not accurate, contact the manufacturer of your glucose sensor. Manufacturers have staff who are trained to help you troubleshoot the problem. In some cases they may also send you new sensors if they feel that the one you are using is faulty for some reason.
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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!
My Lebre 2 is usually 50 points different than my fingers sticks.
So is mine, now I know why
My son’s libre 2 is as much as 150 points lower than his finger stick and if had believed it he would have been in perfect range…instead the finger stick put him at 270
I’m curious so what are you treating the libra2 or finger stick. Trying to help my mom to figure out since she’s on a sliding scale lantus.. Trying to not bottom her out.. please advise. Thank you in advance.
Educators always say to use the fingerstick readings. Remember that there can be a lag, so when in doubt, go with the fingerstick.
This is what we are finding and not sure we can trust it to properly treat my spouse. The doctor says to stick to one testing method and wait for the A1C to be updated in 3 months.
Why is my dexcom reading 46 and finger stick 90?
Susan is this something that is consistent? I would try to recalibrate the sensor and also contact Dexcom support about the issue.
How do you change the Dawn phenomenon? My sensor shows a huge rise at 4 am, low numbers at bed.
I am so sorry Monica! I am not sure how this was overlooked. The Dawn Phenomenon is a huge battle for a lot of people. There are a variety of different tactics including exercise, changing what you have for breakfast or tweaking insulin rates. This is something that you should definitely speak with your diabetes team about. They should be able to offer you a variety of options.
Thank you for writing this article! I had no idea why my libre2 is almost always between 20-60 off from my finger tests.
I have idiopathic gastroparesis and my readings are all over the place. Is there anything that I can do to improve my blood sugar levels?
That is a tough one Christy. In reading a bit about this, it can be a huge challenge and there isn’t a lot of information out there. I did find this document which looks pretty helpful. Everything seems to point to eating smaller meals more often and working with your diabetes team to match your insulin to your gastric emptying.
I’ve only had my first Freestyle Libre sensor on for 5 days. So far, every single day the alarm has gone off telling me my glucose has dropped below 70. It scares the bejesus out of you when it goes off. I run and do a finger stick and the results couldn’t be more different. Sensor says 69, finger stick says 100 to 111. This is a huge difference. The support desk couldn’t really offer an explanation or help — saying call back if it keeps happening or call your doctor. Ugh….. I really do love this thing but I won’t keep wearing it if the alarm goes off every day 2 hours after lunch. They went through the normal check list with me and all is being done correctly.
Have they offered to give you a replacement for them? Some people do seem to have issues while others have them work perfectly.
My husband’s was exactly the same with the replacement one they sent. Abbott told us it’s perfectly normal to have a 20% error on the sensors but we are getting a 40-50% error.
That is a huge error. I have heard of using these devices to watch for trends rather than actual readings, but that is terribly disappointing…and scary!
I have a freestyle libre 2 and I get the same results. I don’t know which to trust anymore as my meter is older. The company replaced a sensor once when I called and told them about the issue but since it is an ongoing issue, I don’t think every sensor is flawed. I don’t know what to think or do anymore.
Are you able to get a newer meter through your pharmacy or your diabetes center? When in doubt, they always say to trust your glucometer.
I too am on day 5 of my very first Libre2 sensor and have been getting mixed readings. I have done a lot of research and understand the potential differences in readings between the libre and my finger sticks. Mine usually read much higher, around 40mg/dl more than my finger sticks but on my triple digit readings. My lower readings can be off 5-20 mg, always higher and never consistent. I get high alarms in the morning saying I’m 181 (180 alarm set) yet my finger stick will be around 140. This happens every morning when my liver is releasing glucose. My readings NEVER get as high as the libre readings, so it’s not a matter of lagging behind real numbers. These readings are definitely where I may want to dose insulin, skip a meal, change my exercise routine, wake up earlier or go to bed later, CONSTANTLY, die to the higher readings! It’s not a good sense of trust for this new user! I hate to think I have 3 weeks of this to endure before trying a new sensor. I can’t afford to waste them. I’m a bit bummed.
That is horrible. Have you tried contacting Abbott to see if they can offer any help?
My sensor reads 86 but when I prick my finger it read 201.
You would definitely want to recalibrate the sensor. If it continues to be off like that, contact the manufacturer for assistance.
I have put a new sensor on and the number are different i tested it on the libra and it was 3.9 and tested it on my finger pricker and it was 4.9 is that bad
I appreciate the above mentioned comments concerning the differences in gathering data regarding numbers received from finger sticking vs using the sensor. I am a new user of Freestyle Libre 2 and was concerned over the difference in glucose readings.
Ugh, I also have readings that are super off. My fasting (i.e., should be stable) CGM readings are 12-20 pts below my finger readings… consistently. If my CGM readings were correct, I’d be in good shape… but the finger stick readings are telling me I’m in need of insulin. It’s super frustrating, and Abbot was horrible on the phone–condescending, kept telling me that a lag was normal–even after I kept telling him that even accounting for the lag, the CGM readings were never peaking even close to the finger stick readings. He also kept telling me that ‘both readings were right’ within margin of error (20 points is well above their margin of error), that he was not a doctor and to talk to my doctor, and my tentative request for a better calibrated sensor resulted in him hanging up on me. So… not sure what to do with the rather-expensive CGMs with readings that make no sense.