By now, you’ve probably come across commercials for the FreeStyle Libre Flash Glucose Monitoring System… “you can do it without lancets”. For those who dread finger pricks or want more frequent feedback data, the FreeStyle Libre, which has been available in Canada since 2017, has been a complete game-changer.
But when it comes to using the FreeStyle Libre for blood sugar monitoring, the more accurate slogan would be “you can do it without lancets...most of the time”.
It’s not uncommon to meet clients who were not aware their Libre reader was built to use finger prick test strips as well! In fact, the Libre reader has a built-in meter for finger-prick blood sugar testing (just like your regular glucometer) and blood ketone testing. The compatible test strips are called FreeStyle Precision test strips.
Why do I still need to prick my finger?
Because there is a time lag between sensor glucose and blood glucose (sugar). Rather than testing blood sugar directly, the FreeStyle Libre sensor monitors glucose (sugar) in the interstitial fluid (ISF) - that’s the fluid surrounding your cells. Glucose in the blood diffuses into the ISF, allowing us to get a pretty good estimate of blood sugar via the sensor. However, sensor readings will not match exactly with your finger-prick blood sugar because there is a lag time of about 5 - 10 minutes.
During times of the day when blood sugar is fairly stable - you can expect sensor readings to be pretty similar to blood sugar readings. However, when blood sugar is changing fast (e.g. after eating, after insulin injection, after exercise), you will likely notice a larger gap between sensor versus finger prick readings.
If blood sugar is dropping fast - sensor readings will be higher than finger prick readings. If blood sugar is rising fast - sensor readings will be lower than finger prick readings.
Across the day, this still provides a good picture of blood sugar trends, and the lag should not impact most routine treatment decisions. However, there are times when finger pricking is recommended.
When do I need to finger prick?
You should confirm blood sugar with a finger prick blood sugar test when:
Your meter displays the “Check Blood Glucose” symbol.
You see a hypoglycemic (low blood sugar) reading (<4 mmol/L). Confirm with a finger-prick blood sugar test.
You are experiencing symptoms that do not match sensor readings.
You need to check blood sugar during the 60 minutes warm-up period for new sensors.
You are using the Libre insulin calculator. Unfortunately, the built-in mealtime insulin dose calculator can only be used with finger prick testing.
Flash Glucose Monitoring. Abbott. https://freestylediabetes.co.uk/freestyle-libre/flash-glucose-monitoring-temp#:~:text=The%20average%20lag%20time%20between,readings%20may%20be%20very%20similar. Accessed May 31, 2021.
FreeStyle Libre Flash Glucose Monitoring System User’s Manual. Abbott. https://freestyleserver.com/Payloads/IFU/2017_dec/ART34745-107_rev-A-WEB.pdf. Published Dec 2017. Accessed May 31, 2021.
FreeStyle Libre FAQ’s. Abbott. https://www.freestylelibre.com.au/faq. Accessed May 31, 2021.
The FreeStyle Libre System. Abbott. https://www.freestyle.abbott/ca-en/home/freestyle-libre.html. Accessed May 31, 2021.
Summary Basis of Decision - Freestyle Libre Flash Glucose Monitoring System. Health Canada. https://hpr-rps.hres.ca/reg-content/summary-basis-decision-medical-device-detailThree.php?linkID=SBD00477. Updated Nov 13, 2020.