Is my sensor running high?
Put your Stelo, Dexcom or Libre numbers beside healthy people on the same kind of sensor, and take the sensor's bias out.
Your day above 140, beside healthy people
In 15 healthy adults, the sensor showed about 4 times as much time above 140 as fingertip blood did, and about 2 times once each person's fasting offset was removed (Hutchins 2025).
Your average
- On the sensor
- 108 mg/dL
- Likely in your blood
- 81 to 103
- GMI
- 5.9%
- Healthy average
- 98 mg/dL
GMI was built from people with diabetes, and a sensor runs high. Healthy people's sensors average about 98 mg/dL, which gives a GMI of 5.7, the first number of the prediabetes range. Trust the lab A1c. In Shah 2019 the average was 98 to 99 overall and 104 over age 60.
Next step. A sensor can't diagnose. If your average or your time above 140 sits well above both healthy groups, book a lab A1c and fasting glucose. Otherwise, use it to compare your own meals with each other.
How it works
A sensor measures the fluid under your skin, not your blood. Glucose reaches that fluid 5 to 6 minutes after it reaches your blood, and in one trial of 15 healthy adults a FreeStyle Libre 2 ran about 16 mg/dL above fingertip blood. It showed about 4 times as much time above 140 as the fingertip did.
The healthy groups are two studies of people without diabetes on blinded sensors. In 153 healthy, non-obese people aged 7 to 80, the average was 98 to 99 mg/dL, 96% of the day sat between 70 and 140, and about 30 minutes a day went above 140. In 560 Framingham adults with normal tests, 87% of the day sat in 70 to 140 and about 3 hours went above 140.
The 70% time in range on many reports is the ADA's goal for many adults with diabetes, counted in 70 to 180. It isn't a goal for people without diabetes. GMI turns your average into an A1c-like number with a formula built from people with diabetes, so a healthy 98 comes out at 5.7.
A sensor can't diagnose anything. A lab A1c and fasting glucose decide where you stand.
6 sources
- Hutchins and others, a FreeStyle Libre 2 against fingertip blood in 15 healthy adults, American Journal of Clinical Nutrition, 2025pmc.ncbi.nlm.nih.gov
- Shah VN and others, Continuous glucose monitoring profiles in healthy nondiabetic participants, Journal of Clinical Endocrinology and Metabolism, 2019pmc.ncbi.nlm.nih.gov
- Spartano and others, blinded sensors in the Framingham Heart Study, Journal of Clinical Endocrinology and Metabolism, 2025pubmed.ncbi.nlm.nih.gov
- Bergenstal RM and others, Glucose Management Indicator (GMI), Diabetes Care, 2018pmc.ncbi.nlm.nih.gov
- Basu A and others, Time lag of glucose from intravascular to interstitial compartment in humans, Diabetes, 2013pmc.ncbi.nlm.nih.gov
- American Diabetes Association, Standards of Care in Diabetes 2026, section 6, Glycemic Goalspmc.ncbi.nlm.nih.gov
