• Posted March 4, 2025

Continuous Glucose Monitors Can Overestimate Blood Sugar Levels, Study Finds

Continuous blood glucose monitors have been promoted as potentially life-changing for people with diabetes -- allowing real-time updates on blood sugar levels without the need for repeated finger pricks.

But a new small-scale study suggests these devices might not be as accurate as many believe, and could lead some to mismanage their diets.

Continuous glucose monitors (CGMs) appear to overestimate blood sugar levels in healthy adults, according to findings published recently in the American Journal of Clinical Nutrition.

The monitors consistently reported elevated blood sugar levels two to four times more often than finger prick tests, which are the gold standard for blood glucose testing, researchers report.

“CGMs are fantastic tools for people with diabetes because even if a measurement isn’t perfectly accurate, it's still better than not having a measurement at all. However, for someone with good glucose control they can be misleading based on their current performance,” senior researcher Javier Gonzalez, a professor of health with the University of Bath in the U.K., said in a news release.

“For healthy individuals, relying on CGMs could lead to unnecessary food restrictions or poor dietary choices,” he added. “If you want to assess your blood sugar accurately, traditional methods are still the way to go."

Continuous glucose monitors were originally designed to help diabetics manage their blood sugar.

However, the devices now are being used by health-conscious folks to help them track how different foods affect blood sugar levels, researchers said in background notes.

The monitors track blood sugar levels via a patch slapped onto a person’s arm, and transmit the data to a smartphone, smart watch or other wearable device.

For the study, researchers tracked blood sugar levels in 15 healthy volunteers after they drank or ate various fruit-based products, ranging from whole fruit to fruit smoothies.

CGMs overestimated the glycemic index hit from a fruit smoothie by 30%, reporting a medium GI of 69 compared to the low GI of 53 shown in a finger prick test, results show.

Whole fruits also tended to be misclassified as medium- or high-GI foods by the monitors, while finger prick testing showed they were low GI.

This could lead people to avoid whole fruits, in the mistaken belief that they might cause spikes in blood sugar, researchers said.

Overall, monitors overestimated people’s time spent above healthy blood sugar levels by nearly 400%, results show.

Even after adjusting for other factors, the monitors still overestimated time above healthy blood sugar levels by about two-fold, researchers said.

"CGMs may be inaccurate because they measure glucose in the fluid surrounding your cells, not directly in your blood,” Gonzalez concluded. “This can lead to discrepancies due to factors like time delays, blood flow, and how glucose moves between different parts of the body.”

The research was funded by innocent drinks, a U.K.-based maker of smoothies.

More information

The American Diabetes Association has more on continuous glucose monitors.

SOURCES: University of Bath, news release, Feb. 26, 2025; American Journal of Clinical Nutrition, Feb. 26, 2025