How accurate are continuous glucose monitors? A study puts them to the test

By Dr. Liji Thomas, MDReviewed by Benedette Cuffari, M.Sc.Mar 4 2025

New research compares CGM and capillary blood glucose measurements, revealing key discrepancies in tracking glycemic responses to food.

Study: Continuous glucose monitor overestimates glycemia, with the magnitude of bias varying by postprandial test and individual – A randomized crossover trial. Image Credit: goffkein.pro / Shutterstock.com

In a recent study published in The American Journal of Clinical Nutrition, researchers determine the accuracy of continuous glucose monitors (CGM) by comparing blood sugar levels in response to test foods and drinks using CGM and capillary blood glucose measurements (CBGMs).

The role of diet in insulin resistance

After consuming a meal, a rise in blood sugar levels is normalized by the body’s homeostatic regulation. Insulin, a hormone secreted by the pancreatic beta cells, is a crucial factor involved in this response to rising blood sugar levels.

Small but sustained elevations in blood glucose concentrations reflect reduced beta cell function. These effects, which are observed within a matter of days, are mediated by insulin resistance, which prevents beta cells from responding to the blood glucose lowering effect of insulin.

Insulin resistance can be managed by monitoring daily caloric intake and reducing the consumption of foods that may increase blood glucose levels.

How are blood glucose levels measured?

Foods are often categorized by their glycemic index (GI), which describes the duration for which post-meal blood glucose levels remain elevated after their consumption, as compared to 100% glucose or any suitable reference meal. CBGMs are the gold standard for measuring GI, as these values are often more sensitive and consistent as compared to venous blood glucose levels.

CGM is actively promoted as an aid to controlling blood glucose levels among both diabetics and health-conscious non-diabetic individuals. Otherwise healthy individuals often utilize CGM to identify post-meal increases in blood glucose levels and modify their food intake accordingly.

CGM values are based on interstitial fluid and, as a result, are derived values, rather than directly measured. Other factors that can limit the accuracy of both CGM and CBGM values include inter-individual differences in the uptake and absorption of glucose after consuming a meal, blood flow differences, and the type of food consumed.

Despite these differences, CGM values are considered comparable with static venous glucose measurements or CBGM obtained throughout the day. To date, few studies have compared the reliability of CGM and CBGM for glycemic responses.

About the study

The current study compared blood sugar levels measured by both CGM and capillary blood sampling after a total of 15 study participants consumed various test foods and drinks. Each study cohort consumed seven oral carbohydrate test meals, following which blood sugar levels were measured every 15 minutes for a total duration of 120 minutes. The GI of the food product was also calculated with reference to the control meal.

Possible food products included the control meal consisting of 50 grams (g) glucose, or whole fruits, blended fruits, commercial fruit smoothie consumed immediately or over a period of 30 minutes, commercial smoothie with five g of inulin to increased fiber content, or commercial smoothie with 30 g carbohydrates. Two days were provided between successive tests to prevent overlap effects.

Glucose concentrations by CGM and CBGM were converted into incremental area under the curve (iAUC) values.

Study findings

CGM produced higher fasting and post-meal glucose measurements than CBGMs by an average of 0.9 mmol/L. Blood glucose levels measured by CGM remained high four-fold longer than CBGMs.

Even after adjusting for baseline differences, CGM produced values that were two-fold higher than CBGM values. The time to peak glucose concentration was delayed by five to 12 minutes with CGM as compared to CBGMs.

The type of food affected the magnitude of variation between the methods. With either method, the highest glucose iAUC occurred with the 50 g glucose meal, followed by the 30-minute smoothie.

With CBGM, the next highest iAUC was the blended smoothie, commercial fruit smoothie, and 50 g whole fruit. The lowest iAUC was for the 30 g carbohydrate smoothie, followed by the fiber-enriched smoothie.

The third highest iAUC measured by CGM was for the commercial smoothie, followed by the fiber-enriched and blended fruit smoothie. The lowest value was obtained for the whole fruit, followed by the 30 g carbohydrate smoothie.

Mean GI values for the commercial fruit smoothie were higher with CGM than CBGMs at 69 and 53 mmol/L, respectively, thus making it a high-GI product, rather than moderate-GI as indicated by CBGM values.

These fluctuations indicate that CGM is not highly reliable for determining GI values. Variations in GI values with the type of food suggests that a common correction formula cannot be applied to ensure the accuracy of this method.

Differences in the physical form of the food, type of food, and the way it is eaten may lead to changes in glucose absorption and uptake by cells. Interindividual differences also affected CGM measurement variation, despite adjusting for fasting glucose measurements, glucose tolerance, or body mass index.

Conclusions

CGM is currently marketed as an over-the-counter aid for monitoring GI and blood glucose responses in healthy people. The study findings demonstrate that CGM consistently overestimates both fasting and post-meal blood sugar levels, while also inflating the duration of time spent outside the physiological range of blood glucose levels in healthy people.

Although CBGMs appear to be more accurate in estimating post-meal blood glucose levels, future studies are needed to compare the precision of different models and brands of CGM devices.