By Christine Craig, MS, RD, CDCES

As more and more people are using glucose sensors as a tool to improve their blood sugars, they may feel frustrated or confused about the mismatch between their sensor and glucose meter readings. Some many even ditch this new fangled sensor, thinking that their old friend, the meter, provides more reliable information.

We invite you to download  CGM Interfering Substances Cheat Sheet. This CGM Troubleshooting Cheat Sheet that provides specific information on different interfering substances and other factors that can affect the accuracy of continuous glucose monitor (CGM).

We often hear that BG does not equal SG, but what does this mean? 

A blood glucose meter measures capillary blood glucose, and CGM sensors measure interstitial fluid, a thin layer of fluid surrounding the cells just below the skin. These values correlate well, but they measure two different types of fluids.

This difference in BG vs. SG at one point in time can be alarming at first and is a commonly reported concern related to sensor accuracy. Education about the difference in measure and understanding that this difference is most significant during rates of change (a rise or fall in glucose values) can help address this concern. Blood glucose values change first, followed by sensor glucose.

The lag time between BG and interstitial glucose (what the sensor measures) can be from 2 to 20 minutes.

We may notice this difference most during increased rates of change (such as >2 mg/dl per min) and during circumstances such as after a meal, after treating a low glucose event, during and immediately after exercise, or after taking diabetes medications that have a more rapid effect on glucose such as meal/correction insulin or sulphonylurea.¹

Educating about the difference between BG and SG and how to interpret glucose trends, sensor glucose trend arrows, and monitoring change over time can improve understanding.  

Interfering Substances

The 2024 ADA Standards of Care recommends that for ALL people who wear CGM devices, education and assessment of potential interfering substances occur. Many of these interfering substances can be a common over-the-counter supplement or pain reliever. The effect for all reported interfering substances is a false elevation in sensor glucose versus actual blood glucose values. ⁴ It is cautioned that this could cause missing a severe glucose event.

Each manufacturer has different identified interfering substances.

• For the Libre 2 and 3 systems, more than 500 mg/day of supplemental Vitamin C can interfere. More than 1000 mg/day can interfere with the Libre 2 Plus. ⁵
• Dexcom G4, G5, G6 ⁶, and G7 ⁷ are all affected by hydroxyurea and acetaminophen,
• However for Dexcom G6 and G7 devices acetaminophen dose is specified as greater than 4 grams per day or 1 gram every 6 hours.
• Medtronic Guardian 3 and Guardian 4 devices are affected by hydroxyurea and any acetaminophen dose⁸
• For more information, refer to our cheat sheet: CGM Interfering Substances.

Studies and reports of interfering substances are not comprehensive, so it is recommended that if suspicion occurs that a medication or supplement is contributing to erroneous CGM values, they should contact the manufacturer’s technical support team.⁹

Calibration

For sensors requiring calibration, poor quality of the blood glucose test can also create inaccuracies with the glucose sensor readings. Most manufacturers with CGM calibration requirements recommend ensuring a “clean calibration,” having individuals wash their hands, taking the second drop of blood when hand washing is unavailable, and calibrating when glucose values are more stable, such as before a meal, insulin, or exercise. Luckily, most devices no longer require calibration, but it is important to review technique when applicable. 

Getting Back to AW

AW reported no recent changes with medications, supplement intake, or fortified food sources, and no changes were noted at the sensor site. However, they did report that the CGM was on the last day of manufacturer recommended sensor wear. Sensor integrity variation based on day of sensor was determined to be the main consideration of cause. After changing to a new sensor, AW’s general range of BG to SB differences were observed. This event served as an important reminder to AW to check BG when SG values were not reading as expected and different than any symptoms.

Education When Starting Sensor Important

As educators, we can support individuals concerned with CGM accuracy by providing education about the differences between SG and BG values. Assessing individual factors such as medical conditions, movement and activity, site integrity and placement, issues of compression, and medication and supplement use (assessing for potential inferring substances). We can educate individuals to check BG anytime symptoms do not match SG readings and report any concern of a faulty sensor to the manufacturer. Reporting can often lead to the replacement of faulty sensor. 

Since the arrival of CGM, benefits including improved quality of life factors, A1c reduction, increased Time in Range, and prevention of hypoglycemia have been found.⁴  We know from working in diabetes care and through real-world studies the overall value of CGMs. Unlike BG, CGMs provide a complete picture showing trends for direction, personalized alerts, and supporting pattern recognition. 

Although not ideal for all, growth of users will continue to rise with approval of over-the-counter product options and understanding accuracy issues is essential to ensure safety and support user experience.

For more information, refer to our cheat sheet: CGM Interfering Substances.