Continuous Glucose Monitoring Overview

The HbA1c has been the gold standard for assessing level of glucose control for years . However, the HbA1c does not reveal anything about glycemic variability or incidence of hypoglycemia . HbA1c values can be affected by red blood cell (RBC) turnover, anemia, iron deficiency, genetic factors, liver disease and race¹ . The HbA1c value only offers an average blood glucose measurement over 2-3 months, which gives the health care provider limited information as to how to adjust the diabetes medications to best serve the person with diabetes.

To thoroughly assess glycemic patterns and make treatment decisions, it is important to know and understand glycemic variability. Glycemic variability is a measure of the extremes in glucose values over time. A larger amount of glycemic variability is associated with more frequent and severe hypoglycemia and may lead to both microvascular and macrovascular diabetes complications. ^2,3

To better assess glycemic variability and patterns, Continuous Glucose Monitoring has become more widely adopted and utilized in recent years . Continuous glucose monitors measure interstitial fluid glucose levels and display numerical and graphic data regarding current glucose status, along with current and projected trends in the glucose . The projected future glucose trend is depicted by directional arrows . This allows users to proactively respond to glucose data rather than waiting for hyper- or hypoglycemia to develop, therefore offering the ability to take preventive action to avoid hyper- or hypoglycemia . ^4,5 

Although not a new technology, CGM devices are constantly evolving and improving. Devices continue to improve in accuracy, reliability and convenience and insurance coverage is more widespread. Continuous glucose monitoring is considered a standard of care for people treated with intensive insulin programs.

Personal CGM vs. Professional CGM

Continuous glucose monitoring devices are either owned by the user for personal use or owned by the health care center for professional use. Some continuous glucose monitors link to other compatible devices, such as insulin pumps, blood glucose meters, smartphones, automated insulin-dosing systems and smart pens. Most CGM devices may be used as stand-alone devices.

Consumers who own personal CGM devices utilize the data in real time to make decisions about their diabetes management. The devices display trend arrows, indicating whether there is a pattern of rising or falling glucose to assist them in problem solving and self-management. Clinicians who offer Personal or Professional CGM analyze retrospective data, sometimes combined with a user diary, log or mobile app information to gain insights into glycemic patterns.

Terms to Know

• Sensor – A glucose sensor is the part of a continuous glucose monitoring (CGM) system that is inserted under the skin and measures your interstitial glucose levels.
• Interstitial glucose level - The glucose found in the fluid surrounding the cells in the tissue.
• Transmitter – A small, reusable or disposable transmitter connected to the sensor allows the system to send real-time glucose readings wirelessly to another device that displays your glucose data.
• Receiver or reader – The receiver (reader) or compatible smart device receives glucose data from the transmitter and displays current levels, historical trends in levels, and arrows to show direction that glucose is heading.
• Real-time CGM – This device automatically transmits glucose data to the receiver/smartphone.
• Intermittently-scanned CGM – This device requires the wearer to swipe the receiver/reader/smartphone over the sensor to obtain glucose data.
• Trend arrows – Trend arrows indicate the direction the glucose is heading and allows anticipatory changes to be made to prevent hyper/hypoglycemia.
• Calibration – Some CGM systems require fingerstick blood glucose (BG) meter readings in order to generate accurate sensor interstitial glucose readings. These BG meter readings are entered into the device and are used for scheduled calibrations or as needed. Calibrations with blood glucose readings are used to ensure that the glucose sensor maintains its accuracy over time. When systems are factory calibrated, fingerstick calibration is not recommended.
• Warm-up time – The amount of time it takes for the sensor to calibrate after it is placed under the skin, before the data is considered to be accurate. The warm-up time varies for different devices. During the warm-up time, the person with diabetes must check a fingerstick blood glucose for treatment decisions.
• Lag time – This refers to CGM sensor interstitial glucose readings lagging behind fingerstick blood glucose readings. This occurs because the interstitial fluid glucose that the CGM sensor measures tends to lag behind the fingerstick glucose that the blood glucose meter reads, especially when the glucose level is changing rapidly. The lag time can be up to 15 minutes but is typically less than that in current CGM systems due to algorithm adjustments.
• Adjunctive indication – A CGM that cannot be used to make treatment decisions. A stand-alone home blood glucose monitor result should be used to make treatment decisions in this case.
• Non-adjunctive indication – A CGM that can be used to make treatment decisions without the need for a stand-alone home blood glucose monitor to confirm blood glucose results. • Standalone device – A CGM that does not require integration with an insulin pump.
• iCGM – Per the U.S. FDA, “An integrated continuous glucose monitoring system (iCGM) is intended to automatically measure glucose in bodily fluids continuously or frequently for a specified period of time. iCGM systems are designed to reliably and securely transmit glucose measurement data to digitally connected devices, including automated insulin dosing systems, and are intended to be used alone or in conjunction with these digitally connected medical devices for the purpose of managing a disease or condition