Continuous glucose monitoring (CGM) is a technology providing a constant stream of glucose information for individuals with diabetes, and its application is expanding for people managing type 2 diabetes. A CGM is a wearable device that tracks glucose levels throughout the day and night, offering a detailed view of how diet, exercise, and medication affect the body. This provides a dynamic picture of glucose fluctuations, moving beyond the single-moment data from traditional finger-prick tests.
How Continuous Glucose Monitoring Works
A continuous glucose monitoring system has three primary components that work together. The first is the sensor, a tiny filament inserted just beneath the skin, typically on the arm or abdomen. This sensor does not measure glucose directly from the blood; instead, it measures the glucose concentration in the interstitial fluid that surrounds the cells in your tissues.
The second component is the transmitter, which attaches to the sensor and wirelessly sends the glucose information every few minutes. The data is then sent to the third component, a display device. This can be a dedicated receiver, a smartphone app, or an integrated insulin pump, which shows the current glucose reading and a graph of recent trends.
Because a CGM measures glucose in the interstitial fluid rather than the blood, there is a natural delay in the readings. Glucose first enters the bloodstream and then moves into the interstitial fluid, creating a lag time of about 5 to 15 minutes between a blood glucose meter reading and a CGM report. This delay is an important factor in understanding the data, especially when glucose levels are changing quickly.
Utilizing CGM Data for Diabetes Management
The continuous flow of information from a CGM allows for a more proactive approach to diabetes management. A powerful metric from CGM is “Time in Range” (TIR), which is the percentage of time a person spends within their target glucose range. For most adults with type 2 diabetes, the recommended target range is 70 to 180 mg/dL, with a goal to spend at least 70% of the day within this range.
Achieving a higher TIR is associated with a lower risk of developing certain long-term diabetes complications. CGM reports often display TIR as a color-coded bar graph, showing the percentage of time spent in low, in-range, and high glucose zones. A common goal is to spend less than 4% of the time below range (<70 mg/dL) and less than 25% of the time above range (>180 mg/dL).
CGM technology also uses trend arrows, which appear on the display screen next to the glucose number. These arrows indicate both the direction and the speed at which glucose levels are changing. A horizontal arrow signifies stable glucose, while arrows pointing up or down indicate rising or falling levels, allowing a user to anticipate and prevent high or low glucose events.
This information helps users make timely decisions. For example, if a user sees their glucose is 130 mg/dL with an arrow pointing up after a meal, they might take a short walk to help manage the rise. If their reading is 110 mg/dL with two arrows pointing straight down, they could consume a small carbohydrate snack to prevent a hypoglycemic event.
Eligibility and Access for Type 2 Diabetes
A prescription from a healthcare provider is required to obtain a continuous glucose monitor. For individuals with type 2 diabetes, access is determined by insurance coverage, which has specific eligibility criteria that can vary significantly between companies and plans.
Medicare and private insurance plans have established guidelines for CGM coverage. Coverage is often available for people with diabetes who use any type of insulin or have a history of problematic hypoglycemia. Problematic hypoglycemia is defined as recurring low blood sugar events or a severe event requiring assistance from another person.
Coverage policies are expanding, but individuals not on insulin may face more hurdles. The process frequently involves a “prior authorization,” where the doctor must justify the medical necessity to the insurance company. If not covered, out-of-pocket costs can be a factor, though manufacturer discount programs may be available.
Practical Considerations and Limitations
While CGM technology offers benefits, it also has practical limitations. The sensor’s adhesive can cause skin irritation or allergic reactions for some users, and the sensor must be replaced regularly, typically every 7 to 15 days depending on the model.
Alarm fatigue is another issue. CGMs can be set to sound an alarm when glucose levels are projected to go too high or too low. Frequent or inaccurate alerts can become overwhelming, leading some users to ignore them, which defeats a main safety feature of the device.
The lag time between blood and interstitial fluid readings is an important consideration. This delay means the CGM may not reflect the most current blood glucose level during rapid changes, such as after meals or exercise. For this reason, a finger-prick test may be needed to confirm a reading before making a treatment decision.