Types of Continuous Glucose Monitoring (CGM) Devices for Diabetes and How to Use

Key takeaways:

• Continuous Glucose Monitoring (CGM) devices provide real-time blood glucose data for better diabetes management.
• There are different types of CGM devices, including real-time CGM (rtCGM), intermittently scanned CGM (isCGM), and the hybrid closed-loop system.
• Real-time CGMs offer continuous glucose levels and automatic alerts for high or low levels of blood sugar. Intermittent scanning CGMs require scanning the sensor to get glucose data.
• CGM systems consist of a small sensor placed under the skin, a transmitter, and a display device (receiver or smartphone app).
• Proper placement and calibration are essential for accurate readings of the CGM devices.

Blood glucose monitoring helps monitor or look for changes in blood sugar levels and how these levels respond to food, exercise, medications, and health conditions like diabetes. This is essential because extremely high or low blood sugar levels can cause serious health problems in the short and long term. 

Blood glucose monitoring is done using devices such as glucometers and continuous glucose monitoring (CGM) devices.

These devices are usually reliable and provide accurate results. However, their reliability may be affected by factors such as proper device placement, insufficient blood samples, environmental factors, etc.

Continuous Glucose Monitoring (CGM) devices are wearable systems that help monitor glucose levels in blood throughout the day and night. They have a small sensor that is placed under the skin.

They track glucose in the fluid between cells, providing uninterrupted data on sugar levels. CGMs are helpful in that they send alerts for high or low blood glucose readings. This helps users manage their glucose levels more effectively.

Different types of CGM

Continuous Glucose Monitoring (CGM) systems come in several types, each with unique features and usage methods for daily diabetes management. These include:

• Real-time CGM
• Intermittently scanned CGM (isCGM)

1. Real-Time CGM (rtCGM)

These systems continuously measure glucose levels and provide real-time data to a user or healthcare provider.

These systems track glucose levels in the tissue fluid and display real-time numbers and graphs. These show current glucose levels, trends, and arrows that indicate how fast and in which direction glucose changes.

How to use:

A small sensor is inserted under the skin (usually on the abdomen or arm) to measure glucose levels in the interstitial fluid. Interstitial fluid is the liquid that surrounds the cells and comes from substances that leak out of small blood vessels called capillaries.

After measuring the glucose level, the data is recorded and sent to a smartphone or insulin pump receiver in real-time.

Examples of real-time CGM devices:

• Dexcom G7
• Medtronic Enlite®

Accuracy:

• MARD (Mean Absolute Relative Difference- the average percentage difference between two sets of values) is typically between 8-10%, depending on the device.

Strengths:

• It provides continuous real-time data.
• It sends alerts for high and low glucose levels.
• It can be connected with insulin pumps.

Limitations:

• It requires regular sensor replacements.
• Calibration and fingerstick testing may be needed (for some models).
• It is an expensive device.

2. Intermittently scanned CGM (isCGM)

Intermittently scanned continuous glucose monitoring (isCGM) devices measure glucose levels continuously in interstitial fluid and store the results. Users need to scan the sensor to get glucose readings.

The sensor provides glucose readings for up to 14 days (10 days in the US) if it is scanned at least every 8 hours. If a scan is not performed, the device automatically retrieves and displays data from the previous 8 hours.

Unlike rtCGM, it is factory-calibrated and does not require daily fingerstick calibration. However, confirmatory blood glucose tests are sometimes required if low glucose is detected or glucose levels change rapidly.

How to Use isCGM:

The sensor is typically applied to the upper arm or abdomen. A small filament is inserted beneath the skin via a painless application device. The sensor is then scanned by a dedicated reader or a compatible smartphone app, which provides a real-time glucose reading and data from the last 8 hours.

Readings include current glucose levels, trends (e.g., rising or falling), and past data. Some devices have hypoglycemia (low blood glucose level) or hyperglycemia (high blood glucose level) alerts.

Examples of isCGM devices:

• Freestyle Libre 2 (Abbott)
• Freestyle Libre 3

Accuracy:

• isCGMs generally have a Mean Absolute Relative Difference (MARD) of 8-12%, and the accuracy can be affected by factors like dehydration, rapid glucose changes, and improper calibration (if required).

Strengths:

• No fingerstick calibration is required (for most models).
• Usually less expensive than rtCGMs.
• Data is only retrieved upon scanning, reducing information overload.
• Some models provide optional alerts for hypo and hyperglycemia.

Limitations:

• Requires frequent scanning as it does not have automatic real-time alerts.
• Records data up to 8 hours, requiring frequent scanning for proper records.
• Accuracy may drop during rapid temperature changes or after strenuous exercise.
• Some models don’t integrate with insulin pumps for automated insulin delivery.

Intermittently scanned CGMs (isCGMs) vs real-time CGMs (rtCGMs): Their differences

Here’s a table summarizing the differences between intermittently scanned CGMs (isCGMs) and real-time CGMs (rtCGMs)

Using CGM with an insulin pump

Certain CGMs connect with insulin pumps, providing real-time glucose readings to streamline insulin delivery. This collaboration allows the pump to adjust insulin doses automatically by identifying current glucose levels and predicting where they are headed.

This helps users maintain more stable blood sugar levels and reduces the risk of high or low glucose episodes.

How the CGM/insulin pump integration works

The CGM provides automatic glucose readings wirelessly to the insulin pump.

Based on the data provided, the pump can automatically give the correct basal insulin (background insulin) dose.

Hybrid closed-loop system

A hybrid closed-loop system, or "artificial pancreas," connects a CGM with an insulin pump to help manage blood glucose. The CGM tracks glucose levels, and a program calculates how much insulin is needed, which the pump delivers automatically. Users still enter the carbs they eat to adjust insulin for meals.

Key features of the hybrid closed-loop system

Here are some key features of the hybrid closed-loop system:

• It adjusts insulin delivery to maintain the required glucose levels.
• It stops insulin to prevent hypoglycemia.
• It reduces the burden of frequent manual adjustments.

Children usually get real-time CGM, while adults can choose between real-time or flash CGM.

Hybrid closed-loop systems can be given to children and adults with type 1 diabetes who have high HbA1c, severe hypos, or are pregnant/planning pregnancy. Consult your diabetes team to see if you're eligible.

People with type 2 diabetes on insulin may get a flash monitor if they meet certain criteria, like frequent hypoglycemia or needing many finger-prick tests. For people in the United Kingdom, keep in mind that insulin pumps and hybrid closed-loop systems aren’t offered on the NHS for type 2 diabetes.

Who should use continuous glucose monitoring?

Continuous glucose monitoring (CGM) is recommended for:

• People with type 1 diabetes to improve glucose control and reduce hypoglycemia/hyperglycemia risk
• People with type 2 diabetes take multiple daily insulin injections, especially if they experience frequent or severe hypoglycemia
• Pregnant individuals with diabetes to closely monitor their blood glucose levels
• Those who are unable to recognize hypoglycemia symptoms.
• Individuals who are trying to improve their glucose management and reduce the need for finger-prick tests.

Consult your healthcare provider to find out if CGM is suitable for you.

Wrap up

Continuous Glucose Monitoring (CGM) includes three main types: isCGM, which needs scanning the sensor to view glucose levels (e.g., FreeStyle Libre); rtCGM, which continuously provides real-time data and alerts of low and high glucose levels (e.g., Dexcom G6); and hybrid closed-loop systems, which integrate CGM with an insulin pump to provide automatic insulin delivery.

To use CGM, a sensor is applied to the skin, and data is viewed via a reader, smartphone, or pump. These systems help manage glucose levels, reduce finger-prick tests, and improve overall diabetes control.