About the Test
Purpose of the test
The purpose of continuous glucose monitoring is to measure blood sugar levels repeatedly throughout the day and at night in people with diabetes.
CGM allows you and your doctor to keep track of blood glucose trends over time and can help you make short-term treatment decisions if your glucose levels are too high or too low. It also reduces the need for frequent fingerstick blood tests to measure your glucose levels, though it may not eliminate fingersticks completely.
CGM devices may be professional or personal:
- A personal CGM device lets you keep track of your blood glucose data yourself. It sends the information directly to your receiver or smart device. It may also be able to send the results to other people, such as family members or your doctor.
- A professional CGM device collects blood glucose data for a set amount of time and transmits it to your doctor. Your doctor can then discuss any observed trends with you. You may not be able to see the information before your doctor collects it.
There are also two basic types of personal CGM devices, real-time and intermittently scanned:
- Real-time CGM devices measure glucose levels every five minutes and send the information automatically to the receiver or smartphone.
- Intermittently scanned CGM devices require you to pass a scanner over the sensor/transmitter in order to read the information.
What does the test measure?
Continuous glucose monitoring tracks the level of glucose in the fluid between the cells, called the interstitial fluid. Interstitial glucose levels correspond closely to levels of glucose in the blood.
CGM devices measure glucose levels with a very small wire sensor, inserted just under the skin of your abdomen or upper arm. A transmitter attached to the sensor sends this information wirelessly to a dedicated receiver or to a device such as a smartphone.
Glucose levels are reported in milligrams per deciliter, or mg/dL.
Finding Continuous Glucose Monitoring
How to get tested
Continuous glucose monitoring devices require a doctor’s prescription to purchase. Your doctor may recommend continuous glucose monitoring for you, or you may wish to bring CGM up with your doctor to learn if it is right for you.
Can I take the test at home?
Continuous glucose monitoring takes place wherever you are while you are wearing the monitoring device. This includes your home, workplace, doctor’s office, and other locations.
How much does the test cost?
Continuous glucose monitors may cost different amounts of money. Prices can depend on the type of health care coverage you have, the model of device your doctor prescribes for you, and the pharmacy or distributor that sells you the device.
Intermittently scanned CGM devices are typically less expensive than real-time models.
If your doctor prescribes continuous glucose monitoring for you, your insurance may cover the costs associated with the monitor and its components, such as replacement sensors. Your insurance provider may require you to cover some amount of these costs yourself in the form of copayments or deductibles. You can learn more about the costs you may be expected to cover from your doctor, pharmacy, insurance company, or from the monitor’s manufacturer.
Using Continuous Glucose Monitoring
Continuous glucose monitors may require setup before use, and some of their components must be periodically replaced. CGM devices do not generally require a sample of blood or tissue themselves. A tiny sensor is typically placed under the skin, often in the arm or belly, and measures the amount of sugar in what is known as the interstitium, the space between cells. Their sensors detect glucose levels automatically. But some devices may need blood samples for calibration and to confirm their readings.
Before the test
You will need to set up and program your CGM device before using it for the first time. Setup instructions will come with the monitor you purchase. The monitor’s manufacturer may have videos available online that you can refer to. You can also ask your medical team for advice and guidance with setting up your monitor.
To begin wearing the CGM device, you will use a special applicator to insert the sensor below your skin. This procedure is painless. The sensor is held in place by an adhesive patch. After you have inserted the sensor, you will attach a reusable transmitter to the sensor patch.
Because CGM models vary, be sure to discuss using your CGM device with your doctor and read the manufacturer’s instructions carefully before beginning to wear it.
During the test
Many continuous glucose monitors need to be calibrated twice a day with a fingerstick and a blood glucose meter to ensure that the results are correct. Some older models may require more frequent calibration, while some newer models may not require daily calibration at all.
For most people with diabetes, fingersticks do not cause too much discomfort. To perform the fingerstick test, you will use a sterile lancet to prick the tip of your finger until a drop of blood appears. Then you will put the drop of blood onto a test strip and insert the strip into your blood glucose meter. The reading on the meter indicates your blood glucose value, which you will use to calibrate your CGM.
Most of the time, CGM devices can be worn throughout your normal daily activities, including showering and exercise.
After the test
Sensors for personal CGM devices must be replaced after a matter of days or weeks, depending on the type of monitor you are using. You will need to take the old sensor out and apply a new one when the sensor reaches the end of its lifespan, in order to make sure that the CGM’s readings are accurate.
Continuous Glucose Monitoring Results
Receiving test results
Because continuous glucose monitoring occurs in real-time, the results can provide insights into how your blood sugar levels change through the day and night. This information can highlight what behaviors or environmental conditions raise or lower your blood sugar.
You can access results from a CGM device by looking at the receiver or smart device app. To get results from an intermittently scanned device, you must pass the scanner over the transmitter. You can do this through your clothing if you need to.
As well as sending glucose level data to your receiver or smartphone, your personal CGM device may trigger an alarm if your glucose level becomes too low or too high. This is a feature of all real-time monitors.
If you are wearing a professional CGM device, your doctor will discuss results with you after the monitoring period is completed. This conversation may take place in person during an office visit, over the phone, or through an email exchange.
Interpreting test results
Depending on the type of personal CGM device you are using, your receiver or smartphone app may display several types of data that you can review at a glance. These often include:
- A number representing your current glucose level in mg/dL
- A graph or line showing glucose levels over time
- An arrow indicating a rising or falling trend
CGM devices may also allow additional data to be downloaded to a computer, to be reviewed with your doctor.
Your doctor is in the best position to discuss the target insulin range for your specific situation and how to interpret your CGM results over time.
CGM devices can help you make treatment decisions, such as changing your insulin dose or treating low blood glucose, in response to results or alarms. But some CGM devices—especially older models—may still require that you confirm the results with a fingerstick blood test before making any treatment changes.
Continuous glucose monitoring can help you record how your blood glucose levels change due to your meals, exercise, insulin use, and other factors. By observing these changes, you and your health care team can work together to adjust your medications, insulin doses, and other aspects of your diabetes self-care.
