Also Known As
Interstitial Glucose Monitoring
Real-time CGM (rtCGM)
Intermittent-Scanning CGM (iscCGM)
Flash glucose monitoring
This article was last reviewed on
This article waslast modified on 14 November 2019.
At a Glance
Why Get Tested?

To monitor your blood sugar (glucose) concentrations; to evaluate changes and trends in your glucose concentration over time

When To Get Tested?

NHS funding for CGM is restricted to people with difficult to manage diabetes mellitus. Short term use may be available from a clinic if they own a device for short term use to help guide insulin therapy. Long term use is restricted to specific individuals as per NICE guidance. For adults this primarily involves difficulty with low blood glucose (hypoglycemia), or high despite testing by finger prick at least 10 times a day. For children the rules are slightly less restrictive. The purpose of CGM is to help the patient and team control blood sugars better and avoid too many finger prick tests.

Sample Required?

A continuous glucose monitoring (CGM) device includes a small sensor wire that is inserted beneath the skin of the abdomen or the upper arm and held in place with an adhesive patch. The sensor measures glucose in the space around cells (interstitial space). CGM measures glucose at frequent intervals and sends the results wirelessly to a device that is attached to your clothing or in some cases to a smart phone. These digital readouts let you know your equivalent blood glucose level in real time.

Test Preparation Needed?

No test preparation is required.

On average it takes 7 working days for the blood test results to come back from the hospital, depending on the exact tests requested. Some specialist test results may take longer, if samples have to be sent to a reference (specialist) laboratory. The X-ray & scan results may take longer. If you are registered to use the online services of your local practice, you may be able to access your results online. Your GP practice will be able to provide specific details.

If the doctor wants to see you about the result(s), you will be offered an appointment. If you are concerned about your test results, you will need to arrange an appointment with your doctor so that all relevant information including age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), can be considered.

Lab Tests Online-UK is an educational website designed to provide patients and carers with information on laboratory tests used in medical care. We are not a laboratory and are unable to comment on an individual's health and treatment.

Reference ranges are dependent on many factors, including patient age, sex, sample population, and test method, and numeric test results can have different meanings in different laboratories.

For these reasons, you will not find reference ranges for the majority of tests described on this web site. The lab report containing your test results should include the relevant reference range for your test(s). Please consult your doctor or the laboratory that performed the test(s) to obtain the reference range if you do not have the lab report.

For more information on reference ranges, please read Reference Ranges and What They Mean.

What is being tested?

Continuous glucose monitoring (CGM) measures the glucose in the space surrounding cells (interstitial space).  Blood, including finger prick, and interstitial fluid glucose levels are not exactly the same, but they mirror each other closely.

CGM Devices

Some features of CGM devices include:

  • A small sensor wire that is inserted beneath the skin of the abdomen or the upper arm and then held in place by a sticky patch for several days (7, 10, or 14) up to 3 months.
  • A transmitter above it on top of the skin. A device to show the results, which may be a smart phone, pump or separate hand held machine.
  • the information can be used in several ways.
    • CGM devices can measure and display glucose readings "real-time" at set intervals, or display the last glucose result performed when they are scanned (intermittent scanning or flash).
    • CGM devices may send alerts and have alarms.

One use of CGM is in conjunction with an insulin pump. When you eat and your glucose level rises, your CGM device measures glucose levels. Controls in the insulin pump react to this result to deliver insulin.

Accordion Title
Common Questions
  • How is it used?

    CGM is not used by a majority of people with type 1 diabetes, but as CGM devices become increasingly accurate and user-friendly, their use is increasing.

    National Institute for Health and Care Excellence (NICE) recommend CGM for a limited number of reasons:

    Children and young people with type 1 diabetes who have frequent severe hypoglycaemia are offered ongoing real time continuous glucose monitoring with alarms.Adults with type 1 diabetes and episodes of disabling hypoglycaemia despite optimal management with continuous subcutaneous insulin infusion.

  • When is it requested?

    Real-time CGM may be used daily and constantly to have better glucose control when you have type 1 diabetes.

    CGM may be used periodically when you have diabetes and your healthcare practitioner wants to collect and evaluate data on your day-to-day glucose variability and control.

    CGM may be used when you have diabetes and hypoglycemia unawareness (no distinct symptoms) and/or frequent low or high blood glucose.

  • What does the test result mean?

    Glucose values from CGM devices generally correlate closely with blood glucose concentrations. Your target range for your glucose levels is determined by your healthcare professional and depends on several factors, such as:

    • How long you have had diabetes
    • Your age and life expectancy
    • Other underlying conditions you may have, including heart disease
    • Whether you do not experience distinct symptoms of low blood sugar (hypoglycemia unawareness)

    CGM results can identify variability in glucose levels throughout the day, identify trends and anticipate when your glucose level is getting too low (hypoglycemia) or too high (hyperglycemia), sending an alert.

    Looking at patterns of glucose data (such as data points collected over several days and graphed in a report) can help you and your healthcare practitioner evaluate variations in your glucose levels and suggest actions that may help stabilise glucose levels and improve glucose control.

  • Can I buy a CGM device over-the-counter?

    Yes, though it may be worth discussing it with your health care team first to advise on the most appropriate system to buy as you will need support in how to use it and interpret the results.

  • Can I use a device for longer than indicated?

    No, they are approved for specific periods of use and need to be replaced as indicated by the manufacturer. It is likely that CGM devices will be able to be worn longer in the future as they continue to improve.

  • Is there anything else I should know?

    It is likely that finger prick glucose tests will still need to be done, but less often, in order to confirm the CGM is measuring correctly.