Continuous glucose monitoring (CGM)

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There are devices that regularly measure the levels of sugar in body tissue (continuous glucose monitoring, CGM). People who use a CGM device hardly need to prick their finger to measure their blood sugar levels anymore. These devices can also be used in combination with an insulin pump.

People who have type 1 diabetes need insulin injections every day. In rarer cases, some people with type 2 diabetes need them too. They either inject the insulin several times a day or are regularly supplied with insulin from an insulin pump.

In intensive insulin therapy, the amount of insulin used varies according to the body’s needs: A fixed amount of insulin is injected first to provide the body with a basic supply. Additional injections are given flexibly based on your blood sugar levels, the amount of food you eat and how physically active you are. So your sugar levels need to be measured several times a day. This can either be done using conventional blood glucose meters or a continuous glucose monitoring (CGM) system.

How does CGM work?

A CGM system is made up of the following two small devices:

  • A sensor with a small transmitter that is placed on your skin using a sticky patch. The thread-like sensor is inserted into fat tissue just under your skin (subcutaneous fat), for instance in your belly or upper arm, using a special insertion device. The sensor needs to be replaced regularly.
  • A small device that you can wear on your belt, for example, receives and stores the data sent by the transmitter. The information is displayed on its screen.
Illustration: CGM system: Continuous glucose monitoring in subcutaneous tissue

The system measures the sugar levels in the body’s tissue every few minutes and can alert you if the readings get too high or too low.

Unlike monitoring your blood sugar levels by pricking your finger, CGM devices keep tabs on your sugar levels 24 hours a day. They let you know how your blood sugar levels are developing over time, which can help to prevent hypoglycemia or hyperglycemia. The data the device saves can be sent to a computer and printed out. This allows doctors to check how well your blood sugar is controlled and, if necessary, make changes to improve it.

There are special patient education programs where you can learn how continuous glucose monitoring works.

Can CGM systems be combined with an insulin pump?

Many CGM systems can be used together with an insulin pump. Sometimes both of the devices can be controlled using the same display. One of these combination devices can stop the insulin pump whenever sugar levels fall below a certain threshold, in order to prevent hypoglycemia. This is known as the low glucose suspend (LGS) function.

Illustration: CGM system with an insulin pump

What is flash glucose monitoring (FGM)?

In this approach, a scanner takes the readings from a sensor that measures your sugar levels. The scanner is either inside a special measuring device or used through an app. This allows you to see your blood sugar readings on your smartphone, for instance. The sensor is placed on your upper arm using a sticky patch. Like in CGM, there's a small thread-like sensor that is inserted into the fat tissue beneath the skin.

You have to check the sugar levels yourself every eight hours by holding the measuring device or smartphone against the sensor. It is important to remember that the readings aren't automatically sent to the device or smartphone! And you can only see the sugar levels that were measured over the last eight hours.

Arrows show you whether your sugar levels are going up or down, or whether they are stable. There's also an alarm function to alert you if your sugar levels are rising or dropping too fast.

Are conventional blood glucose meters still needed?

Although it's becoming less common for people who have diabetes to measure their blood sugar by pricking their finger, they can't completely do without this conventional approach. They are generally still advised to carry a blood glucose meter (with a lancet and test strips) on them at all times. Some people who use a continuous glucose monitoring system should measure their blood sugar levels once a day as well, for instance. CGM devices have to be calibrated regularly too. Because the readings on the device don't correspond directly to your blood sugar levels, CGM systems are adjusted on a regular basis – for example, every twelve hours – using the information from blood sugar readings. What's more, it's good to have a back-up in case one of the devices stops working properly. Flash glucose monitoring devices don't need to be calibrated.

Also, there is a delay in how quickly the CGM device can display blood sugar fluctuations, such as those that may be caused by doing sports or eating a meal: It can take 5 to 20 minutes for the changes in blood sugar levels to be detected in subcutaneous fat tissue. So if you want to know immediately how much insulin you need, or whether you are at risk of hypoglycemia, you will have to measure your blood sugar levels as well.

Advantages of continuous glucose monitoring

Research has found that additionally monitoring sugar levels in tissue improves the regulation of blood sugar levels. Compared with conventional blood sugar monitoring, continuous glucose monitoring allows you to lower blood sugar levels without more episodes of severe hypoglycemia occurring. Serious cases of hypoglycemia are a problem because they can affect your health.

CGM is particularly suitable for adults who have type 1 diabetes because it reduces their risk of severe hypoglycemia. But research suggests that the additional use of CGM could help children with type 1 diabetes better regulate their blood sugar levels, too. It's not clear whether people with type 2 diabetes also benefit from this additional monitoring.

The studies didn't last long enough to be able to compare the effects of the two approaches on cardiovascular disease, number of deaths, or complications of diabetes caused by damage to small blood vessels.

Are the costs covered by statutory health insurers?

Yes. The statutory health insurers in Germany cover the costs of CGM devices if you need intensive insulin therapy and have already learned how to use the device properly. The doctor has to give you a prescription for medical aids ("Hilfsmittel"). You send this to your health insurer, together with the application for reimbursement of costs. The devices are available from pharmacies and shops that sell diabetes products.

Using CGM devices in everyday life

Most people have positive experiences with their CGM devices. They say that continuous glucose monitoring helps them to feel more relaxed and less worried that their blood sugar levels will get too high or too low. Parents also feel more at ease if their child uses a CGM device. They're more likely to let the child sleep over at a friend's house, for instance, and worry less about blood sugar fluctuations when they're away. CGM devices can also make it easier to plan activities because you don't need to measure blood sugar levels regularly.

The devices allow more flexibility at work too. For instance, you no longer need to prick your finger to get a blood sample during your working hours.

But they may also take some getting used to. The sensor can irritate your skin and affect your sleep. The regular readings can be unsettling, and so can the alarm function. But they can make you feel safer too – for instance, while you're sleeping – because the device alerts you if your sugar levels rise or drop a lot.

It may be easy to see the device, particularly in the summer. Some people feel uncomfortable about others "seeing" their disease in this way. But many don't mind.

Boucher SE, Aum SH, Crocket HR et al. Exploring parental perspectives after commencement of flash glucose monitoring for type 1 diabetes in adolescents and young adults not meeting glycaemic targets: a qualitative study. Diabet Med 2020; 37(4): 657-664.

Deutsche Diabetes Gesellschaft (DDG). S3-Leitlinie: Therapie des Typ-1-Diabetes. AWMF-Registernr.: 057-013. 2018.

Institute for Quality and Efficiency in Health Care (IQWiG, Germany). Continuous interstitial glucose monitoring (CGM) with real-time measurement devices in insulin-dependent diabetes mellitus: Documentation and evluation of comments on the preliminary report; commission D12-01. 2015.

Messer LH, Johnson R, Driscoll KA et al. Best friend or spy: a qualitative meta-synthesis on the impact of continuous glucose monitoring on life with Type 1 diabetes. Diabet Med 2018; 35(4): 409-418.

Ontario Health (Quality). Flash Glucose Monitoring System for People with Type 1 or Type 2 Diabetes: A Health Technology Assessment. Ont Health Technol Assess Ser 2019; 19(8): 1-108.

Scharf J, Nguyen XQ, Vu-Eickmann P et al. Perceived Usefulness of Continuous Glucose Monitoring Devices at the Workplace: Secondary Analysis of Data From a Qualitative Study. J Diabetes Sci Technol 2019; 13(2): 242-247.

IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

Our information is based on the results of good-quality studies. It is written by a team of health care professionals, scientists and editors, and reviewed by external experts. You can find a detailed description of how our health information is produced and updated in our methods.

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Created on December 8, 2021

Next planned update: 2024


Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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