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Continuous glucose monitoring (CGM)

Scientific support: Andreas Vosseler M.A.

Frequent blood sugar level testing can be painful and annoying. Also, blood sugar level testing provides only a momentary snapshot of the current blood sugar level. This means that, despite testing, blood sugar fluctuations resulting in low or high blood sugar levels may remain unidentified.

To ensure that the glucose levels remain well managed and to appropriately counteract any increase or fall in blood sugar levels, a new measurement system known as CGM (continuous glucose monitoring) was developed.

CGM systems measure the sugar levels in the subcutaneous fat tissue and aim to contribute to better insulin therapy management. In particular, they make living with diabetes easier for people who need to regularly test their glucose levels and adjust their amount of insulin and generally lead to an improvement in diabetes therapy. Individually adjustable alarm functions enable the early detection of low and high blood sugar levels.

There are 2 different types of systems for glucose monitoring: There is real-time CGM, rtCGM, and intermittently scanned continuous glucose monitoring, or iscCGM. In contrast to rtCGM, iscCGM only shows the current glucose levels when actively requested by the user. Another term for iscCGM is FCG, flash glucose monitoring.

Good to know:

rtCGM is the abbreviation for real-time continuous glucose monitoring.

iscCGM is the abbreviation for intermittently scanned continuous glucose monitoring. Alternatively, the term FCG, flash glucose monitoring, is used - both refer to the same thing.

What does CGM mean?

CGM is the term for continuous glucose monitoring systems that test glucose levels every few minutes. However, it is important to note: CGM systems do not test the glucose levels in the blood but rather in the subcutaneous fat tissue. This means the “tissue sugar level” is determined and not the sugar level directly in the blood. It should be taken into account that the tissue sugar level can be roughly 10 minutes behind the current blood sugar level. Deviations can appear especially in cases of rapid changes in the blood sugar levels, e.g., after a meal or exercise. In situations with stable blood sugar levels, both values are comparable.

How does rtCGM work?

rtCGM systems are comprised of 3 components: a senor, a transmitter, and a receiver. Depending on the product, the sensor is attached either to the abdomen or upper arm. The sensor is attached to a sensor wire that is placed under the skin using an insertion device. The sensor wire measures the sugar content in the tissue fluid approx. every 5 minutes.

The sensor can be worn when bathing, showering, or swimming. The sensor must be replaced every 6 to 14 days, depending on the system, and this is carried out by the user. There are also systems that require the sensor to be inserted under the skin of the upper arm by specially trained medical professionals. The transmitter is placed above the sensor on the skin and wirelessly sends the glucose levels to the receiver device. Using this system, the sensor can remain under the skin for up to 6 months. Replacements are also carried out by specially trained medical professionals.

Other types of rtCGM systems have the transmitter attached directly to the sensor. For example, the receiver device is a small wearable device or a smartphone, smart watch, or tablet that has an app to display and store the tested glucose levels. An insulin pump can also be used as a receiver device.

The current glucose level can be shown at any time on the receiver device display. Alongside the level, the display also shows trend arrows. Using the trend arrows, it is possible to determine whether the glucose levels are currently increasing, falling, or remaining stable.

If there is a threat of the blood sugar levels becoming too high or too low, an acoustic alarm is triggered to enable timely counteraction to, for example, prevent dangerously low blood sugar levels.

The stored blood sugar levels taken during the day and night can be electronically evaluated. This allows users to obtain a long-term overview of the progression of glucose levels.

How does iscCGM work?

iscCGM systems consist of a sensor and a reader device or an app. The sensor is usually attached to the back of the upper arm. Similar to the rtCGM, the measurement wires extend into the subcutaneous fat tissue to enable continuous monitoring of the glucose levels. The sensor can remain in place when showering, bathing, or swimming. The sensor remains in place for 14 days.

In contrast to rtCGM systems, iscCGM systems do not automatically send the glucose readings to the receiver device. Instead, the user can access the sensor glucose levels by scanning using a reader device or smartphone. This requires waving the reader device or smartphone above the sensor. This also works when wearing thicker clothes.

iscCGM devices store the readings for a period of 8 hours as a progression curve on the sensor. To obtain a complete overview of the progression of the glucose levels throughout the day, the data contained in the senor must be retrieved using the reader device at least every 8 hours. Similar to rtCGM systems, iscCGM systems also display trend arrows to show the current glucose level progression, i.e., if the levels are increasing, dropping, or are stable.

Also, like rtCGM devices, iscCGM devices have alarm functions to warn when there is a risk of high or low blood sugar levels. In the event that an alarm is triggered, the user must scan the sensor to view the current glucose level on the reader device.

The use of a CGM system reduces the number of blood sugar measurements using a blood glucose monitoring system. However, the use of this conventional type of blood glucose testing cannot be completely avoided. For example, daily blood sugar testing may be useful to estimate the sensor-dependent measurement differences between the tissue and blood glucose readings. Blood glucose measurements can also be necessary in the event of suspected low blood sugar levels, persisting elevated blood sugar levels, or in certain exceptional situations. Some rtCGM devices require at least twice daily blood glucose testing using a lancing device and test strips to help calibrate the system.

No regular calibration of iscCGM systems is possible. Nevertheless, after placement of the sensor, the blood glucose reading should be checked using a lancing device and test strips during the familiarization phase. This enables the user to ensure that the iscCGM reading actually does match the blood test results. For all practical purposes, a blood glucose test device is already integrated into the iscCGM reader.

When using an rtCGM or iscCGM system, it is generally recommended to also carry a conventional blood sugar test system (measurement device, lancing aid, test strips). This allows blood sugar level testing if the rtCGM or iscCGM systems malfunction.

Good to know:

rtCGM or iscCGM systems can malfunction. For these reasons, it is recommended to also carry a conventional blood sugar test system (measurement device, lancing aid, test strips).

Before a continuous glucose monitoring device can be requested from a health insurance provider, it makes sense for the user to inform themselves of the available CGM systems and any potential advantages or disadvantages. Furthermore, special training programs on the use of rtCGM or iscCGM systems are available:

The manufacturer-independent training program for rtCGM systems is known as SPECTRUM.

FLASH is a manufacturer-specific training program for an iscCGM system recognized by the German Diabetes Association (DDG).

Since 2016, rtCGM systems with alarm function are listed as an aid in the catalog of services from the statutory health insurance providers. In addition, in June 2019, the FreeStyle Libre 2 iscCGM was added to the catalog of services.

However, this does not automatically mean that the statutory health insurance providers (GKV) agree to cover the costs for the systems in all cases. GLVs are not obligated to cover the costs. After the treating physician has issued a prescription for a medical aid, an additional application for cost reimbursement must be completed by the patient and the physician together, or the physician alone. The health insurance provider will then examine the application and make a decision.

Good to know:

An application for cost reimbursement can be submitted to the statutory health insurance provider. They are not obligated to agree to cost reimbursement.

Private health insurance providers (PKV) are only obligated to provide the services listed in the insurance contract. When the contract includes an open medical aid register explicitly listing, for example, rtCGM systems, the PKV is obligated to reimburse the costs of such systems in the event of medical necessity.

The costs for rtCGM or iscCGM systems with alarm function will be reimbursed by the statutory health insurance providers after submitting an application and if deemed medically necessary.

The requirements each patient must fulfill for cost reimbursement are as follows:

  • The patient suffers from diabetes mellitus.
  • The affected person requires intensified therapy using insulin.
  • The affected person has already participated in intensified insulin therapy training.

Patients who are eligible for this program are those for whom

  • the therapy goals for metabolic control are not achieved despite maximum efforts.
  • severe hypoglycemia has occurred in the past and/or a hypoglycemia perception disorder is present.
  • a pregnancy is planned or there is a pregnancy with inadequate metabolic control despite therapy.
  • more than 10 blood glucose measurements per day are necessary to achieve the targeted metabolic control.

Good to know:

A diabetes diary can help with the decision regarding the medical necessity of an rtCGM or iscCGM system.

The completed prescription for a medical aid and the application for cost reimbursement are submitted to the competent health insurance provider or specialist diabetes store.

TIP: The patient can submit the diabetes diary and a personal statement to justify the necessity along with the application. It is also recommended to seek the advice of a diabetologist.




Automatic display of glucose levels


No (The levels have to be individually accessed)

Calibration is necessary

Yes / No§


Alarm function warning of low blood sugar levels



Blood sugar level testing using a lancing aid and test strips

At least twice per day for calibration/occasionally§

Occasionally (for example, once daily)

Sensor replacement frequency

Depending on the system, between every 6 days and every 6 months.

Every 14 days

Cost reimbursement by the GKV

Yes, when the application for cost reimbursement is approved

Yes, when the application for cost reimbursement is approved

§For a CGM model with factory calibration.

*First generation iscCGM systems without an alarm function


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Arbeitsgemeinschaft Diabetes & Technologie der Deutschen Diabetes Gesellschaft: Gewebezuckermessung (CGM). (Letzter Abruf: 02.04.2020)
Bundesverband Medizintechnologie e. V.: Sensor-basierte Glukosemessung bewährt sich im Alltag. 2019 (Letzter Abruf: 02.04.2020)
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As of: 23.04.2020