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What is a Continuous Glucose Monitor (CGM)-How does it work

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Maria Jacob

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Friday, November 24, 2023

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Regular blood glucose monitoring is essential for individuals with diabetes to manage their condition effectively and make informed decisions about lifestyle and medication adjustments. Continuous glucose monitoring devices are revolutionizing diabetes management.

By providing real-time insights into blood sugar dynamics and trends, continuous glucose monitoring devices have been shown to increase the safety and efficacy of diabetes treatment. As continuous glucose monitoring devices continue to gain accuracy and simplicity, this technology is being used more and more.

What is a Continuous Glucose Monitor (CGM) device: How does it work?

Before we jump into the understanding of a CGM monitor device, let us understand a brief history of the development of glucose tests over time.

It is now more than 100 years after the first use of commercial urine glucose testing which utilized copper reagent and was known as Benedict’s assay. The same small modification was used for more than 50 years to quantify glucose in urine (Hirsch 2018). As time progressed, we have seen numerous developments and attempts to provide the best and most accurate methods to monitor blood glucose. The concept of self-monitoring of blood glucose (SMBG) became available starting in the 1980s (Olczuk 2018). During this time, SMBG became the standard of care, especially for patients with type 1 diabetes (insulin-depended diabetes). In 1999, the monitoring of blood glucose was revolutionized when the US Food and Drug Administration approved the first commercial CGM device (Hirsch 2018). Since its professional inception, CGM device has proved better with technological advancement in the management of diabetes. 

A continuous blood glucose monitor is a cutting-edge device that offers real-time monitoring of blood sugar levels, providing individuals with diabetes valuable insights to proactively manage their condition. A CGM device is a wearable medical device that is used to assess a patient’s glucose level. This device provides data that can be used to make rational decisions about making changes to treatment and lifestyle, thus helping to achieve a normal glucose level. The CGM device usually records the blood glucose level at a certain time interval (every 5-15 minutes) throughout the day and night (Irace et al. 2020). Typically, a CGM monitor device system comprises a sensor, transmitter, receiver, application software, and insertion tool. The sensor is placed under the skin (often on the stomach or arm). This sensor penetrates the top layer of the skin and is held in place with the help of an adhesive patch. This sensor, at a certain period, measures the glucose levels in the interstitial fluid (fluids surrounding the body cells). This glucose level data is transmitted by a CGM device and can either be received or downloaded to see, or monitored in real-time on devices like smartphones, depending on the type of CGM device (Elian et al 2023). Occasionally, CGM devices can set alarms to alert users whenever the glucose levels are dangerously high or low. The CGM sensor/device can be used anywhere between 7 to 14 days. Currently, newer generations of CGM devices have improved in accuracy, ease of application, and durability. There are the availability of CGM devices, which can last up to a 90-day wear period (Sanchez et al. 2019). 

Note that CGM is not a diabetes treatment. It is just a tool that continuously assesses the level of glucose and assists the clinician/individual to manage and bring the glucose level into a healthy range. Blood glucose monitoring is a key component of diabetes management, providing valuable insights into how dietary choices and physical activity impact blood sugar levels throughout the day. 

Who Should Use CGM?

  • Broadly speaking, CGM can be recommended by clinicians to adults with any type of diabetes or children (ages > 2 years) with type 1 diabetes. 

  • Individuals who cannot sense hypoglycemia (low blood glucose) can benefit from CGM timely alert (van Beers et al 2016).

  • The use of CGM to monitor blood glucose levels during gestational diabetes has also been reported in studies. This assists in keeping the glucose level under control thus ensuring the safety of both the pregnant woman and the baby (Chen et al 2003; Polsky 2017).

  • Advancement in sport has also led to the inclusion of medical devices like CGM for real-time monitoring of blood glucose levels in athletes. The data obtained from CGM can help in developing nutrition plans to optimize endurance and overall performance (Pujalte et al 2022).

Tracking blood sugar levels

Tracking and monitoring of blood sugar levels is a fundamental part of diabetes care to ensure that the condition is under control, as both low and high glucose levels can have deleterious effects on the body (Khan 2019). Currently, for tracking and monitoring of blood sugar levels SMBG and laboratory blood/urine tests are used. Lately, SMBG has been preferred for convenience and rapid results (Saudek 2004). 

Typically, SMBG includes:

  • Blood Glucose meters (Glucometers)

  • CGM device

  • Flash Glucose Monitoring (FGM)

A glucometer requires a small blood sample obtained by pricking a fingertip, followed by placing the blood sample in a test strip, which is subsequently inserted in the glucometer to get the blood glucose level result. The results are quick, but finger pricking is often uncomfortable and not encouraged in minors (Heinemann 2008). This can be overcome by using CGM devices. CGM offers less frequent pricking, thereby making it less invasive. Also, CGM offers continuous monitoring and data sharing and can be configured for alerts and alarms. FGM is a type of CGM that is less invasive than traditional CGM. Each type has its advantages and disadvantages and can be used in diabetes monitoring plans under the guidance of a clinician. 

Benefits of CGM

As technology, research, and treatment modalities progress further, the management of diabetes will evolve. CGM diabetes technology can promote glucose and weight control, help reduce the risk of hypoglycemia (low glucose level) and hyperglycemia (high glucose level), and improve quality of life (New et al 2015). Studies have shown that CGM increases time spent at normal glucose levels and reduces the chances of severe hypoglycemia in patients with type 1 diabetes and impaired awareness of hypoglycemia (van Beers et al 2016). CGM technology will likely advance further, bringing even more precise and convenient devices. Better predictive algorithms and data analytics may offer clearer insights into blood sugar changes. The prospective trend information from CGM might help encourage and assess the behavior change of an individual (Block, 2008).

Dynamic Results

CGM has proven features of continuous data collection and real-time monitoring. The trajectory of glucose fluctuations is also provided by CGM, allowing users to see the direction and rate of change in glucose levels. Patients and healthcare professionals can use this data to quickly address and avoid expected abrupt occurrences. Additionally, it enables patients to make better decisions about their prescription requirements and other facets of their daily diabetic self-care (Galindo 2020). The reader/receiver of the device or a smartphone application can also be used to retrieve historical data, enabling diabetes patients and their healthcare practitioners to undertake retrospective studies.

Precise Results

Patients and their healthcare professionals can now fine-tune therapy and lessen the burden of diabetes thanks to continued advancements in CGM and insulin delivery technologies. Continuous monitoring, multiple data points, advanced CGM sensor technology, trend analysis, and retrospective and prospective data have enhanced the precision of diabetes management decisions. 

However, CGM results are considered highly reliable for daily diabetes management and treatment decisions. Users should follow the instructions of healthcare providers and use CGM data in conjunction with other aspects of their diabetes management plan to maintain optimal glucose control.

Pros and Cons of Using a CGM


  • CGM can provide real-time glucose data and trends.

  • CGM requires minimal or no fingerstick testing, making it less painful.

  • Features like alerts and alarms for high or low glucose can help in enhancing timely management.

  • Historical data helps clinicians to analyze patterns and make informed decisions.

  • The use of CGM during gestational diabetes and in sports for athletes is opening doors for greater acceptability.

  • Also, correct usage of CGM helps an individual to lower their need for frequent glucose testing, thereby improving their quality of life.


  • Repeated calibration by fingerstick sampling

  • The patient reported barriers like pain or discomfort during sensor insertion in some individuals.

  • Concerns about the high cost associated with CGM.

  • Accidental removal of the CGM device or adhesive strip and skin irritation during prolonged use

  • Skin reactions or irritations related to sensor adhesion

Difference Between Flash Glucose Monitoring (FGM) and CGM

CGMs can be categorized into two groups based on their data reporting methods: "real-time" and "flash." Real-time CGMs provide glucose readings every 1 to 5 minutes and transmit the data to a receiver. In contrast, Flash CGMs, also known as intermittently scanned CGMs, record glucose levels every 1 to 15 minutes and require user-initiated scanning to access intermittent data. During this flash, the previous 8 hours’ worth of data can be downloaded for review (Forlenza et al 2019). Typically, FGM is factory-calibrated and does not need daily calibration (or finger sticks) by the user. Regarding cost, flash CGM costs less than that of traditional CGM (Heinemann 2015). Also, FGM has been shown to have a longer sensor life of 14 days and does not require frequent calibration (Kashimada 2021). 

Limitations with CGM and FGM

Although traditional CGM devices are generally convenient to use, they also come with certain limitations. CGM is associated with pain or discomfort during sensor insertion, shorter sensor life, cost and affordability, doubts regarding the accuracy of CGM compared to glucometer readings, etc. (Markowitz 2012). On the other hand, despite having remarkable benefits, FGM also has some aspects to improve. FGM does not provide real-time alerts or alarms when blood glucose levels fluctuate or hypoglycemia is suspected; simultaneous fingerstick sampling is advised because sensor glucose values can occasionally be inaccurate. FGM is not appropriate for toddlers younger than 4 years old, as it is only intended for use by people 18 years of age and older (Kashimada 2021).

In conclusion, CGM has been used as an important tool to manage diabetes; however, individuals and clinicians should carefully discuss the benefits and limitations of CGM diabetes technology for better management.


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