A Continuous Glucose Monitor (CGM) tracks glucose levels throughout the day and night. Unlike a traditional blood glucose meter, a CGM measures the concentration of glucose in the interstitial fluid—the fluid found between cells—rather than directly in the blood. This constant data stream allows users to observe trends and patterns in their glucose fluctuations. Understanding the correct and approved locations for sensor placement is necessary for ensuring the device’s accuracy and optimal function.
Primary Approved Placement Sites
The primary locations approved for CGM placement are defined by the manufacturer and vary between different brands and models. For many adults, the two most common approved sites are the back of the upper arm and the abdomen. Some systems, such as the FreeStyle Libre, are approved exclusively for the back of the upper arm, requiring consultation of specific device instructions.
When using the abdomen, the sensor should be placed in an area with sufficient soft tissue, typically at least two inches away from the navel. It is important to maintain a distance of at least one to three inches from any insulin injection sites or pump infusion sets to prevent interference with glucose readings. For children and adolescents, some manufacturers additionally approve the upper buttocks as a primary placement site.
Regardless of the body area selected, the sensor must avoid bony areas, scars, tattoos, or irritated skin. These restrictions ensure the sensor filament sits correctly within the subcutaneous layer for reliable measurements. Placing the sensor too close to areas of high friction, such as the beltline or under tight clothing, can cause the device to dislodge or lead to inaccurate readings.
Factors Influencing Site Selection and Accuracy
The selection of a proper site is linked to how the sensor functions beneath the skin. Consistent readings are obtained in areas with a good supply of subcutaneous adipose tissue (body fat). This fatty layer is rich in the interstitial fluid the sensor requires and offers a stable environment for the sensor filament.
Areas with significant muscle movement or very little fat should be avoided because they can lead to erratic readings and discomfort. Physical activity can also temporarily affect sensor accuracy, as changes in blood flow and interstitial fluid dynamics occur around the sensor. For example, placing a sensor on the buttocks may lead to greater changes in accuracy during intense exercise compared to abdominal placement.
Another factor influencing accuracy is pressure applied directly to the sensor, often referred to as “compression lows.” If a person sleeps directly on the sensor site, sustained pressure can temporarily impede interstitial fluid flow, leading to falsely low glucose readings. Choosing a site less likely to be compressed for extended periods is a practical consideration for maintaining data reliability.
Essential Tips for Application and Site Rotation
Proper skin preparation is essential for maximizing sensor adhesion and minimizing infection risk. Before applying the sensor, the chosen area must be thoroughly cleaned with an alcohol wipe and allowed to air-dry completely. Residual moisture or oils on the skin can compromise the adhesive, leading to early sensor failure. If the site has significant hair, shaving the area beforehand ensures the adhesive patch sticks directly to the skin.
Site rotation is necessary for long-term sensor use and skin health. Using the same location repeatedly can lead to the formation of scar tissue or hard, raised lumps called lipohypertrophy. These skin changes interfere with the sensor’s ability to measure interstitial fluid accurately, leading to unreliable data.
A rotation plan involves selecting a new, unused site for every sensor change, which typically occurs every seven to fourteen days depending on the device. Users should map out the approved area into quadrants and systematically move the placement site with each application. This structured rotation allows previous sites adequate time to heal, maintaining skin integrity and ensuring future sensor readings remain accurate.