A Continuous Glucose Monitor (CGM) is a medical device that continuously tracks glucose levels, providing a detailed picture of trends and fluctuations throughout the day and night. The system uses a tiny sensor inserted just under the skin to measure glucose in the interstitial fluid, which is the fluid that surrounds cells. Because the sensor’s accuracy and longevity depend heavily on its placement, understanding the best application sites is important for effective glucose management. This article examines the officially recommended sensor locations and explores the common question of placing a CGM sensor on the leg.
Standard Placement Guidelines
Manufacturers conduct rigorous clinical trials to determine the most reliable sensor locations before receiving regulatory approval. These official placement guidelines are based on sites that demonstrate optimal signal stability and accuracy across a broad user population. For most CGM systems, the primary approved site is the back of the upper arm. Some devices also list the abdomen or the upper buttocks as approved areas, particularly for younger age groups.
These recommended sites are selected because they typically offer a consistent layer of subcutaneous fat and are less prone to physical trauma during daily activities. Adhering to these guidelines ensures data accuracy validated by the company’s research. It is recommended practice to rotate the sensor insertion site within the approved areas each time a new sensor is applied to prevent skin irritation and the buildup of scar tissue.
Practicality of Leg Placement
While the leg is not listed as an approved site on the label for many popular CGM systems, many users choose to place the sensor on the upper thigh as an “off-label” practice. This location is often chosen for comfort, discretion, or to avoid interference with clothing, beltlines, or activities like exercise. For instance, people who sleep heavily on their side may find that the upper thigh, placed slightly toward the front, helps avoid the compression that can cause inaccurate readings.
The upper thigh is preferred over the lower leg because it typically contains sufficient subcutaneous fat, necessary for correct sensor function. However, choosing this site introduces practical risks, the most common being sensor dislodgement. The natural movement of the thigh during walking, running, or sitting can place stress on the adhesive, making it more likely for the sensor to be ripped off prematurely.
Users often mitigate this risk by using supplemental adhesive patches or protective covers designed to keep the sensor secure. Any placement not specified in the manufacturer’s instructions means the sensor’s accuracy is not guaranteed or clinically validated. Although some independent studies suggest that the accuracy of certain sensors on the upper thigh can be comparable to the upper arm, this varies widely and depends on the individual’s body composition and activity level.
Biological Factors Influencing Accuracy
Sensor location affects accuracy because the device measures glucose in the interstitial fluid, which acts as a middleman between the bloodstream and the sensor. The sensor filament must be situated within the subcutaneous fat layer, not in underlying muscle tissue, to ensure stable access. Factors that disrupt the flow of fluid or blood to this area can cause temporary inaccuracies in the glucose reading.
One major biological factor is tissue compression, which can lead to what are called “compression lows.” Applying prolonged pressure onto the sensor site, such as lying on it while sleeping, can temporarily restrict local blood flow. This restriction causes the sensor to report a falsely low glucose value until the pressure is relieved. This phenomenon explains why locations prone to pressure, like the abdomen or buttocks, can be problematic.
Tissue movement or muscle strain can also impact sensor performance. Areas with high muscle density or significant mechanical movement during exercise may cause the sensor to shift or disturb the tissue-sensor interface. This can lead to signal fluctuations or a temporary loss of accuracy. Accuracy differences between sites are often due to variations in local blood flow and glucose delivery to the interstitial fluid.