A Continuous Glucose Monitor (CGM) tracks glucose levels throughout the day and night. Unlike traditional blood glucose meters that measure sugar in a drop of blood, the CGM measures glucose in the interstitial fluid, which is the thin layer of liquid surrounding the cells just beneath the skin. Proper placement of this sensor is fundamental, as the chosen location directly influences both the comfort of wearing the device and the accuracy of the readings it provides. Selecting the right spot ensures the sensor remains securely attached for its full wear time while minimizing interference from daily activities.
Identifying the Ideal Upper Arm Location
The best anatomical location for a continuous glucose monitor on the arm is the posterior, or back, of the upper arm. This area is generally preferred because it offers a stable, flat surface with less movement compared to other body sites. Manufacturers designate this spot as it provides the most consistent access to interstitial fluid, which is where the glucose readings are taken from.
The key to accuracy is placing the sensor within the subcutaneous fat layer, the soft tissue between the skin and the muscle. You should aim for a spot roughly halfway between the shoulder joint and the elbow crease, ensuring the area is not overly lean. Pushing the sensor into dense muscle tissue or placing it over a bone can lead to both pain upon insertion and less accurate data. To avoid muscle, gently flex your arm to identify the firm tricep muscle, then place the sensor slightly forward into the soft, yielding area.
The sensor should be positioned high enough on the arm to avoid catching on things, but low enough to remain on the fleshy part. A simple technique is to choose a location that can be pinched slightly, confirming the presence of adequate subcutaneous tissue. Selecting a flat area free from any skin folds, scars, or moles is necessary for optimal adhesion and consistent readings.
Lifestyle Considerations for Placement Selection
Once the general anatomical area is identified, personal lifestyle factors dictate the exact spot to maximize comfort and reliability. A major consideration is your preferred sleeping position, especially for side sleepers who risk experiencing compression lows. A compression low is a temporary, inaccurate reading that shows a false low glucose value when sustained pressure is applied directly to the sensor site. This occurs because pressure temporarily restricts blood flow and interstitial fluid circulation.
To mitigate this issue, side sleepers should place the sensor on the arm they do not typically sleep on. If you tend to sleep on both sides, positioning the sensor slightly to the outer side of the posterior arm may help keep it off the mattress. Physical activity is another factor, as the sensor must be placed where it is unlikely to be bumped or rubbed by clothing or equipment. Active individuals should avoid placing the sensor near the armpit or in a spot where a backpack strap or sports gear might cause repeated friction.
For daily routines, consider how your clothing interacts with the device. Choosing a site that is covered by a standard t-shirt sleeve can provide a layer of protection from accidental knocks. However, the clothing should be loose-fitting, as tight sleeves can cause friction that irritates the skin and potentially dislodges the sensor prematurely.
Ensuring Sensor Longevity and Accuracy
Achieving the full wear time and receiving reliable data requires careful preparation of the skin before insertion. Begin by thoroughly washing the site with non-moisturizing, fragrance-free soap to remove body oils and residue. After washing, the area must be cleaned with an alcohol wipe and allowed to dry completely before the sensor is applied. If there is significant hair at the chosen location, carefully trimming or shaving the area beforehand will help the adhesive form a stronger bond with the skin.
To maintain skin health and reading accuracy, it is important to rotate the insertion site with every sensor change. Using the exact same spot repeatedly can lead to skin irritation, scar tissue formation, and poor absorption, which compromise sensor function. Ideally, you should alternate between the left and right upper arms, and even slightly vary the position on the back of the same arm.
Once the sensor is applied, securing it properly will ensure its longevity. Many users find supplemental adhesive products, such as skin barrier wipes or over-patches, helpful for securing the device against water and friction. After applying any over-patch, firmly rub the edges for several seconds to activate the adhesive and create a secure, waterproof seal. Finally, remember that all continuous glucose monitors have a warm-up period, often lasting 30 minutes to two hours, during which the device adjusts to the interstitial fluid.