Monitoring blood glucose is routine for individuals managing diabetes, providing data for treatment decisions. While the fingertip is the most common and reliable testing location, many seek less painful or more convenient alternatives. Blood glucose can be monitored from other locations, but utilizing these alternative sites requires understanding how the readings may differ from the standard fingertip check.
The Standard Method Fingertip Testing
The fingertip is the traditional and preferred site for blood glucose testing because of its rich supply of capillaries. This dense network allows a small drop of blood to be easily obtained. Capillary blood accurately reflects the current glucose concentration in the main bloodstream, offering a near real-time snapshot of the body’s glucose level.
To minimize discomfort and the formation of calluses, proper technique involves lancing the side of the fingertip, rather than the sensitive pad, and rotating the fingers used for testing. Despite its accuracy, the frequent lancing required is the main drawback, leading many to explore less sensitive areas.
Exploring Alternative Site Testing
Many modern blood glucose meters are approved for Alternative Site Testing (AST), which involves collecting a blood sample from locations other than the fingertips. These areas are less sensitive than the fingertips, offering a reduction in pain for people who test multiple times a day. Lancing devices designed for AST often use a different cap to accommodate the thicker skin layers found in these body areas.
Common sites for AST include:
- The palm
- The forearm
- The upper arm
- The thigh
- The calf
The Difference Between Testing Sites
The primary distinction between the fingertip and alternative sites lies in the type of fluid being measured and the speed at which it reflects glucose changes. Fingertip testing uses capillary blood, providing a fast and direct measurement of the blood glucose level. Alternative sites draw a sample from the interstitial fluid, the thin layer of fluid surrounding the body’s cells. Glucose must travel into this fluid, creating a physiological delay known as “lag time,” meaning the reading can lag behind the actual blood glucose level by up to 20 minutes.
This lag time has safety implications, especially when blood glucose is changing rapidly. For example, after eating, during intense exercise, or when treating hypoglycemia, the bloodstream shifts quickly, but the interstitial fluid cannot keep pace. Therefore, AST should not be used when a precise, current reading is necessary, such as when low blood sugar is suspected. The fingertip remains the only reliable site for testing during times of swift glucose fluctuation or in an emergency.
Advanced Glucose Monitoring Technology
Beyond traditional lancing, advanced devices offer a way to measure glucose elsewhere on the body continuously. Continuous Glucose Monitors (CGM) use a small sensor, typically placed on the back of the upper arm or the abdomen, to track glucose levels around the clock. Similar to Alternative Site Testing, the CGM sensor measures glucose in the interstitial fluid, providing readings every few minutes. The technology offers trend data, which indicates the direction and speed of glucose change, providing a less invasive way to monitor glucose dynamics than a traditional blood strip test.