Dr. Dennis Van Hoof, PhD, CLC
“Don’t blindly trust your continuous glucose moniter (CGM).”
False glucose readings
Continuous glucose monitors (CGMs) have become an essential tool for excellent diabetes management. The sensor probe that is inserted underneath your skin measures glucose levels for several days, and displays these values every few minutes in a graph. This allows you to see what your glucose level currently is, but also if it is going up or down, so that you can take the appropriate action to keep your glucose levels in the safe zone (see Blog post “Continuous glucose monitoring (CGM)“).
A continuous glucose monitor (CGM) does not measure your blood glucose, but the glucose in your interstitial fluid.
The lag — Unfortunately, CGMs have some practical limitations, as they do not measure blood glucose, but the glucose level in the interstitial fluid underneath your skin. Glucose from your blood first enters the space in between cells, which is filled with this interstitial fluid, before the glucose reaches the cells that need the energy (see Blog post “Insulin and Diabetes“). This means that CGM measurements are always lagging behind the true glucose value in your blood. Under normal circumstances, this lag is hardly noticeable; the glucose concentration in your blood is practically the same as in your interstitial fluid. But when you eat or drink something sweet, or when you start exercising, the glucose concentration in your interstitial fluid may be different from the concentration in your blood. Your CGM could still be showing a flat line, while in reality, your blood glucose is going up (from the sweets) or down (from the physical activity).
The prediction — The lag is mostly taken care of by a complicated calculation (algorithm) made by the receiver. The algorithm takes the average of several new measurements made by the sensor, and then looks at what the previous values were to predict what the new value most likely is. But because the algorithm cannot look into the future, it has to go by what the earlier values were, and will not be able to accurately anticipate what your blood glucose truly is when things change rapidly.The pressure dip — Since all the nutrients in your body are moved around from one place to another by liquids (blood and interstitial fluid), physical pressure can easily disrupt that flow. Check this out yourself. Just press your thumb for a few seconds on the back of your hand. When you remove your thumb, you see a white spot that quickly disappears. By pressing on your skin, you literally squeeze the blood (and interstitial fluid) away. As soon as you remove your thumb, blood (and interstitial fluid) come rushing back in that area. This can happen to the area where you inserted your sensor too; for instance when you roll over in your sleep. When lying on top of the sensor, fluids (and the glucose therein) are squeezed out of the area underneath your skin and away from the sensor probe. The little bit of fluid that is still surrounding the probe has only a small amount of glucose left, and the sensor will think that your glucose level is dangerously low. But it is only in that small, local area; the glucose concentration in the rest of your body hasn’t changed.
The shower spike — The opposite of a pressure dip can happen too. When you get hot quickly, for instance from taking a hot shower, your body tries to cool down by increasing the blood flow to your skin to release heat. This hot flush temporarily increases the amount of fluid (containing glucose) located just underneath your skin. Because of that, the sensor probe will detect a lot more glucose too, even though this is only underneath your skin surface, and the average blood glucose throughout the rest of your body did not change.
The take-home message is that you should not just blindly trust the readings of your CGM, but keep in mind what affects the interstitial fluid that the sensor probe is measuring. If in doubt, a finger-prick measurement will probably give you the most accurate answer.
Keep an eye out for my next blog, and I hope to see you soon to get you started on the journey to your new life!
Dr. Dennis Van Hoof is a Certified Life Coach (CLC) with an academic PhD degree in Biochemical Physiology. By combining 20 years of first-hand personal diabetes experience with his in-depth scientific background, he developed a method to efficiently manage his own diabetes in a sustainable way. To learn how you can do this too, reach out for personal Diabetic Lifestyle Coaching or follow a group workshop that is specifically tailored to people with Type 1 or 2 Diabetes as well as pre-Diabetics and those at risk due to being overweight or obese. His clients thrive with their challenges and become an inspiration™ to others — with or without diabetes.