Continuous Glucose Monitor

A Continuous Glucose Monitor, or CGM for short, is a small measuring device worn on the skin. A thin filament sits underneath the skin, continuously measuring how much sugar is present in the tissue. Sugar in the blood, also called glucose, is the body’s most important source of energy. In the disease diabetes, this value gets out of balance and must be monitored. The sensor sends a new reading to a phone or a small reader device every few minutes. Instead of individual measurements taken during the day, this produces a continuous curve.

From finger pricks to a continuous curve

In the past, people with diabetes had to prick their finger several times a day. Each prick delivered a single value, like a photograph. What happened between two measurements remained unknown. A CGM, by contrast, delivers a movie: you can see whether the value is currently rising, falling, or holding steady.

This is more than just convenience. A dangerous drop in blood sugar at night often goes unnoticed with isolated measurements. A CGM can sound an alarm before the value becomes critical. Many devices already warn when the curve is pointing sharply downward. In individual cases, this early warning saves lives.

CGMs are also of interest to tech companies because they generate a great deal of data. A single sensor produces around 300 readings per day. Over months, this adds up to data series from which software can learn patterns. This is precisely where medical technology meets artificial intelligence.

What the sensor actually measures

The filament under the skin does not sit in a blood vessel but in the fluid between the cells. An enzyme, a biological helper substance, sits on this filament. It reacts with the sugar and in doing so generates a tiny electrical current. The more sugar present, the stronger the current. The electronics in the patch convert this current into a sugar reading.

This leads to an important limitation: the sensor does not measure directly in the blood. The tissue fluid lags behind the blood by roughly five to fifteen minutes. After a sugary meal, the CGM therefore shows the rise with a slight delay. Modern devices partly compensate for this using computational models. A common misconception is that any deviation from the blood value automatically means a malfunction.

Depending on the manufacturer, a sensor lasts about 7 to 15 days before it is replaced. The data is sent wirelessly to an app, usually via Bluetooth, a short-range wireless connection. There, it is translated into curves, daily patterns, and statistics. Increasingly, algorithms also suggest how much insulin would be needed.

From the hospital to the fitness market

CGMs are most commonly encountered among people with type 1 diabetes. There, they often work together with an insulin pump. The sensor reports the value, the software decides on the dose, and the pump delivers it. Such systems are referred to as an artificial pancreas because they mimic an organ. They are a good example of how algorithms take over medical decisions.

For a few years now, manufacturers have also been selling CGMs to healthy people. Athletes and nutrition apps advertise them as a way to optimize one’s own metabolism. In the US, the first sensors are available without a prescription. Whether the benefit for healthy individuals has been proven is a matter of controversial debate among experts.

For this reason, CGMs regularly appear in business news. The market is considered a billion-dollar business, dominated by a few manufacturers such as Dexcom and Abbott. At the same time, tech companies are working to integrate such sensors into watches. A pain-free measurement without a needle would be the next big step, but this has not yet been reliably solved technically.