A continuous glucose monitor (CGM) is a small sensor worn on the body that measures blood sugar every few minutes, day and night, and sends the readings to a phone app. It replaces most of the finger-prick tests that people with diabetes otherwise do several times a day. In India, CGM sensors are sold at chemists and online. They are used by people with type 1 and type 2 diabetes, and more and more by people with prediabetes, the stage before diabetes.
The sensor is a small disc, about the size of a coin, stuck to the back of the upper arm or to the belly. A thin, flexible filament, a few millimetres long, sits just under the skin. It does not measure the blood itself. It measures the sugar in the fluid between the cells, which follows blood sugar with a delay of about 5 to 15 minutes. A chemical on the filament reacts with glucose and produces a tiny electric current. The sensor's chip converts the current into a glucose value, in mg/dL. A sensor lasts 10 to 15 days, and then the user replaces it with a new one.
Take, for example, a 14-year-old girl in Guwahati with type 1 diabetes, who needs insulin injections every day. Her sensor sends a reading every five minutes to the app on her phone. That is 288 readings a day, each stamped with its time, which makes them time series data. The app draws the readings as a curve. At 2 a.m. one night, her sugar falls below 70 mg/dL. The app sounds an alarm on her phone and on her mother's phone, so that her mother wakes her to drink some juice. Without the sensor, the low sugar might not have been noticed until morning.
The app also calculates "time in range": the share of the day for which sugar stays between 70 and 180 mg/dL. Doctors commonly aim for more than 70% of the day. Her doctor reads a two-week report before each visit.
The same readings go to other products. An insulin pump can use them to adjust its insulin dose automatically. Coaching apps, such as a CGM-based diabetes reversal programme or a digital diabetes management app, mark meals on the curve. In the United States, doctors' practices bill for reviewing the readings through remote patient monitoring software.
The software that converts the current into a glucose value and sounds the alarm is regulated as a medical device. So each change to it must be approved, as explained in software as a medical device (SaMD).