# What a CGM measures (CGM)

A thread-thin filament under the skin reads glucose in the fluid between cells, every few minutes, for 14 days; it is not blood, which is why it lags a finger-prick.

Type: technique · Unit: mmol/L · Domain: Metabolic Health · Accuracy: Measured · Importance: Core

Also called: continuous glucose monitor, glucose sensor, Sibionics GS3, Libre, interstitial glucose, how a CGM works

## What it is

A continuous glucose monitor is a coin-sized sensor worn on the back of the upper arm. Under the adhesive patch, a soft filament thinner than a hair sits a few millimeters into the tissue beneath the skin and measures glucose in the fluid between the cells there, every five minutes, day and night, for fourteen days. A transmitter sends each reading to your phone by Bluetooth; the sensor stores two weeks of readings if the phone is away.

## How it works, in a sentence

The tip of the filament carries an enzyme that reacts with glucose and produces a tiny electrical current in proportion to how much glucose is in the fluid around it; the electronics turn that current into a glucose figure using the maker's factory calibration. The sensor VT distributes at launch, the Sibionics GS3, needs no finger-prick calibration from you.

## What it is not

It is not a blood glucose measurement. The fluid between cells follows blood with a delay: measured directly with tracers in eight healthy adults in the fasted state, glucose took about five to six minutes to cross from the blood into the tissue fluid (Basu 2013), and after a meal, when glucose is rising fast, the sensor runs further behind. That is why a sensor and a finger-prick rarely agree to the decimal, and why the sensor's own figure for how far it sits from the laboratory, a mean absolute relative difference of 8.83 percent in adults for the GS3 (the manufacturer's figure), is a measure of ordinary disagreement, not of fault.

## Strengths

It is the only instrument that watches your glucose while you eat, train, sleep and get stressed, 288 times a day, for weeks; a laboratory test is one point, a finger-prick a handful. It costs less than a month of coffee. And it reads your responses, not the average person's: in 800 adults wearing sensors for a week through 46,898 meals, the same food produced widely different glucose responses in different people (Zeevi 2015).

## Weaknesses

The lag; the first day's unsettled readings; the night you lie on it; the sensor that fails early; the fact that it reads one arm of one body, and the other arm may read differently. The gotchas entry takes each of these in turn. And it is a medical device made for people with diabetes, whose label says so; wearing one without diabetes is a choice to look, not a treatment.

## What Vitality Tracker does with it

VT reads your readings from your sensor account (the Sibionics account at launch; Libre 2+ through LibreLinkUp, and Ottai, for members who own them), keeps every reading with the sensor it came from and the site it was worn on, draws your meals, fasts, training, drinks and the time you woke on the same line, catches impossible readings and the warm-up hour before they reach your averages, and builds the fourteen metrics on this shelf from what is left. Every sensor you have worn is one history, with your Baseline Month as the reference.

## How to get one in New Zealand

From the VT store, without a prescription: the Baseline Month pack holds two GS3 sensors, enough for 28 days, and single sensors follow. Delivery within New Zealand. If you have diabetes, read the sensor's intended-use statement and talk to your doctor first; the sensor and its app are built for you, and VT is an addition, not a replacement.

## Sources

1. Basu A, et al. Time lag of glucose from intravascular to interstitial compartment in humans. Diabetes 2013;62:4083–7. [doi:10.2337/db13-1132](https://doi.org/10.2337/db13-1132)
2. Zeevi D, et al. Personalized nutrition by prediction of glycemic responses. Cell 2015;163:1079–94. [doi:10.1016/j.cell.2015.11.001](https://doi.org/10.1016/j.cell.2015.11.001)
3. Sibionics GS3 product page (specifications, intended use), read 19 October 2026
4. Vitality Tracker, CGM page brief (19 October 2026).

## Related

- [What a healthy adult's sensor shows](https://vitalitytracker.app/knowledge/what-a-healthy-adults-sensor-shows)
- [Why a month](https://vitalitytracker.app/knowledge/why-a-month)
- [Reading a CGM without fooling yourself](https://vitalitytracker.app/knowledge/reading-a-cgm-without-fooling-yourself)
- [Mean glucose](https://vitalitytracker.app/knowledge/mean-glucose)
- [Sensor coverage](https://vitalitytracker.app/knowledge/sensor-coverage)

Reviewed October 2026 · v1

Source: https://vitalitytracker.app/knowledge/what-a-cgm-measures