An Introduction to CGM
You may have heard that CGM can help your family to identify + prevent lows, but what exactly is CGM, and how does it do this? We'll also explain the benefits of CGM compared to fingersticks, the parts of a CGM system, the difference between BG and SG, what glucose trends are, and how CGM alerts and alarms can help.
What Is a Continuous Glucose Monitor (CGM)?
A Continuous Glucose Monitor (CGM) is a medical device for measuring glucose levels just underneath the skin; it is used most often by individuals who have type 1 diabetes. It performs the same function as a “fingerstick” blood glucose meter (that is, it gives you information about glucose level). There are, however, a few critical differences between CGM and fingerstick checks. Let's explore those differences...
If you're interested in Flash (Libre):What is Flash Glucose Monitoring?
- fingerstick checks involve piercing the skin of the fingertip or toe each time you check blood glucose
- CGM involves inserting a very small wire under the skin for several days
- fingerstick readings measure glucose in a drop of blood. Therefore the resulting number is called "blood glucose" (BG).
- CGM measures glucose in the interstitial fluid just underneath the skin, where cells get oxygen and nutrients (including glucose). Since the amount of glucose is measured by a sensor (see below), the resulting number is called "Sensor Glucose" (SG).
- fingerstick glucose monitoring is like a snapshot: it captures a moment in time
- CGM is like a movie: gives you a series of readings in real-time, as it repeatedly measures and reports glucose levels (the equivalent of performing 288 finger pokes each day). This gives you information about what’s happening between fingerstick checks.
In short, fingerstick blood glucose testing gives a number for a single point in time; CGM shows where glucose is, where it's going, and how fast it's getting there.
The Component Parts of a CGM
A Continuous Glucose Monitoring system consists of:
A sensor is a thin, flexible wire coated in glucose-sensing enzymes in a bio-compatible material (to camouflage the sensor from the immune system so your body doesn't reject it). The sensor sits just under the skin. It’s inserted using an automatic insertion device, including an introducer needle (which is removed after insertion, leaving only the thin sensor behind); the sensor is then held in place by an adhesive patch, in much the same way that an insulin pump infusion set is held to the skin. Once in place, the sensor is the part of the system that actually measures (“senses”) glucose levels, taking a glucose reading every 5 minutes. Sensors are disposable: one is worn for 6 to 10 days (depending upon the manufacturer) and then thrown out.
The transmitter clips into the sensor housing; it takes the glucose reading obtained by the sensor and sends it out (via wireless signal) to a separate receiver.
A transmitter lasts 3-6 months (or more) until the internal battery dies. Different manufacturers/models have different batteries: Medtronic transmitters are rechargeable for a year or more; Dexcom transmitters are not rechargeable, the transmitter is discarded every 90 days. These companies offer different transmitter warranty periods accordingly (for more information, see Comparison of CGM Systems Available in Canada).
The receiver “receives” the data from the transmitter and displays it in one of a few different user interfaces:
- a stand-alone hand-held device, or
- integrated into the display of an insulin pump, such as the Tandem t:slim X2, the Medtronic MiniMed 770G/670G/630G (or some of you may remember the Animas Vibe, previously available in Canada), or
- an electronic device (smartphone or tablet) with the relevant app.
The receiver must stay within range of the transmitter for data to be transmitted in real-time. Early CGM systems had an unreliable and short range of only a few feet but current systems do much better than that: Medtronic CGM’s have a range of up to 2 metres (~6 ft); Dexcom's will maintain transmitter-to-receiver communication for up to 6 metres (~20 feet).
The battery-type for receivers depends on the manufacturer and model, and varies if you use a stand-alone receiver, a pump as the receiver, or a smart device. For more information see the Comparison of CGM Systems Available in Canada.
Glucose Reading + Trend
The receiver displays the following information:
- the current glucose reading
- past readings, displayed as a line graph, or glucose “trace” (up to 24-hours on screen, and all past data via computer download)
- glucose trends: show if your glucose is stable, rising or falling, AND how quickly glucose is changing. Trends are important information, because if glucose is 5.5 and headed down rapidly, that’s very different from 5.5 and rising! Trends may be illustrated visually using arrows. There are differences in how each manufacturer and model illustrates glucose trends, but it may look something like this:
The receiver also gives you information about your current and predicted glucose readings, plus a "heads up" about how everything is functioning:
- High Glucose Alert tells you when your glucose crosses an upper threshold that you set. The threshold is adjustable, and can be set as high as 22.2 mmol/L.
- Low Glucose Alert tells you when your glucose crosses a lower threshold that you set. The threshold is adjustable, with a different minimum for different CGM manufacturers.
- Rate of Change Alerts tell you is your glucose is rising or falling rapidly.
- Out-of-Range or Loss of Signal Alert tells you when the receiver and transmitter are too far apart to communicate with each other.
- “Replace Sensor” Notification tells you that the sensor life is almost over.
In terms of low glucose, the different manufacturers have slightly different features to notify users regarding low blood glucose:
- Dexcom (in addition to the Low Alert that you set) has:
- Urgent Low Glucose Alarm that tells you when glucose is very low (at or below 3.1 mmol/L); it cannot be adjusted, and cannot be turned off; it will sound every 30 mins until glucose is above this fixed threshold.
- Urgent Low Soon Alert that lets you know that BG is falling fast and is predicted to be low within 30 minutes.
- Medtronic ( in addition to the low alert) has:
- A predictive Alert Before Low, that is triggered when you approach the low limit you’ve set – it’s hard-wired to go off 30 mins before the algorithm predicts you will cross the low threshold you’ve set, and uses the current glucose reading, plus trend info.
Both Dexcom and Medtronic also offer features that help prevent low glucose, when you use CGM integrated with an insulin pump: the pump will automatically adjust or suspend insulin delivery when sensor glucose is low, as well as when it is predicted to become low. For more information on these features, check out More on Medtronic and More on t:slim.
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