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:
- fingerstick checks involve piercing the skin of the fingertip or toe each time you check blood glucose
- CGM involves inserting a fine electrode under the skin for several days
- fingerstick readings measure glucose in a drop of blood
- CGM measures glucose in the interstitial fluid just underneath the skin, where cells get oxygen and nutrients (including glucose).
- 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:
1. The Sensor:
A sensor is a thin, flexible wire (electrode, or filament) covered in a bio-compatible material (to camouflage the sensor from the immune system so it’s accepted by the body). 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 the 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 or 7 days (depending upon the manufacturer) and then thrown out.
2. The Transmitter:
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 (non-replaceable, non-rechargeable) battery dies, and then you throw it out. Different manufacturers and models have different transmitter warranty periods (for more information, see Comparison of CGM Systems Available in Canada).
3. The Receiver :
The receiver “receives” the data from the transmitter and displays it in one of a few different user interfaces. The receiver could be:
- a stand-alone hand-held device (Dexcom G5), or
- integrated into the display of an insulin pump, such as the Medtronic MiniMed 630G, or the Animas Vibe (previously available in Canada), or
- an electronic device (smartphone or tablet) that contains the relevant software, such as the Dexcom G5 Mobile 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 2 metres (~6 ft); Dexcom G4 and G5 will maintain transmitter-to-receiver communication for 6 metres (~20 feet).
Glucose Information & Trends
The receiver displays the following information:
- the current glucose reading
- past readings, displayed as a line graph, or glucose “trace” (24-hours on screen, and all past data via computer download)
- glucose trends: uses trend arrows to 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!
The receiver also gives you alerts:
- 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. (see CGM chart)
- Rate of Change Alerts tell you is your glucose is rising or falling rapidly.
- Out-of-Range Alert tells you when the receiver and transmitter are too far apart to communicate with each other.
- “Replace Sensor” Notification tells you that the 6 or 7 days are almost up, it’s soon time to remove the current sensor.
In terms of low glucose, the different manufacturers have different features to notify users regarding low blood glucose:
- Dexcom G4/G5 (in addition to the Low Alert that you set) has a Low Glucose Alarm that goes off when glucose drops to 3.1 mmol/L or lower. This alarm cannot be adjusted, and cannot be turned off; it will sound every 30mins until glucose is more than 3.1 mmol/L (to give you an extra “heads up” that you need to take action).
- Medtronic 630G ( in addition to the low alert) has:
- A separate 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 – uses current glucose reading, plus trend info (used to be called “Predictive Alerts” in the Veo – you could choose 5-30 minute-warning – now hard-wired for 30 mins)
- Suspend On Low feature that turns off insulin delivery if the user doesn’t respond to the low alert (when glucose has dropped to or below the low threshold set by the user)
The battery-type for receivers depends on the model, and vary from replaceable (non-rechargeable) AA Lithium to a custom rechargeable (not replaceable) battery, which may need to be recharged every 3-5 days; it can be recharged for about a year until it will no longer accept a charge as so the entire receiver needs to be replaced. Battery warranties vary by manufacturer and model, for more information see the Comparison of CGM Systems Available in Canada.
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This material has been developed from sources that we believe are accurate, however, as the field of medicine (in particular as it applies to diabetes) is rapidly evolving, the information should not be relied upon, as it is designed for informational purposes only. It should not be used in place of medical advice, instruction and/or treatment. If you have specific questions, please consult your doctor or appropriate health care professional.