FUDiabetes

How does a Dexcom Continuous Glucose Monitor work?

A Continous Glucose Monitor (CGM) inserts a glucose sensor in your skin, and allows you to obtain an ongoing readout of your glucose on a device somehow connected to the sensor (generally through radio waves) until the sensor must be replaced.

Dexcom, founded in 1999, supplies the most common Continuous Glucose Monitoring system in the world.

The components of a Dexcom CGM System

A Dexcom CGM has three components:

  • a sensor inserted in your skin

  • a transmitter, bedded on top of the sensor, that transmits sensor information on an ongoing basis

  • a receiving device, which receives and displays the information transmitted by the sensor. While Dexcom provides a dedicated receiver, many users of the latest generation of Dexcom systems use their cellphone as a receiver.

In addition, Dexcom provides online analytics that allow the patient to review CGM data online and analyze it for patterns.

Use in combination with a pump

Some insulin pump manufacturers (Animas, Tandem) have provided or provide systems that combine a pump with a Dexcom CGM system. In general, these systems do not include a Dexcom receiver, but combine display units.

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The Dexcom Sensor

The same Dexcom G5 / G4 Platinum Sensor is used with both the Dexcom G4 and G5 systems. The sensor frame is approximately 2" x 1" x3/8", barely large enough to wrap around the transmitter. The sensor is a narrow wire, quite flexible, which is the only part that will be inserted under the skin.

DexcomSensorFinger_2017-10-07

The sensor frame comes surrounded by a large adhesive pad, and attached to a kludgy inserter:

DexcomSensorWithApplicator_2017-10-07

The user uses the applicator to insert the sensor fiber:

DexcomSensorInjected_2017-10-07

Once the sensor is inserted, the user removes the applicator, which leaves the small sensor and sensor patch on the skin:

DexcomSensorOnSkin_2017-10-07

The sensor wire, an enzyme electrode, gives the transmitter access to glucose levels immediately under the skin, within the interstitial tissue fluid. It does not directly measure blood glucose levels.

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The Dexcom Transmitter

The Dexcom sensor is a simple analog way to reach into the interstitial fluid. The transmitter holds the microprocessor brain of the system. It is the part of the system that actually measures the glucose level in the interstitial fluid located right by the end of the sensor wire, applies the signal processing algorithms to the measurements, and transmits them over the air to a receiver.

Despite its importance, the transmitter is deceptively small, 1.9" x 0.9":

DexcomG4Transmitter_2017-10-07

Both the G4 and G5 transmitters fit into the sensor bed, so their length and width are the same.

The more modern G5 transmitter is a bit thicker than the late model G4 pictured above (G4 “slim”, a 2nd-generation follow-up to the original G4 “non-slim” transmitter), due to the extra Bluetooth hardware.

https://aws1.discourse-cdn.com/standard14/uploads/fudiabetes/original/2X/e/e21a5f1f755d34ab204847647d7884a5d3b254b2.jpg

Both transmitters end up embedded into the sensor:

The transmitter transmits each new measurement, every 5 minutes, over the air to the receiving device. The range of the receiver is limited to tens of feet when outdoors, sometimes feet when indoors and separated from the receiving device by a wall.

Numbers of DIYers have replaced G4 batteries: the procedure is well documented on the web. Changing a G5 battery has proven to be a harder task, although some have done it and do it. It is often possible to find, online, services that will change G4 or G5 batteries – these services come and go.

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The Dexcom Receiver

Within each of its CGM systems in existence, Dexcom provides a receiver, to receive and display the signal from the transmitter:

The receiver, however, is not the only device that can receive Dexcom data.

Integrated insulin pump and CGM systems

Both the Animas Vibe G4 and the Tandem t:slim X2 insulin pumps can be part of an integrated system with a Dexcom CGM – the Animas with a G4 only, and the t:slim X2 with a G5 only. When purchased in that configuration, they can function as replacements for the Dexcom Receiver. These systems do not adapt insulin pump deliveries to CGM output – they simply consolidate the control and display unit for both parts of the system.

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Dexcom G4 and G5 systems: costs and differences

Dexcom has kept two generations of CGM systems alive in parallel. These two generations have different abilities: the newer generation is not automatically better for all users.

Sensors

The G4 and the G5 systems use the same sensor. A single Dexcom G4/G5 Platinum sensor can be purchased either directly from Dexcom or through various distributors in the price range of $70 ~ $80. The Dexcom G4/G5 sensors are typically sold and distributed with four sensors per box.

The Dexcom G4/G5 sensor is FDA approved for 7 days of continuous use. Many patients can make a sensor last 2 weeks, some can go up to 3-4 weeks.

Transmitter: cost and duration

The G4 and the G5 systems use different transmitters. The transmitters’ footprints are the same, since they must fit in the same sensor bed, but the G4 transmitter is a bit thinner than the G5.

The G4 transmitter costs $600, and has a long-lasting battery, that is guaranteed for 6 months, but can often last a lot longer, up to a year. More importantly, although the manufacturer does not recognize this capability, it is possible to change the G4 battery doing heavy DIY, or have it done by a third party, and reuse the G4 transmitter. This represents very significant savings for self-funded patients.

The G5 transmitter costs $300, half the price of the G4 transmitter, but its duration is limited by software to between 105 and 112 days – even if the battery is changed (a non-trivial DIY procedure) an old G5 transmitter won’t be accepted by a Dexcom G5 system. So, while the nominal cost appears the same, the G4 transmitter system cost can be made a lot lower than the G5’s.

Transmitter: Bluetooth capability
The G4 transmitter cannot broadcast over Bluetooth, while the G5 transmitter can. This means the G5 transmitter can send data directly to cellphones, while the G4 transmitter cannot.

Transmitter: backfilling
Over the course of a day, for one reason or another there is often no receiver within range of a transmitter for some period of time. With a G4 system, the data is lost. However, the G5 transmitter, when communicating with an iOS or Android app (but not with a regular receiver), can backfill up to 3 hours of information – a feature that I find significant in practice. For instance, it makes sports events for kids a lot more manageable.

Receiver

The receiving devices are significantly different between the G4 and the G5 systems. The G4 system has two receivers: the regular G4 receiver, and the G4 receiver with Share (not available in all countries). The G4 receiver with Share can rebroadcast the CGM data over Bluetooth to a cellphone. In turn, the cellphone can relay the data to Dexcom data servers.

The G5 receiver does not have share capability, but the G5 transmitter can be received directly by any iOS phone and by some Android phones, any of which can relay data to the Dexcom servers. So, with a G5 system, you don’t need to use a G5 receiver – instead, you can use your phone as a receiver.

Regular G4 and G5 receivers have an official price of $600, but for patients paying out of pocket Dexcom makes it possible to obtain them for $300.

Receiving CGM info on a watch

If you have an iOS phone, your phone can relay CGM information through Bluetooth to an Apple Watch, in which case you can check your CGM info directly on your Apple Watch.

If you have an Android phone, a combination of opensource applications (NighScout and xDrip) also allow you to receive data on specific Android watch models.

The value of relaying data to Dexcom data servers

There are two valuable benefits to being able to relay real-time CGM data to the Dexcom data servers:

  • Dexcom can provide powerful analytics to the user, who can review the data of days and weeks, and make adjustments based on trends. For that purpose, the data is made available with a 3-hour delay. When using a receiver that cannot use Bluetooth, it is possible to upload the data to the Dexcom servers by connecting the receiver to a computer through a USB port.

  • Dexcom can make that data immediately available to other phones through a Dexcom Follow app. This way, other family members, such as parents or spouses, can have access to the PWD’s real-time CGM data. This is invaluable to parents of CWDs for instance.

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Sharing

As the father of a T1, my life changed when my son started wearing a G5 transmitter, and when I started receiving in real time my son’s CGM data. I don’t believe that I can describe the difference it made to my world to any other person than another parent of a T1 child.

G4 and G5 systems make sharing happen in two different ways. The G5 transmitter can natively broadcast CGM data over Bluetooth. Any iOS phone and many Android phones can receive this data and relay it to the Dexcom servers. So, a PWD wearing a cellphone can, at the same time, receive CGM data on his/her phone and share it with the Dexcom servers. If the Dexcom servers receive the data in real time, any person invited to share the PWD’s CGM data can access it in real time.

DexcomG5TransmitterShare_2017-10-07

The G4 transmitter does not have Bluetooth capability. Instead, a special G4 receiver called the G4 receiver with Share has Bluetooth capability, and can relay the CGM data to the PWD’s iOS phone. In turn, the PWD’s cellphone can relay the data to the Dexcom servers, from which information is available to anyone invited by the PWD to share their real-time CGM data.

DexcomG4TransmitterShare_2017-10-07

Both systems can do the job, The G4 system is a bit kludgier, since it requires the PWD to carry both a receiver with Share and a cellphone.

Nightscout

For those ready to use opensource apps, Nightscout offers the ability to create a cloud repository of CGM data, accessible and shareable. It works with both G4 and G5 systems, and with both iOS and Android, but offers slightly more functionality for Android devices. It does require some amount of setup, as well as some technical knowhow – but a lot of information is available online.

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Analytics

For those PWDs who are able to upload their G5 system data to Dexcom servers (such as G5 users with a cellphone receiver), Dexcom provides a powerful analyitcs app called Clarity (at clarity.dexcom.com):

On Clarity, you can review your data day by day, look at superposed daily pictures of your CGM data, review averages, and review patterns detected by the app. To be frank, the app finds fewer patterns than an experienced user would – but I am sometimes surprised by the app finding a pattern I have not detected.

DexcomClarity_2017-10-07

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Resources: official Dexcom info

Dexcom: what is Continuous Glucose Monitoring?

Dexcom: the Dexcom G5 CGM system

Dexcom: Dexcom Clarity

Dexcom: Dexcom Studio

Dexcom: Moble apps

Resources: Nightscout

Nightscout: Welcome to Nightscout

Nightscout: How do you get your CGM in the sky?

Resources: changing transmitter batteries

iFixit G4 battery replacement guide

Joern’s guide to G4 battery replacement

Youtube: Vince P’s G4 battery replacement demo

Thingiverse: G5 battery cover




End of wiki ---------- comments start here

This is a lot of information. Nice work. This had to have taken a huge amount of time to put together.

I would suggest not to use the word “implants”. As I think of it, this implies the Senseonics approach.

Where you say:
"Dexcom, founded in 1999, supplies the most common Continuous Glucose Monitoring system in the world."
Do you know how many cgm patients there are for Abbott, Dexcom and Medtronic?

"While Dexcom provides a dedicated receiver, many users of the latest generation of Dexcom systems use their cellphone as a receiver."
Some people also use their pump as the receiver.

"As of 10/2017, the current Dexcom sensor is the same for all Dexcom systems (G4 and G5)."
It looks like this was already rephrased. I would suggest another rephrase to make it look not rephrased. lol. I assume this was worded to bear in mind that the G6 will use a different sensor but without making mention of G6 at this time.
“The same Dexcom G5 / G4 Platinum Sensor is used with both the Dexcom G4 and G5 systems.” - Maybe?

"The so-called transmitter also holds the microprocessor brain of the system."
The word/phrase “so-called” seems a bit out of place with no additional explanation. It might mean the transmitter does much much more than simply transmit but I would suggest simply remove “so-called”.

"Transmitters from all systems fit into the sensor bed, so their length and width are the same."
Similar to above, write for anticipation of the G6. Suggest rephrase as:
Both the G4 and G5 transmitters fit into the sensor bed, so their length and width are the same.

"The transmitter transmits each new measurement, approximately every 5 minutes, over the air (normally using direct line of sight) to the receiving device.“
Two suggestions. Remove “approximately”. Just call it 5 minutes. Remove direct line of sight as that implies IR and the signal easily passes through many things. Well except apparently a human body.
The transmitter transmits each new measurement, every 5 minutes, over the air to the receiving device.”

"Within each of its CGM systems in existence, Dexcom provides a receiver, to receive and display the signal from the transmitter:"
With the Animas Vibe G4 systems, no Dexcom Receiver was provided. The pump was the sole receiving device although SmartPhone usage was/is/?? also allowed.

"Dexcom has kept two generations of CGM systems alive in parallel – the reason is that these two generations have different abilities."
Perhaps. But also perhaps due to the fact that once a pump is integrated with a particular Dexcom device, I believe Dexcom is required (??) to continue to provide that cgm for at least the users of those pumps for at least the duration of the pump warranty? Unless of course they pull a J&J and refuse to fully honor the warranty period for which users signed up for and expected. But I think Dexcom is a much better company than J&J. Blah. Sorry. Sideways rant.

"A Dexcom sensor can be found for approximately $70."
I would suggest rephrase from “found”.
A single Dexcom G4/G5 sensor can be purchased either directly from Dexcom or through various distributors in the price range of $70 ~ $80. The Dexcom G4/G5 sensors are typically sold and distributed with four sensors per box.
"Dexcom recommends it be used up to 1 week. " Suggest revision to point to the FDA.
The Dexcom G4/G5 sensor is FDA approved for 7 days continuous use.

"The G4 transmitter costs $600, and has a long lasting battery, that is guaranteed for 6 months, but can often be made to last a lot longer, up to a year."
Remove “be made to” as no action is required. It simply runs until it dies.
The G4 transmitter costs $600, and has a long lasting battery, that is guaranteed for 6 months, but will often last longer up to 9~12 months.

"The G4 system has two receivers: the regular G4 receiver, and the G4 receiver with Share (not available in all countries) which broadcasts data over Bluetooth to a cellphone. "
Minor rephrase.
The G4 system has two receivers: the regular G4 receiver, and the G4 receiver with Share (not available in all countries). The G4 receiver with Share can rebroadcast the cgm data over Bluetooth to a cellphone.

In general where receiver is discussed, add some points about both the Animas Vibe G4 and the Tandem t:slim X2 can integrate and function as replacements for the Dexcom Receiver.

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This is outstanding feedback, @Thomas, thanks so much!

4 posts were split to a new topic: FUD wikis: how to display them better?

No. I have heard it mentioned that about 1M patients wear a CGM worldwide. On T1D exchange, a metrics about the registry is that 15-20% of T1Ds wear CGMs in the US – but that is self-reporting with self-selection, hard to see what is real. The penetration around the world is much lower than in the US.

However, since the US are the #1 CGM market, and Dexcom is what basically everyone wears as far as self standing CGM, I figure there is no doubt Dexcom is it.

I had wondered how many people were using Dexcom. But anywhere close to 1M would certainly put Dexcom as the leader by far.

"May 8, 2017
Abbott’s FreeStyle Libre system was introduced across Europe in 2014, and is now available in more than 30 countries and used by more than 300,000 people with diabetes around the world. "

I believe the G5 transmitter is actually a bit thicker than the G4 because it has extra gadgetry inside for Bluetooth capability

Unfortunately, I can’t find a good quotable source :frowning: I am not even sure the number is true, since I can’t verify sources.

I don’t think that the Libre counts as a CGM though, since you need to handscan.

Can someone validate that? I don’t have a G4 transmitter, but went by what I read from users moving from G4 to G5 – so my info is second hand.

I assumed G5 thinner made sense because of smaller battery.

I can’t find a good reference but if I recall correctly when I was shopping for them around a year ago the sole remaining perceived benefit of the G4 was its slightly smaller size of the transmitter… for people who are bothered by the siZe of the device itself— for the tradeoff of lack of Bluetooth. Terry on tud explained a fair bit about the differences between the two to me back then and eventually I ended up with the g5

Here’s a review that mentions size

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Got it, thanks so much – I am making the change now.