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CGM test method - interstitial or blood??

AJB_81

Well-Known Member
Messages
55
Type of diabetes
Type 1
Do all CGMs measure interstitial fluid glucose? Or are there some on the market that measure blood glucose directly? Any experts out there know?
 
They all measure interstitial fluid, the fluid in which all body cells are 'bathed'. As far as I am aware there are no CGMs measuring blood glucose directly, which I suspect would be a rather different kettle of fish, just in terms of getting it inserted in the right place, and having the sensing technology constantly in contact with blood. Also, and I'm not a techie person, I wonder if it might be affected by the different rates of blood flow between people, given the extent to which heart rate can vary between individuals, even throughout the day, whereas interstitial fluid just sits there most of the time.

@tim2000s is the ranking expert on all things tech, and I'm sure will be able to correct me if I've just spewed a load of rubbish.
 
Thanks for the info.
Currently use Libre but thinking of giving Dexcom G5 a go as the low alarm function must be tremendous, particularly for night hypos. Libre is amazing but often shows rather concerning trends, again at night!
Thought I'd check if technology only measures interstitial, which I suspected it did, but thought it worth checking, before buying the Dexcom
 
andybraith said:
Libre is amazing but often shows rather concerning trends, again at night!
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Are you checking your libre results against a blood reader? It works very well for many (most?) people, but can under read very badly for others and give false low readings...
 
Yeah, always checking highs & lows (if awake!) with blood tests. My experience of sensors is that they are pretty close, close enough to spot trends, trim basal and observe general BG trends to predict highs/lows to treat before effects are felt
 
I've dealt with most of the large venous blood access ports doing heart caths and pacemaker implants over the years. The problem would be having the smaller veins clot off, which rarely every happens with a pacemaker (subclavian vein is big, pacer leads pretty thin), or getting in and out of the large veins with the sensing lead multiple times. With a pacer you do it once and the leads stay there forever usually. Not sure a sensor would ever do that. And if one could leave a sensor lead in there for years it would have to be put in there by a doctor. So that would increase the price.
 
Thanks, I'm no medical expert but what you have said makes sense to me.
I have been confused by people saying Libre isn't CGM and Dexcom is... so wondered if I was missing something. For me, now I now they measure the same thing, they are essentially the same thing - CGM devices whether the device pushes info, or you scan it.
 
As you may well know the Libre is a Flash Glucose Monitor - because you have to 'flash' it with a reader, although it does continually monitor the glucose levels in interstitial fluid (which lags behind blood glucose), so although not generally called a CGM it is, it's just missing the transponding bit, which brings me nicely to blucon/nightrider - which fits over the libre and uses Bluetooth to ping the results to your phone, scott-c has an amazing thread about it so do a search for it.

On another note the Libre can sometimes show a compression low - when you're laying on it. So don't take the night time graph as gospel.
 
I've used both the libre and the dexcom. I gave up on the libre because the accuracy was just so hit and miss, and I also wanted the low/high alarms without having to scan - I do a lot of lab work, and so by the time I've taken my gloves off and washed my hands to scan, I might as well just have done a finger prick. Generally, I find the dexcom to be a lot more accurate, although it too is subject to compression lows, like the libre. The 'follow' option has also been wonderful for me - boyfriend has the follow app on his phone, so if I'm having a hypo and it isn't coming up he gets an alert and can check on me. The dexcom is more expensive than the libre, primarily because of the transmitter, but although they advertise the sensor as being only for a week, I normally get 2 - 3 weeks out of mine. In terms of insertion, the libre is definitely easier, as the dexcom can be a bit of a fiddle, but for me the dexcom seems to stay down better, although that seems to vary from person to person. Also, it seems to me as though the dexcom isn't *quite* as far behind as the libre, though that could just be my interpretation.
 
andybraith said:
I have been confused by people saying Libre isn't CGM
Click to expand...

A monitor is expected to have alarms that will alert when BG gets too low or too high, and that the alarms can be depended on to prevent death. I would not trust my life to a Libre, but it is a great tool for learning how your BG responds to food/meds.

(Remember that an alarm that goes off when it should not, is very harmful as people start to ignore the alarms and that Libre is well known to give invalid readings of low BG.)

PS, I don't know if intensive care units have CGM working from blood yet. Once faster "insulins" are on the market, there may be a case for CGM being implemented so as to enable better "closed loop" pump systems.
 
ringi said:
PS, I don't know if intensive care units have CGM working from blood yet. Once faster "insulins" are on the market, there may be a case for CGM being implemented so as to enable better "closed loop" pump systems.
Click to expand...
They don't. They do fingerpricks and use glucometers. Then the nurse enters the reading into the EMR (electronic medical record) manually so everyone can see it.
 
Libre, as standard, doesn't alarm. This is a shame as lots of people that use them for the first time identify night hypos and it would be handy if they were woken up bulky the event, not finding out in the morning. Dexcom alarms when BG drifts low so maybe leading the field for readily available CGMs. Maybe Abott are better at PR and have the NHSs ear at the moment, just a shame the device doesn't alert user when things are drifting off...
 
Libre wins hands down on cost! And hence is the only option for lots of people. Hence a lot more people have experience of using them.
 
andybraith said:
just a shame the device doesn't alert user when things are drifting off...
Click to expand...

Pop a reusable £100 Blucon transmitter from Ambrosia Systems on top of it, pair it with the free xDrip+ app on an android phone, and libre can be calibrated to improve accuracy to a comparable extent to dexcom and provide customisable alerts...been using this set up for about 4 months, very happy with it, full on cgm for 100 quid.
 
Not sure Libre is cheaper... can get set up with G5 device and a sensor for £200, sensors are then £50 thereafter - so the same as Libre. The benefit from Dexcom is the sensors generally last longer 3/4 weeks isn't unheard of and can link easy with most devices, mainly Apple in our house. Be great to glance at my watch to see BG.
Still a big fan of Libre, will have a look at this Blucon thingy, thanks!
 
@andybraith you do have to replace the transmitter on the dexcom every 120 days or so, at £200 a pop, which is where the dexcom really becomes more expensive. If you are on android, i believe you can get longer out of them by using xdrip, though.
 
Ah, didn't realise there was additional costs for transmitters - I assumed once I had one the only cost would be new sensors. I have details from Dexcom but didn't notice that. Might try a blucon, but from a quick read it doesn't look like this will alarm either??... more research to do.
 
EllsKBells said:
@andybraith you do have to replace the transmitter on the dexcom every 120 days or so, at £200 a pop, which is where the dexcom really becomes more expensive. If you are on android, i believe you can get longer out of them by using xdrip, though.
Click to expand...
True the transmitters have to be replaced every 3-4 months with Dexcom. But don't you have to replace the "Readers" with the Libre every few months? Either way you have something else to buy. Or does that $100 Bluecom transmitter make it so you don't have to buy the transmitters at all and use your cell phone. How long does the 3rd party Bluecom transmitter for the Libre last? All this sensor/reader/transmitter stuff gets confusing right.
 
No, you don't have to replace the libre reader. It's rechargeable. The libre doesn't have a transmitter, which is why it doesn't give you a constant output of information. I have no idea how bluecon works though.
 
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