Continuous glucose monitor - is it reliable?

Is a CGM reliable? i asked my new endo about it and he said they arent reliable and do not improve glucose control. He said they are "delayed" in readings and provide false blood glucose because its not from your finger. I have read numerous studies that state otherwise and say it definitely improves glucose control - any feedback on this?

I have been on one since late april. The CGM is not to be used for giving a bolus or to "monitor" or diabetes. What it is there for is for when you go high, or are going low. Sometimes my CGM is right on only off by a few points. Other times it's off by 15-20 points. There was a recent study released saying that it does help improve control for most people who use it. I have only been on it a month and feel lost with out it on. I think it is a great tool for anyone who can get it.

The readings are delayed but they it's only about a five minute lag time. Which can help you if you are getting high and the alarm goes off before it gets real high. Or if you are getting low so you can see that you are trending low.

Any more questions ask!


oh wow - good to know! my doc said it was like a really long lag time and that it was NOT good for highs or lows! I know it doesnt replace a prick or so throughout the day but from what i have read it helps continually in getting your blood sugar readings. Thanks!

Josh is right.

The lag of the CGMS depends on what you are using. The Minimed System is about 15 minutes delayed where as I believe the Dexcom is five minutes. I might be wrong on that. While the accuracy flactuates I would never rely upon it soley for anything. As Josh stated sometimes it is dead on other times it can be off. The times though that the CGMS tends to be off is when you have either a rapid increase or decrease of what your Blood Sugar is. Than you cannot rely on it for perfect information. However, it has been very helpful to me this past year. I have been more aware of when my sugar spikes and especially when it goes low which is quite important. It has helped with my a1c but I was never really worried about it. Right now the CGMS is still not 100% but from what I have read the accuracy is usually pretty close somewhere around 75% - 85% of the time.

[quote user="venere1"]

oh wow - good to know! my doc said it was like a really long lag time and that it was NOT good for highs or lows! I know it doesnt replace a *** or so throughout the day but from what i have read it helps continually in getting your blood sugar readings. Thanks!


I use the MInimed system and there is a lag time when your BG is rapidly changing, like during exercise, after a meal or after a correction bolus.  I find that between meals during the day the readings are close to my meter.  CGMS isn't meant as a replacement for fingersticks and I still test often, though not as much as I used to unless I think a sensor is acting up, or I've just started a new one and want to make sure it's working okay.

It is great for spotting trends.  You can see if/when your numbers start to rise or fall overnight.  It's shown me that it's best to bolus before a meal instead of right when I'm about to eat (unless my BG is already low to begin with).


I've been on the Freestyle Navigator for about 4 months now and I love it.  It's great to be able to see which direction my blood sugar is's REALLY saved me in more than a few situations.  And I haven't noticed any lag time at all.  There have been occasions when I've checked my BG with it, and then done an actual blood test, and they've been really close, if not exact.  The most it's ever been off is about 25 points (on the low side), when I've actually been a bit higher.

Since I've been on it my last A1C was 5.8.  I never had bad A1C's...always in the low 6's, but now I can keep even tighter control on it.

Some of the CGMS are unreliable. Others are quite reliable. There are a lot of variables that factor into reliability - the most important of which is pt use and expectations. It takes understanding that there will be times when it is not within 5 points. It also takes the acceptance that some sensors won't function as well.

All of this being said, I can tell you that I rely on my almost entirely. In fact, I relied on my Dexcom (STS - SEVEN - MANUAL CALIBRATION - PLUS) throughout my last 2 years of medical school as basically a glucometer. Yes, this is far outside the FDA-approved use, but it was within 5-10 points most of the time. Using the trending and the numbers, the Dexcom CGM enabled me to travel the wards freely. I did have sensors fail and some sensors were less accurate. However - they have a liberal replacement policy and if ever a sensor was not functioning optimally, I received a free new one.

Also important for women out there - the CGMS are a MIRACLE for pregnancy. Tight control and hormonal fluctuations were much more easily managed using the Dexcom through my second pregnancy than without it for my first.

Its an incredible device. If you meet anyone who uses the Dexcom system - 99.9% of them will say they feel completely LOST (as Josh mentioned) without their system. Even if you've had diabetes for 50 years, once you have the system, its virtually impossible to go without it. I think a real testament to that is the number of people who paid and continue to pay out of pocket for the device and sensors each month.

If your doctor would like more information, refer him to the studies performed by Satish Garg out of the Barbara Davis Center. He is very well-known in the diabetes field and has performed many studies using CGM.

Feel free to PM me if you have any specific questions!

thats great to know! i dont know why my dr. was so against it?? even if it WAS 20 min lag time it is still better knowing many different blood sugar levels over 24hrs than say just 3-4 blood sugars a day - and if it seemed questionable what the cgm was reading than just prick your finger...needless to say i was not impressed with my new endo and didnt go back to him but when i find a new dr i am definitly going to refer him to the studies you referred to...thank you so much! this is all very helpful! 

have you ever heard of the guardian real time cgm by medtronic? that was the one i was really interested in getting....are you on the pump? 

The MM Gauradian Real Time System is their CGMs for those that do not use a pump. It has all the same functionality as the Paradigm pump, it just does not do any pumping. As I mentioned earlier I really like the Minimed system and the CGMS with my pump for the past year or so has been great. While I am trying to switch to the Dexcom system, it is only because I am switching pumps. Again, I have no issues with Medtronic I was just really impressed with the Ping, so I am switching.

Hi venere1 -

I've tried the MM CGM that communicated with the Paradigm pump (as Brian said - same as the Guardian). In my opinion, the CGM technology is not as robust as that of the Dexcom. The lag time is longer and the accepted accuracy is lower. Yes, there are so many different studies comparing the two but until you've worn them, its impossible to know what the studies mean.

Personally, I would discourage you from wasting the money to go with Minimed before trying both the Minimed and the Dexcom. In my experience, so many patients start out with the Minimed and get so frustrated it ends up in a drawer. Alternatively, the CGM is used just for retrospective trending and thus offers no benefit for the immediate, life-threatening emergencies that face us (hypoglycemic-related events are the leading cause of diabetes-related death for diabetics 16-40). 

If you don't have a minimed pump - it would be a collossal waste to purchase an independent CGM using the MM technology.

I have very strong feelings about this (clearly).