Anybody else getting CGMS readings that wildly disagree with meter BG's?

Hi, I just started the CGMS last week and things have gone well overall. I had an instance last night where it said “WEAK SIGNAL”, but moving my pump closer helped. My problem has been with my meter BG’s radically differing from the CGMS readings. My trainer told me to expect discrepancies of up to 20 points, but I’ve had several that differed by as much as 50 points! I just did a meter BG and it was 241, but my CGMS showed 194…I haven’t eaten in 3 hours. I’m trying to not overcorrect my BG’s for a few days on my trainer’s advice. I realize I need to still rely on meter BG’s for my actual numbers, but are large discrepancies like this normal?

Hi Angi !

Difference comes from point where your measure BG. If you measure from finger you get real value, but since CGMS reads from different place, you get different value… Theoretically after 15-30 minutes, value on CGMS should be the same as measurement from finger. As for difference between finger and CGMS in points… This difference will never be the same… For example if you BG is falling very rapidly, difference can be bigger… And since CGMS is experimental device, this things can happen.


Yesterday, it was amazing! The CGM and my bloodsugars were exact, that only happens once in a while for me. Usually they are pretty close BUT some days I wonder why I am wearing it because they are so far off. I guess it really depends on when you calibrate. But Andy was right saying that these things can happen because it isn’t 100% yet…

Hi Angie. For me (Dexcom), the first day is MUCH less reliable than than the following weeks. It seems as if the “warm up period” really needs much longer than two hours to stabilize.

I never got good results from MM when I tried it (about two years ago). But you might be having the same effect which I have on Dexcom. Usually, when I’m having really bad results after the warm-up, it’s a lot better to just restart the thing than it is to enter additional calibrations. new calibrations make adjustments to the “flakey” numbers, the flakey results are still being used. A full restart (with a fresh two-hour warmup) clears things up instantly.

And, have you tried some alternate sites? my frontal abs never worked as well as my love handles or butt cheeks. I know that for appearance and clothing issues butt cheeks might not be viable, but love handles (near the belt line, straight below the ribs) might work better for you too. If you sleep on your side, though, you’ll need to make sure that your Sensor is on the “up side”, and not being squashed against the bed.

Slightly OT: After you get used to CGMS, and get somewhat more reliable results, you can correct FAR more aggressively than you used to. This is because you can look at the graph 2 hours later and see the trend (slope of the curve)-- if it’s falling too fast, you just pile on some quick carbs to prevent the hypo from ever happening. Sometimes I add carbs at bG levels as high as 130-140, because I see that I’m gonna fall through the floor in the next half hour without them. Dexcom has been found to be faster at showing lows during drop off periods by several people who have used both, but it is fast enough-- you’ll just need to learn watching the trend, rather than just the bG value (as you’ve always done with finger sticks.)

I’ve found that my Minimed sensors are not the best at tracking high BGs and that’s when I see the biggest difference. During the night it’s even worse (for catching highs) and I’ve woken up close to 300 while the sensor said 180, but the same sensor would track fine during the day.

I believe other people have mentioned that their sensors just don’t catch highs very wekll. I have my High alert set to 160 because if I set it any higher, I’m afraid my BG will be out of control by the time the alarm sounds.

Also, I think the trainer was supposed to tell you to expect differences up to 20 percent, not 20 points. Everything I’ve read and have been told said 20%. So, unfortunately, the 194 vs. your meter BG of 241 falls into that +/- 20% range (barely). I find that most of mine, other than when I go high during the night, are much closer.

Oh, I also think the sensors don’t catch lows too well at night either! I have my low alarm set to 75.

Liz, you might be pushing the Sensors for too long: When they’re dying from insuficient reagents left, MM Sensors do pretty well on normal bG but refuse to cross a line, say 150 mg/dL, to match a high. No matter how much higher your bG really goes, it barely budges above that erronious “maximum”.

Lots of us think MM is slower than Abbott, or even Dex, at alarming the lows it DOES catch, and more likely to miss lows without alarming at all. My Dex seems almost right on bG without delay when I’m going down, but it’s late/delayed when I’m going up. MM seems to have about the same delay in both directions, I’ve no idea why.

If you end up sleeping on the Sensors, maybe try an alternate site? “Stale” ISF leads to totally whacked numbers, and I think that compression on the site tends to prevent the ISF from getting “refreshed” via blood flow.

I had an experience yesterday (thursday)…just after my wonderful day, I changed my sensor and the new one gave me problems from the beginning. I think I hit a blood vessel or something. After 5 Cal Errors during the day I finally decided to remove the sensor after it told me I was 70, 200, 350…all within 10 minutes! When I removed it, it was a gusher… the blood had definitely traveled up the little tubey thingy and thats why all the readings were so off. Today seems to be a better day…keep your fingers crossed.

my alarms are set higher and lower for the same reason…to catch them early. My low alarm is set at 90 (I have no low symptoms) and my high alarm at 200 (it used to be 180)

yep, that sort of thing happens from time to time. I’ve asked for two “freebies” from Dexcom during my two years of usage, and they’re happy to comply, even though the CS woman asked me to please plug it in the computer and send in the data over the Internet once.

My experiences have been very similar to this. Almost every time my CGMS says 70-75, I’m really in the 50s. Maybe it’s just not quick enough. A lot of the time it’ll be spot on again after I come out of my low!

Even a brand new sensor with a good Isig will lag at night if my BG rises. I believe Minimed has told people that inactivity can lead to the sensor flatlining, or not tracking well. I pretty much like to sleep at night, so yeah, I’m pretty inactive then!

I know a sensor is dead when the Isig drops to a low single digit and stays there even when I push my BG a little higher. I change it then. I’m amazed when I heard that some people can get 3-4 weeks from a single sensor.

The sensor I had in up until yesterday was working great. Right after eating dinner and for the next two hours, it kept telling me I was low and the Isig was in the 4 range, so I knew it was dead and removed it. That one was on day 9.

I usually insert the sensors on my lower abdomen but in September I tried my arms for the first time. My left arm was so-so but every sensor I’ve inserted in my right arm was a winner right from the start. I wish I had a few extra right arms to use! I try not to use it for 2 weeks after removing a sensor. I’ve tried my upper abdomen but it wasn’t tracking well, and my one leg site so far didn’t work out.


I also have a alot of discrepancies with my sensor but my doctor mentioned if your sugar is really high or really low the sensor could be way off when your sugar is in a good range the sensor is more accurate, So basically when you need it the most when you are high or low it does not work as well hmmmmmm. I also heard not to put adhesive patch over the whole sensor to cut in in half it keeps in flatter I have not tried it yet but my friend does it that way she uses her sensors on her leg I tried that but whenever I would change or have to use the bathroom I would hit it or pull it out no matter how carefull I was.
I tend to use my stomach for the pump and sensor it is sensitive there though and that stinkin needle to put the sensor in is way tooooooo fricken big excuse my language it hurts like hell and it think and way to long I hope they change that soon. I have had a bad batch of sensors too I am not a fan of mini meds customer service. I have to say when I had a animas pump they were always very nice and helpfull I am looking forward to them working with dexcor for the sensor next year I think I will switch to them.

Sometimes it takes 48 hours for me to get good on track readings. Sometimes i get good readings from the start. When i get a sensor that is consistantly off…i do my best to try to get it in line…sometimes it just does not work. It could be placement - i have found that certain sensor locations work better than others - but that was thru trial and error.

good luck…once you get the kinks worked out, you will love it.

They happen with me also. Usually the #s above 240 and ;the # below 80 have the biggest differences in CGMS versus blood sugar. The CGM is basically a trend unit and is not presently an exact device.

Someday they may be more accurate…All I am asking for is an alert that above 200 I get receive a vibration, I can test and correct; I have set low sensor alarm at 80 so that I can test and not approach a dangerous low. Once the unit said 74 and it was 44 by blood sugar. Great difference but a wonderful "wake up " call in at 2 am…


Angi - This is why I gave up on the CGMS. If its not accurate, what’s the point? In the month I used it, it allowed me to go into a serious crash failing to warn me. Several nights it beeped me until I woke up telling me I was low. Meter tests showed I was perfectly normal. I hope you have a better experience than I did. Jan

Hey Angie, I never replied to your question-- until now, when I received an email notification from Jan’s new post. There’s FOUR distinct possibilities:

(1) your Minilink Transmitter is faulty, like many others. Minilink is far more prone to failure than the Receiver (R/T or Guardian unit), and many have been replaced by Minimed. The Transmitter actually does two things: it both measures the ISIG and sends to your Receiver. If you had “weak signal” after less than 5 days from charging, then it’s very possible your Minilink is leaking current and discharging itself-- and exactly HOW it is leaking internally could be messing up the ISIG measurement.

(2) More likely, if the Sensor was more than 7-8 days old and you had a high bG average during it’s usage: Minimed Sensors, unlike Dexcom and Abbott, “die of old age” by failing to show high bGs. When old, they can remain quite accurate for normal and moderately high readings, but there’s a line at which they just won’t generate more ISIG (no matter how much higher your bG really is). The most Obsessive-Compulsive users “challenge” their aging Sensors by eating a bunch of carbs and then seeing if Minimed can track the resulting high: A particular finicky expert in Greece who usually gets about a month from each Sensor, but he sees that after 2 weeks it won’t show a reading above about 250; after 3 weeks it won’t show a reading above about 190, and after 4 weeks, it won’t show a reading above 140-150 (and then he replaces it).

(3) Also more likely, Laura Jo’s theory: Minimed (and Dexcom, but NOT Abbott) uses a straight-line graph to do their calculation, even though the actual ISG vs. bG relationship isn’t purely linear. And if you haven’t entered a calibration at a reading of 200+, the high readings become very likely to be systematically wrong. (But I think that #2, “failure to show highs due to old age”, is a more likely cause, for both you and Laura Jo.)

(4) And finally, you’re not gonna like the other possibility (because you’ve already paid. or made your insurance pay, for MiniMed): MiniMed seems to have a much higher rate of “I’m doing everything right, and the durn numbers aren’t reliable!” than the competitors. And these users ARE doing everything right-- I know of a nursing student, AND an Endo (my own!), AND myself, whose skill with lab techniques are absolutely professional-research-lab quality. Here’s my SWAG, from lots of Internet chat on other boards: about 20% of Minimed buyers are dissatisfied with the reliability of the readings (dissatisfied enough to absolutely HATE having bought it). Dexcom seems to have a lot less dissatisfaction, maybe about 1/3 less (thus 12-15%). The winner for accuracy in adults, however, seems to be Navigator: everyone I know who’s switched from either MM or Dex to Navigator is simply amazed at how much better the accuracy is (even when they were “pretty satisfied” with the one they had before.) The problems with Abbott right now are (a) a much higher rate of failure at both ends Receiver AND Transmitter, than anyone else (even including MiniLink)-- they’re doing LOTS of part swapping, and of course, the users get real frustrated with “puttering around” under Support’s instructions before Abbott finally knuckles under and replaces the failing component; (b) the larger, somewhat uncomfortable sits-on-a-hard-plastic-plate Sensor design; and © the shortest lifespan of all 3 products among careful and attentive adult users (those maintaining A1c below about 6.2).

BTW: Dexcom dies by becoming flakey at ALL levels of bG, the readings start “jumping around”. Abbott either fails to restart after day 10 (because it’s more strict with requiring a certain “minimum” voltage/current reading on a “new” Sensor), or it dies like Dexcom-- all the readings start jumping around. Only Minimed dies with the “good readings at below 130, but utterly refuses to show any value above 140” behavior.

I haven’t had any issues with weak signals or anything, so #1 doesn’t seem to be it. And I don’t wear my sensors more than 6 days and I always recharge before reinserting, so #2 doesn’t either. But I am hearing lots of feedback saying that people tend to “flatline” a lot and it stops detecting highs and lows. I get that this is still pretty cutting-edge technology and I’m willing to be patient as the technology develops, no problem there. But it’s a bit frustrating to keep hearing the explanation that the CGMS “isn’t intended to reflect actual BG levels, it’s meant to indicate trends.” So how am I supposed to make any real decisions based on these trends if the highs and lows aren’t even remotely accurate? Do I adjust my basal by 0.1 or 0.3? And what about the poor souls who have lousy hypoawareness and rely on CGMS to help detect lows? Shoot, my endo and I had to arm-wrestle my insurance company before they’d cover it, because they were initially only covering it for people with poor hypoawareness, and I can’t imagine this is working well for that, either. I dunno, I will keep up with it and hopefully a next generation will be here soon.

I’m tried the CGM system and had the same problems with discrepancies btw the CGM and the meter—up to 100 points different! The CGM would wake me up and night saying I was low, but the meter would say 180. I returned my CGB to MiniMed and since it was not covered by my insurance and I paid for it myself, I was refunded the price of the CGM, but lost $500 on the non-refundable supplies (which no one ever mentioned before I purchased it.) Since you have to do the fingerstick and meter to confirm CGM BG, what’s the point?

I, too, find it ironic that I’ve been hearing about a “virtual pancreas” system that would not need human input for the past 10+ years, and we still don’t have one that works, yet computers and video games have improved dramatically. I understand that the risk involved with diabetes control is much greater than for games, but I can’t help but wonder if someone in the diabetes treatement industry is slowing down the progress untill they can switch their business from strips, meters, syringes, etc. to the new technology so that they won’t loose $$$millions.

I’ve been using Medtronic’s GCM for almost a month. I got ten sensors to start with and so far have had only one work exactly as I’d like it to. Two of them mostly worked, one mostly at the beginning and the one I’m on my fifth day with now at the end.

I am a thin male and I think that makes a big difference. When I found out I was Type I I was down to 138 lbs so I suspect I couldn’t have ever gotten this thing to work, but even now at about 185 (I’m 6’4") I’m having a heck of a time finding spots where my sensor will work. The second sensor I installed on my back love handle at exactly the angle recommended and it worked like a charm, but I have not been able to reproduce those results on the next 7 sensors.

The fattest spot on my body is my belly, and there’s a lot of fat there, so I tried putting the sensor near my belly button away from the scar tissue caused by my insulin catheter. I can’t get predictable results. Even if I leave it in “wetting” for six hours I still tend to get flat lines (as mentioned in another post).

I had tried this once before when they transmitter was huge and ungainly and gave up after three sensors, the last of which woke me up four times in a night for no reason.

This new system is small, elegant, goes in nicely but does not work.

I have little fat on my arms or my butt. I have man boobs, will they work? I suspect it will hurt so I’m not going there yet.

I have three left to try and I’m hoping you guys can make a suggestion. My trainer (who’s been using one of these for two years and has no problems with them) suggests my arms or legs but to be honest, I’m convinced I’ll hit muscle because I don’t think there’s that much fat there.

So from this thread and other sources, here are the places I’ve heard used:

love handles. Tried one on left side. Didn’t work (simply not accurate, but I’m more aware of the types of failures now and this was my first try). Probably failed due to catheter scar tissue. Tried one on left back love handle (sensor number 2) and it worked beautifully. I wore it for six days and it was awesome. Except that’s the side I sleep on and the pump kept losing signal and waking me up. So I tried three on my right back love handles which failed miserably. Every time I calibrated it was as much as 80 points off. I sense a low, look at the reading of 100, test and find out I’m 47. Useless. Actually worse than useless, dangerous because if I went low and believed the damn thing because my head was cloudy I could be in big trouble. Its as if the signal would change a bit but not represent the entire swing. So if I calibrated to 120, then it might go up to 160 when I was 200 and down to 100 when I was 80.

At this point I talked to Medtronic support. Greg was great, and we figured out that I was increasing the angle of insertion because the trainer told me to (Lisa, she’s great too but was wrong about increasing the angle on a skinny guy. Works fine on an average woman though because women have more fat than men). He also suggested choosing an area with more fat and suggested my butt because that’s where it worked with him. Greg sent me a free sensor to help me out. Like I said, great guy.

I tried my belly because its the fattest part of me. I shaved the fur, don’t know if that’s necessary but I did anyway. I’ve used two sensors and both have been sorta working. First off they didn’t work right away. I wore the first over night (Greg’s suggestion) before hooking up but still had problems with flat lining. Then it seemed to work sorta for a while and then started to get flaky again so I pulled it out and put another one in my belly aimed left instead of down like the first one.

That one is working pretty well now, but I know its still of on the highs and lows. And it gets less accurate if I calibrate during a high or low (because I don’t know I’m high or low because its 20 points off).

Here is a list of possible sites:

  1. love handles. Been there done that, one in four tries is a bit expensive, don’t you think?
  2. Stomach. I don’t think there’s enough circulation there. Even with all the massaging in the world its just not accurate enough for me.
  3. Arms: I’m certain I’ll get muscle.
  4. Legs: muscle?
  5. Butt: How can I tell how much fat is on there, the skin is tight?
  6. man boobs: I get this feeling of dread when I think about it.
  7. chest: too skinny and too painful. I tried the old system there it was a disaster.

Can someone give me some ideas of what works for you?



Um. The number of sensors I quote above is off. After the one that worked I’ve tried five more sensors…