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

Calibrations determine so much. Make sure you calibrate when your sugars are stable. (I know, not always easy to do)


Thanks Amy. I always calibrate when I’m stable. The pump then sets the center point based on that reading and then the farther my sugar veers from there, the less accurate it is.

The sensor I have in now is a prime example of that.

Angi -

Your experiences closely mimic mine. I did a 2 week trial of the minimed cgms and frequently got weak signal errors and wildly inaccurate readings. Over time I learned that the trick was to be very selective about when I calibrate - ONLY during times of great stability and NEVER when my glucose is below 90.

Frankly I was pretty disappointed with the product and I’ll probably go with the Dexcom which I’ve heard is more accurate. I’ll be starting a trial on the Dexcom over the next week or two.

Good Luck!

As pointed out… there may be multiple different factors for why a CGMS may not be “exact” with your BG.

  1. Lag time. CGMS lags about 15 or so minutes behind what your BG shows. For example, if my CGMS indicates a glocose level of 150 mg/dL, that is about where my blood sugar was 15 minutes ago. It takes some time for the glucose to transfer from the blood vessels and end up in the interstitial fluid where the CGMS take their readings.
  2. Activity level will have an impact. When you are sleeping, your circulation is slower and therefore you get the lower than actual values which quickly correct themselves once you get up and moving, ie lower values when you sleep.
  3. CGMS’s also have to calibrate to your BG, and if you calibrate during less than optimal times, ie times when your blood glucose is rapidly changing, the calibration itself can be off slightly.

I’ve been using the Navigator CGMS for about 8 months now. Even coming from a technical background in clinical chemisty, I found it challenging to remember that sneaky lag time. But I learned after watching the graphs on my meter, that if I just waited about 15 minutes or so, the CGMS usually was right on par with my blood glucose. I’ve often checked my blood sugar when there was a nice steady no-change in my readings and have found the CGMS to usually fall within +/- 10mg/dL of the actual finger stick.

Scott -
Thanks for your input, but the lag time, flatlining, and calibration at optimum times are standard aspects of the training. I’m referring to numbers that disagree significantly in spite of these things, on a daily basis. Many of us with the Minimed CGMS are apparently having problems with this.

I had used MM CGM. It simply does not work.

Yes their readings wildly disagree with meter BGs. MM CGMs are at least 15 minutes delay. It is in fact pretty useless
let alone its heavy bleeding sensor insertion needle of giant size.

Try the latest Dexcom Seven Plus which just came out in March 2009, with much improved accuracy of within less than
20% range of meter readings. Its accuracy amazes me so far. Also Dexcom’s sensor needle size is so tiny that you
can feel nothing during insertion and the rest 10 days in place. You can calibrate Dexcom anytime you want. Yes, that
makes a huge difference from MM’s CGM.

Dexcom gives you 30 days to try with a full refund.

I feel so much better after reading all these posts. I, too, have had wild fluctuations between my meter and my sensor (and yes, I use the Minimed and I change it every three days like I've been told to do). My CDE and Medtronic rep (Both Type 1s) and my endo (also a Type 1), tell me that when my BG is moving quickly in one direction or the other the numbers may be very different. The latest example, from yesterday, sensor says 124, meter says 54. I'm just glad to know that others have this same problem.

I had tried both the Minimed and Dexcom CGM’s several years ago, and neither gave me accurate readings, despite working extensively with their tech support. I finally agreed to “test drive” the MM Enlite recently since I needed a new pump anyway, and had significantly better results, so I’m going to try it again. I hear people complaining about both, so I’m not convinced that one is better than the other. Keep in mind that the CGM’s, no matter the brand, are measuring the electrical impulses in our interstitial fluid as our BG fluctuates, and it tries to convert that to a BG reading using an algorithm. It’s not actually measuring your BG, so it’s far from ideal. But hopefully by calibrating only during fasting periods and only 2-3 times/day, you can at least get some useful trending information. It unfortunately doesn’t take the place of checking your BG several times per day.

Thanks for that last note re Dexcom behavior. I’ve only had mine for a few months and am just experimenting with extending it beyond the statutory 7 days (not to save on sensors but because finding decent “real estate” for all this stuff is getting harder!) and I haven’t known exactly what to expect when the sensor starts giving up.