Dexcom G4 Accuracy

By DrJohn Latest Reply 2014-08-02 11:42:01 -0500
Started 2014-07-19 07:20:28 -0500

I'm disappointed with my new Dexcom G4 Platinum. During the first 24 hours, it's accuracy is about 50%. It gets better with time. It is the most accurate after 1 week at which time the sensor expires.

The manufacturer states: "Your sensor glucose readings are meant to be used for trending only."

Don't I already know that my blood glucose will go up after I eat. Then, come down?

I wanted this CGM for sports so I don't have a sever hypo and pass out. But, can I rely on it?


11 replies

DrJohn 2014-08-02 11:42:01 -0500 Report

The latest: At 3 am the dexcom woke me with alarms. It said my BG was 206. I'm never low in the middle of the night. So, I tested my BG. It was 58.

DrJohn 2014-07-28 12:28:44 -0500 Report

FYI, Dexcom is waiting for FDA approval to release a more accurate algorithm, which you can upload to the receiver from your computer.

Type1Lou 2014-07-22 07:45:05 -0500 Report

It sounds to me that you are trying to make your CGM take the place of your glucose meter readings. This is not the function of the CGM since there is a 10 to 20 minute lag between the glucose levels obtained with CGM and those obtained via self-monitoring. The variance might be especially pronounced during higher levels of activity or after a meal. Depending upon the rate of change in the blood glucose, the CGM values are generally within 20% of the blood glucose values. Medtronic, in their user's guide "Insulin Pumps and Continuous Glucose Monitoring" cautions that we should "make treatment decisions about insulin doses and hypoglycemia from SMBG (Self Monitor Blood Glucose) readings only."

DrJohn 2014-07-19 14:33:20 -0500 Report

Here is another problem, I will likely call the FDA about.
Finger stick = 44
Dexcom = 59
After calibration of 44, Dexcom reads 64.

This is crazy. The dexcom should read somewhere between 44 and 59, not go to 64. This is one of many occurrences.

In the more than 30 years I've been in medicine, if you calibrate an instrument, it should read the calibration value. Otherwise, you need to calibrate again or send the instrument in for repair.

I know that the Dexcom is reading the interstitial fluid vs blood in the finger stick, which is why it is being calibrated so that they will match at at least match closely.


diabeticdummy 2014-07-19 14:55:35 -0500 Report

Wow. If it reads higher After the two are calibrated then that throws off you matrix. So your pump corrections will be off due to it shows higher bg. that's not good.

diabeticdummy 2014-07-19 15:07:52 -0500 Report

Yes thats scary let us know what you find out I never thought to look that closely as I don't use cgm very often just every now and then to give my endo a 24/5 look at my numbers instead of just basal rate bolus and bg stick be careful doc good luck

DrJohn 2014-07-19 14:56:56 -0500 Report

or one can pass out thinking it is 64 when it is 44 and going lower during sports

MarkS 2014-07-21 09:12:34 -0500 Report

Come on Dr. John, don't you remember your physiology course? The level of sugar in the blood will be different than in the interstitial fluid, which is what the CGM measures. And when calibrating the CGM it is not an instantaneous change; plus, CGM shows the trend. You can then tell if your BG is rising or falling, so when you are working out or whatever physical activity you are participating in, you have an overall indication of which direction your BG is heading.

DrJohn 2014-07-21 10:09:49 -0500 Report

I disagree. If you are calibrating the interstitial fluid from the blood value, it should now display that value. It doesn't.

diabeticdummy 2014-07-19 11:52:27 -0500 Report

I don't know about dexcom cgm but the medtronic cgm sounds like it is the same why I don't use it you still have to stick to get accurate bg reading due to fact that cgm reads body fluid sugar not bg so it sleeper to show changes that's why even wearing the cgm you still have to finger stick at least four times a day so you can see the trend and programs can calibrate the two so there close so like my cgm says I'm 110 but finger stick bg says I'm 90 so now when pump figures it it knows 110 cgm is around 90 bg my endo said when doing sports before hand chk bg adjust accordingly to 120 bg and then I can take pump off and chk bg if not feeling well or at 1 hr and readjust if needed I only got in about a week of lifting then had another surgery but I worked well for me I'm hoping to be up and lifting again in about a week but physical therapy lady keeps telling me slow down I'll get there but good luck

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