I’ve been using the Abbott Freestyle Libre Continuous Glucose Monitor (CGM) for about 9 days now and I’ve had the opportunity to compare its blood glucose measurements with the Livongo stick meter that I’ve used for a couple of years now.

The following graphs show the comparison of blood glucose measurements between the Freestyle Libre and Livongo meters.

Line represents zero delta between measurements of the two meters

I have several brands of stick meters that I routinely compare and there is always a delta of at least 10-20 mg/dL so I’m not surprised that these two meters don’t measure the exact same blood glucose levels.  Blood glucose meter accuracy is about 20% so even multiple measurements with the same meter won’t line up exactly and can vary by 20%.

A blood glucose meter is no different than other meters that measure things like temperature, pressure, flow rates, etc.  There is an accuracy stated for the meter but that accuracy may not hold true for the entire range that the meter can measure.  Usually the meter manufacturer will have a tighter accuracy in a range where the measurement is more important (for example, with temperature it would be in the middle of the range where the temperature control system would expect to control).  While I can’t find this information in the literature for blood glucose meters, I’d expect the accuracy of these meters to be tighter at measurements <75 mg/dL because low blood glucose level can kill you quickly.

So while I’m not surprised that the Freestyle Libre and Livongo meters have deltas in their measurements, I am surprised that the delta between the Freestyle Libre and Livongo is always positive.  I would expect some to be positive and some to be negative (as the natural variation in measurement accuracies play out) but the fact that 12 of the 13 comparisons were positive (1 comparison had a delta of 0) leads me to assume that one of these meters has a consistent offset.

Now it should be stated that I have no idea which meter is closest to my actual blood glucose levels but that really doesn’t matter as long as the deltas continue to be consistent. With frequent blood glucose monitoring, the absolute value of these measurements isn’t what’s important; what you should be looking for is trends in the measurements that can be correlated with a diet or lifestyle change.

As I stated in an earlier post, I intentionally went off my typical Paleo diet for the first week of using the CGM so I could see what my blood glucose levels were after non-Paleo meals but next week I’ll return to my normal Paleo diet and I will be interested in seeing if the delta between these two meters decreases as more of my measurements are <100 mg/dL. If my theory about blood glucose meter accuracies being tighter at the lower range is true, this should play out in the data as my blood glucose levels drop to those lower ranges.