Tuesday, 23 June 2015

Sensor (1-3) Performance

It was time to change sensors again, starting my fourth sensor today. I had some accuracy issues with my 3rd sensor, so I decided to have a closer look at the data. From the exported .csv file, I wrote a small piece of software that find pairs of blood glucose and Libre scanned values. Normally I scan with the Libre just before or after I take my BG, so extracting the data was not a big issue. Ideally, the BG and Libre values should match almost perfectly. The performance of these kind of sensors are usually measured by the Mean-Absolute-Relative-Difference (MARD). Abbott claims that the Freestyle Libre have a MARD of 11.8% after the 3rd day. The lower the value, the better.

The BG values can be seen in the x-axis, while the y-axis shows the Libre values. For a perfect sensor and BG combination, all the points should be on the y=x line. My first sensor results shown in red, second sensor in green and third in blue.

The following plot shows the values for all 14 days for each sensor. (click on it for a larger view).
Clearly sensor 3 seems to always measure a bit low. Values below the line indicates that the Libre measured a value lower than the BG value, the inverse applies to values above the line. Sensor 2 seems to be measuring little too low in the [2,8] range, but too high in the [8,16] range. Sensor 1 measured a little too high on most cases. Abbott should really add the option to calibrate. The device already have a BG meter build-in, so why not add an optional calibration / offset process ???

The following plot shows the values for 13 days, skipping day 1 for each sensor.
You can see that most of the values from day 1 are outliers when comparing the two plots.

The MARD values can be seen in the following table:
Day 1-14 Day 2-14 Day 3-14 Day 4-14 Day 1-7 Day 8-14
Sensor 1 14.7% 15.0% 15.3% 16.2% 14.5% 15.0%
Sensor 2 12.1% 12.0% 12.6% 12.6% 13.2% 10.9%
Sensor 3 18.4% 14.7% 13.3% 12.1% 20.26% 11.8%
This shows that the accuracy do improve over time. Although sensor 3 was looking very bad in the early days, the accuracy quickly improved and overall performed better than sensor 1.

The mean error can be seen in the table below:
Day 1-7 Day 8-14 Day 1-14 Day 2-7
Sensor 1 -0.20 -0.68 -0.36 -0.30
Sensor 2 -0.07 0.15 0.03 0.30
Sensor 3 1.39 0.58 1.21 1.05
The mean error table clearly shows that sensor 1 was giving values slightly high and sensor 3 gave very low values, a simple calibration process could have improved the accuracy a lot.

Let's assume a simple calibration procedure: After 7 days, take the mean error (of day 2-7) and use that as a simple calibration offset. If we use the mean error as the offset (100%) or half of the mean error as the offset (50%), we get the following MARD from day 8-14: (0% no calibration)
100% 50% 0%
Sensor 1 12.4% 13.4% 15.0%
Sensor 2 9.17% 10.0% 10.9%
Sensor 3 12.7% 9.68% 11.8%

The system can definitely get some benefit from a calibration process.

Thursday, 11 June 2015

Issues with new sensor and the Dawn Phenomenon

It was time to remove my second sensor a few days ago, and time to start up my third sensor. Image taken right after I let my wife pull off the sensor. Again, no issues with keeping it stuck for 14-15 days. The extra tegaderm over the sensor do help and you can see the ripple effect it caused on my skin.

The dawn phenomenon could be clearly seen in action during many nights. Around 4am, my glucose levels suddenly starts to rise. This is due to the liver releasing glucose in order to prepare me for the day (If I understand it correctly). The graphs below show the sudden rise (+-4am) while I was still sleeping. It gets kind of difficult to manage this, you need to perfectly time the long acting insulin to peak at 4am or start the night on a lowish glucose level (which increase the probability of a hypo during the night), or just make sure to wake up and eat breakfast asap after 5am. I really like my sleep, getting up early is not my favourite option.

My 3rd sensor is giving me some issues. It seems to be always lower than my BG readings (offset issue). There also seems to be some scaling issue. Abbott is going for a calibration free option with the FreeStyle Libre, but it would have been great if they just added an option to calibrate the sensor in case something goes wrong or is a bit off. The readings still follow my BG trend, but needs an adjustment.
Due to my limited stock of sensors and no Libre support in South-Africa, I'm keeping the sensor in, hoping that the offset might improve over time. The asterisks (*) on the graph shows the BG readings (done with a finger prick test). 

Yes, I'm not having a good BG day today, but the readings from the Libre seems to improve a little, hopefully it will be as good as the previous two sensors soon.