A common complaint about CGM systems is the lack of accuracy. I frequently see posts on Facebook or Diabetes forums from people who are frustrated that they can’t get reliable results from their CGM. I believe that the problem is often down to calibration; it is very easy to get inaccurate numbers if you calibrate too frequently, or at the wrong time. Since starting on the CGM, I have managed to develop a calibration routine which gives me pretty accurate BG readings, at least with my Medtronic device.
The ‘rules’ are as follows:
- Do not calibrate too often. You need to calibrate at least two times a day, although three times a day may be more appropriate, as it gives more flexibility with timings and won’t involve testing in the middle of the night.
- Choose the time you calibrate carefully. CGM readings tend to lag further behind meter readings when your BG is falling or rising quickly. Ideally do not calibrate within 2 hours of a bolus and calibrate at least 15 minutes before food. I find that lying down can have an impact on my CGM reading, so I try to avoid calibrating in the 15 minutes before I go to bed. The 15 minutes is because that is how long it takes the CGM to calibrate.
- Do not use “Auto Calibrate”. This means any BG you take with a link meter will be used to calibrate the CGM, whether your BG is stable or not.
- Try not to calibrate when your BG is too far outside of your normal range. If I calibrate when my BG is much higher than usual (e.g. 15) the CGM tends to be more inaccurate when it drops back to my normal range.
I find the most convenient times to calibrate are when I wake up (around 9am), before dinner (around 6pm) and before bed (about midnight). As long as you don’t spend more than 12 hours asleep, or have more than 12 hours between breakfast and dinner, this sort of scheme should work as well. I know that my blood sugar is usually fairly flat at these times so they are the best times for me.
I have found that trying to follow these rules gives me a Mean Average Deviation of 3%-6% each day according to the Carelink software. This means that my CGM readings are within about 6% of my meter readings. The exception to this is when I have a very high BG, as the CGM tends to under-read in these situations (e.g. the CGM might read 17 when my BG is 22). In practice, I find that this isn’t too much of a problem, because if the CGM claims my BG is 17 I’ll be paying quite close attention and testing frequently anyway.
I hope that someone finds this useful and manages to get better CGM readings as a result.