Predicting the future (use of CGM trend arrows)
Most people who start continuous glucose monitoring therapy love it. Why? Because it provides real-time AND predictive information about blood sugars. Knowing how to make use of this information is the key to success with a CGM. So, let’s get into it.
What is the therapeutic difference between a CGM reading of 220mg/dl with two arrows up, and a CGM reading of 220mg/dl with two arrows down? For me, it is the difference between taking a 5.25U or 0.75U correction.
Let me explain. According to the above chart published in Endocrine Society 2017 (see below), trend arrows are best used to predict BG in 30 minutes time, and that predicted BG (not the current reading) is what should be used to calculate insulin doses.
For example, my correction factor is 1 unit for every 40mg/dl >100mg/dl.
If my CGM read 220mg/dl with two arrows pointing upwards (rising by 3mg/dL/min), I would calculate a correction for a BG of 310mg/dl (220mg/dl + 90mg/dl). My correction would then be 5.25U [(310mg/dl – 100mg/dl)/ 40mg/dl].
If my CGM read 220mg/dl with two arrows pointing downwards (falling by 3mg/dL/min), I would calculate a correction for a BG of 130mg/dl (220mg/dl – 90mg/dl) and would give a correction of 0.75U [(130mg/dl – 100mg/dl)/ 40mg/dl].
What a difference!
Of course, this method does not necessarily apply to people using hybrid closed loops system because much of the rate of change is accounted for in the algorithm’s adjustments. It is, however, very impactful for patients on injections and standard insulin pump therapy.
Remember, when changing the way you dose your insulin, always start conservatively and ALWAYS run your changes by your clinician first!