Wednesday, April 8, 2015

Some Libre peculiar behavior you should know about: temperature

Have a look at the following strange ISG pattern (sorry for the poor quality, that was a quick cell phone shot). The sequence of events is roughly as follows: a low around 17:00, probably over-corrected. A bit before 18:00, we decide to go for our evening dose and meal since we probably would have had to correct, then stack insulin for the meal. Best to combine the two and assess the situation. At 19:00, we seem to be on our way down and Max decides to take a warm bath. Max, much to my despair since I am the one who checks for adhesion, loves longish warm baths...

As soon as he goes into the water, his ISG seems to start rising sharply again! One possibility could be that his stomach suddenly offloads food to the duodenum and that he enters a "secondary food absorption phase"  ( since a gravitationally lensed mirror ISG image is unlikely ;-) ). The problem is that I have never seen such a bi-phasic digestion pattern.

A bit before 20:00, a spot check gives a 254 mg/dL reading and the question of an additional dose of insulin becomes legitimate. Did we under-estimate the meal? Are we seeing some fancy cheese induced digestive delay?

While we usually have no issue giving corrections based on the Libre, once we have established it is tracking accurately, this isn't an obvious decision. The sensor has been perfect, but it is only in his first day, approximately 40 hours after insertion (we do pre-insert to avoid the first day low sensitivity phase).

But we remember that we have, in the past, already seen anomalous high readings after warm baths. A BG meter check indicates 137 mg/dL close to the top of the second bump where the Libre displays 254 mg/dL. A second BG check indicates 129 mg/dL a few minutes later and, around 21:00 the Libre and the BG checks are back in sync in the neighborhood of 100 mg/dL.

What happened?


See that protruding thingie in that opened Libre sensor? You could think it is an antenna. But it is not: the antenna is the concentric circle around the circumference of the board. What you see is a thermistor (tests indicates that it is either a TCS651 or a component with a similar response curve). Temperature plays an important role as far as the Libre system is concerned. You may have experienced the "too cold, scan later" message while jogging outside in cold weather. Or may have noticed a loss of historical data (gaps in the retrospective BG curve) in the same circumstances. What we did notice is that our usually extremely accurate Libre would often be a bit enthusiastic, even trigger happy after a bath.

Given the fact that we have three blood checks that are perfectly in line with the expected insulin induced downtrend, that the Libre progressively caught up and that we have raw data corroborating the scenario, we can safely affirm that what we have seen is a temperature induced artifact. It is also, for some reason, an artifact that the Libre's algorithm considered plausible and therefore did not flag as "not available".

Going Deeper

I am a sucker for that kind of thing and can't help looking for rational explanations. Unfortunately, since Max does not share my enthusiasm for investigations and since I can't decently expose him to successive cycles of heating and cooling, I have to limit myself to a somewhat theoretical approach: looking at and interpreting accessible raw data, understanding and measuring devices statically.

Here's a closer view of the system (capacitors unmarked, but they shouldn't be too far from TI reference designs)

A biological explanation would be that heat increases the blood flow through vasodilation and probably the speed of the GOx reaction which could lead to inflated results. But, in that case, shouldn't the role of the thermistor be to compensate? Could that mean the thermistor isn't filling its role? Or is it delivering correct data to an overly conservative compensation algorithm? Does the predictive algorithm whose existence I suspect become confused? No idea. If I knew everything, I would probably be building my own sensor. But exploring is fun!

In practice?

Even without understanding everything, there are a few practical lessons here. When the temperature situation is unusual or changes, too cold, too warm, consider your Libre results with some caution. The system does a decent job at detecting when its data is potentially unreliable, but it is not perfect. The Summer might be interesting. What impact will sunbathing have? The Libre is white and reflective. But what will happen if you stay for hours in the sun and are the proud owner of a Libre sensor that has been decorated with a black sticker? Just out of curiosity, I'll be watching.

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