We had a small typical incident today, frequent occurrences in the life of every T1D kid.
Max takes a relatively standard meal around 18:30 - the meter check (red dot as usual) is on the Libre Trend (red line). The Libre spot check (orange dot) matches the Dexcom spot value (small green dot). It is a draw at this point. We inject Novorapid and eat. Then, real life interferes: Max doesn't feel like eating more and takes his warm bath. Taking a warm bath soon after a Novorapid injection is a mistake. We know it and yet keep making it....
Warm baths cause vasodilation. Vasodilation causes the insulin to be picked up faster. Faster means that the timing is probably off. Vasodilation also slows digestion because blood is diverted from the guts to the periphery. An hypo is on the way.
The bath starts at 19:00. The Dex loses the signal, the Libre keeps recording to its sensor memory. At 19:20 Max feels low, the Libre spot value is 56 mg/dL and so is the meter. The Dex hasn't picked up the signal yet, but when it does 10 minutes later, it still indicates a healthy 90 mg/dL. We aren't extremely aggressive in treating lows and decide to double check the trend 10 minutes later (we also know that food is being digested, even if the vasodilation retarded that. The next spot check is below 50 mg/dL and the Dex has begun to notice the fall, time to treat with 15 grams of fast carbs. The Dex will beep exactly twenty five minutes after the fall in BG was felt by Max and confirmed by both the meter and the Libre. In this case, since the Dex missed an important data point, it is probably a worst case.
In the subsequent rise, we confirm once more that the Libre runs roughly 10 minutes ahead of the Dexcom. I should have the raw dexcom data in my Nightscout mongo database as it was connected throughout the episode but I am less and less motivated to go through the hoops to see what it would have told us.