Tuesday, January 13, 2015

Speed matters: tennis and the Libre

The Dexcom delay issue in sports


The chart below, of a typical tennis training session, shows that the Dexcom G4 can be both utterly wrong (in terms of reality) and perfectly correct within its well defined limits. Dexcom values are blue dots, glucometer values are red dots. We start carb loading for the training at 15:15. At this point, the Dexcom and the meter seem to agree. The Dexcom will match the 15:15 meter value at 15:25, a decent delay. But in fact, the Dexcom is still averaging the drop it saw earlier, when the additional carbs are already acting. At 15:45, it gets ugly: the dex is around 60, the meter is at 140. The Dex, averaging the climbing values, almost catches up at 16:10 but the reality is that BG is already in the danger zone. At 16:40, the fast carbs we took at 16:20 are already working.




In practice, the Dexcom consistently gave incorrect information. It saw a low when we were high, and a high when we were actually crashing. But if you shift the Dexcom's curve 15 minutes back, it almost perfectly fits the BG meter. Sometimes, it is very very hard to warp our heads around what we see. If you don't have a mental image of the curves, at 15:45, when you see 60 vs 140 you simply want to throw the Dex on the wall. To add insult to injury, at 16:25, that wasn't the Dexcom that prompted us to do a meter test and a carb reload: it was just the fact that Max missed 3-4 easy plays in row.

We have a problem...

The case above is almost a textbook example of CGM delay. However, it is not always that simple. The training may be harder than expected, we may have underestimated the required carb amounts. The match may be easier, we may be overshooting. If the match has been delayed, we'll be at another point on our insulin action curve... In practice, the Dex helped very little during sports, and we returned to lots and lots of finger pricks.

But, here's the good news, or at least a reason for hope. When we started using both the Freestyle Libre and the Dexcom, we noticed the Libre reacted much faster to BG level changes. Further analysis showed that, in general and on average, the Libre ran 9-10 mins ahead of the Dexcom.

Is that 10 mins bonus valuable? Incredibly.

Real life test.

Could we rely exclusively on the Libre and forget finger pricks for a whole real game? We had the opportunity to test during the Christmas Holiday. The third round adversary had literally destroyed Max in the Summer. If we crashed based on wrong information, that wouldn't be a problem. The match turned out to be intense: Max lost in two hours and three sets. His opponent went on to win the tournament without losing another set - we wanted a tough game, we had one. The video below shows a few of the second set exchanges so you can judge the intensity of the effort by yourself (and also because dad enjoys making videos of his kids - Max is the white one).









And here are the traces of the day (red - historic / orange spot checks) and Dexcom (green - calibrations in red). I won't go into a longish detailed analysis but that chart again shows that the Libre is significantly faster than the Dexcom when conditions are changing rapidly. It picked up the carb pre-loading more quickly than the Dexcom before the game began at 13:30 and also picked up the transient 14:30 intense carb-reload increase that the Dexcom averaged out. As soon as the game was over, at 15:30, the Libre picked up the small post game increase. The rest of the day also included a double at 20:00 that Max couldn't really handle very well as he was still low from the afternoon. And, unfortunately, we saw a pattern of strong delayed hypos, to be expected after a long intense game.


But the good news was that we could maintain good glucose levels through most of the game, sliding under 100 mg/dL in the third set only. Every Libre spot check (big orange dots) led to a decision and Max was able to maintain an ideal range for an extended period. The Dexcom (green) would have been totally useless for that purpose: its curves looks decent here because it retrospectively averaged a decent evolution. It would unfortunately have not been helpful, as shown in the initial chart, in obtaining that relative stability.


Both devices tracked the post match situation correctly, the Libre being slightly more correct than the G4 compared to the nightly BG meter tests, but not significantly so.

Thanks to the Libre, we almost had a normal game. 

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