When I started with Lumen, I assumed that if a blood test shows that I’m in nutritional ketosis (ketone levels above 0.5 mmol/L), that means my cells are using ketone bodies for energy (unless you’re supplementing with exogenous ketones). While that’s true most of the time, it doesn’t have to be. For example, during intense physical activity, my body releases glycogen — the carrier mechanism of glucose that’s stored in the liver and muscle tissue. When that happens, my blood sugar spikes and my ketone levels go down. However, that free glucose is used up quickly, in particular by those cells that only use glucose for energy. Since I usually exercize in a fasted state, my body has no other choice than to burn fat for energy. That’s why I usually have very low Lumen levels after a workout. And since my body needs all the ketones it can get at this point, my blood levels are low as well. That’s one example where low ketone levels and low Lumen levels can happen at the same time.  Remember, low Lumen levels indicate that you’re burning (mostly) fat. So you’d normally expect higher ketone levels. In most (if not all) other cases, elevated ketone levels in your bloodstream are a clear indication that your cells are using fat for fuel and not carbs. So if you see ketone levels of above 0.5 mmol/L but a Lumen score of 4 or 5, then Lumen's results are inaccurate.