I had that same assumption, but I'm no longer sure it is that clear. When I first started testing, I got a lot of negatives, leading me to believe I was not in ketosis. I then started testing in the afternoon and got much higher readings. This led me to the conclusion that the ketones were from dietary fat, rather than body fat. I went through a experimenting phase, where I tested urine, blood and breath ketones, and found that even when the urine or breath showed no ketones, I still showed some level of blood ketones. Not saying that answers your question, but it convinced me that measuring ketones is not always a clear indication of what's going on. The way I now use them is I only test in the afternoon, when the reading is highest, and only to see the results of changes I may have made to my menu. The level of my ketones does not seem to correlate with my weight loss. I guess that's a long answer to say I don't know.