Ketones in the urine or breath are being wasted, and that tends to diminish with time, for various reasons, apparently. Similarly, even ketones in the blood are what your body hasn’t used yet, so it’s not a great indication of how many ketone bodies you are actually metabolizing.
Any of the measurements can confirm that you are indeed in ketosis, and that may be useful, but you can save yourself a bit of cash by asking yourself these two questions:
- Am I eating less than 20 g/day of carbohydrate?
- Am I breathing in and out?
If you can answer Yes to both questions, then the odds are virtually certain that your liver is producing ketone bodies.
That said, it is true that serum β-hydroxybutyrate at 0.5 mmol/dL is the definition of nutritional ketosis, but even Dr. Phinney says it is arbitrary. That was just the level at which he and Prof. Volek saw benefits of ketosis occuring in their study subjects. Once you achieve fat-adaptation, this will all become much clearer to you.