Yes, you may get different readings at different times of the day. It has to do with time you eat or drink and the food gets processed and maybe also with when you drink water.

Ketones are lost through the breath from the lungs, and also via the intestines. Testing the urine for ketones is the cheapest, easiest (and least messy ) way of checking. The Atkins Center in NYC uses a big expensive machine to measure ketones in the breath as the most reliable measure.

Atkins recommends the evening in some of his books, but the morning was always my strongest reading. You can also lose weight successfully while NOT apparently putting out many ketones. A Trace reading of pale pink IS ketosis.