Edited to add this first para:
Sorry, just re-read my post and decided it was a long winded way of beating around the bush.
Basically it all boils down to this:
Does your daily total energy (calorie) intake of carbs+protein add up to more than your daily energy expenditure? If not, then you are in ketosis, whether your pee strips show it or not.
Not that that will tell you whether your ketosis is due to dietary or body fat useage.
Now, back to the long winded version...
I agree with what the others have said about urinary ketones dropping after a while.
There is a lot of gumph online about measuring nutritional ketosis, and whole flocks of ketoers seem to think that the higher your ketones, the better. But the reality is, that once your body has 'learned' to quickly and easily switch to keto as a fuel source, it then gets more and more efficient at it. Experts such as Phinney and Voleck suggest that 'chasing high ketone numbers' is a waste of time.
Ketones are used as fuel. So the better your body gets at using that fuel, then the better is gets at, well...
using them. Which means there are fewer ketones floating about to be measured.
So pee test strip ketones often drop off dramatically after a while.
You could switch to blood testing or breath testing, but that means buying extra equipment, and the blood test strips that I tried were around £10 for 10 strips. Which is a waste of money when they read ketones, or no ketones (because I ate carbs the day before). Seems pointless relying on a strip when my I remember eating that potato, and can predict the reading, doesn't it?
When you get right down to it, I don't believe there is much need for people to spend money testing for ketones when in nutritional ketosis. I mean, lets face it, if someone is eating exceedingly low carb, and getting their dietary energy from mainly fat and protein, and they are feeling well, then it doesn't matter if they are in keto 24/7 or skimming in and out of it like a stone skipping the surface of a pond.
They
feel well, and their blood glucose is under control (hopefully they test bg now and then).
If they don't feel well, then that is a whole different ball game, and worth investigating, but I doubt it will be nutritional ketones causing the problem.
It is a large and complex subject, but frankly, if you aren't eating enough carbs for their energy to fuel your daily activities, then you are in ketosis. Simples. The one caveat on that is if you are eating silly-high amounts of protein. But appetite and common sense usually stops people from that, and the body is very good at limiting excess protein, by just making it unappetising to eat more than is appropriate.