Simple:
Not needed, they are inaccurate and when they do work it is only for a short period of time.Harder:
Ketostix measure ketone levels in urine, these are primarily for diabetics to ensure they are not in danger from ketoacidosis. For the first few days or weeks of a ketogenic diet your body isn't as efficient at using ketones as it is producing them, so the extras come out in your urine and the sticks change colors based on the concentration of ketones (hydration level drastically affects this). As your body adapts to being in ketosis it is better able to utilize all of the ketones you produced and you will therefore have fewer to lose through urine.So, Ketostix can be used in the beginning, though they are not needed, to confirm you are producing ketones, but later won't tell you anything at all.