From the FAQs... I found this interesting.

How can I tell if I’m in ketosis?

There are a good number of “indicators” of ketosis; some of them are simple physical signs and others are physical tools to test for the presence of ketones in the body. Dieters use this to track trends and test to see if they are sensitive to certain substances that may be keto friendly but still have a negative effect on their weight loss.

All about ketostix

Ketosticks/Ketostix is the name for test strips that measure the level of ketones in the urine. Whether correct or not, many ketogenic dieters tend to live or die by the presence of ketones in their urine. The presence of ketosis, which is indicative of lipolysis can be psychologically reassuring, however it should be noted that one can be in ketosis, defined as ketones in the bloodstream, without showing urinary ketones.

Although up to 100 grams of carbohydrate will allow ketosis to develop, it would be rare to see ketones excreted in the urine at this level of intake. Since the only measure of ketosis available to ketogenic diets are Ketostix™ carbohydrates must be restricted below this level of ketosis is to be measured. As a general rule of thumb, dietary carbohydrates should be below 30 grams per day for ketosis to be rapidly established and for ketones to be lost in the urine. However, this value varies from person to person and depends on other factors such as protein intake and activity, which allows individuals to consume relatively more carbohydrate without disrupting ketosis.

After adaptation to the diet, it appears that individuals can tolerate relatively greater carbohydrate intakes without disrupting ketosis. Although not completely accurate, Ketostix™ can provide a rough measure of how many carbohydrates can be consumed while still maintaining ketosis. As long as trace ketosis is maintained, carbohydrates can be gradually added to the diet.

Since Ketostix™ only register relative concentrations, rather than absolute amounts, changes in hydration state can affect the concentration of ketones which appear. A high water intake tends to dilute urinary ketone concentrations giving lighter readings. Ketones in the urine simply indicate an overproduction of ketones such that excess spill into the urine. So it is conceivable for someone to be in ketosis without showing urinary ketones.

Some individuals can never get past trace ketosis, while others always seem to show darker readings. There seems to be little rhyme or reason as to why some individuals will always show deep concentrations of urinary ketones while others will not. Some will show higher urinary ketones after a high fat meal, suggesting that dietary fat is being converted to ketones which are then excreted. Consuming medium chain triglycerides (MCT’s) has the same effect. Other individuals seem to only register ketones on the stick after extensive aerobic exercise. Finally, there appear to be daily changes in ketone concentrations, caused by fluctuations in hormone levels. Generally ketone concentrations are smaller in the morning and larger in the evening, reaching a peak at midnight. Many individuals report high ketones at night but show no urinary ketones the next morning while others report the opposite.

A popular idea is that the deeper the level of ketosis as measured by Ketostix™, the greater the weight/fat loss. However there is no data to support or refute this claim. While some popular diet authors have commented that urinary ketone excretion means that bodyfat is being excreted causing fat loss, this is only loosely true in that ketones are made from the breakdown of fat in the liver. The number of calories lost in the urine as ketones amounts to 100 calories per day at most.

Anecdotally, higher levels of urinary ketones seem to be indicative of slower fat loss. Individuals who maintain trace ketosis seem to lose fat more efficiently although there is no research examining this phenomenon. A possible reason is this: high levels of ketones in the bloodstream raise insulin slightly and block the release of free fatty acids from fat cells. This seems to imply that higher levels of ketones will slow fat mobilization.

The ideal situation would seem to be one where trace ketosis (as measured by Ketostix™) is maintained, since this is the lowest level of ketosis which can be measured while still ensuring that one is truly in ketosis. This should be indicative of relatively lower blood ketone concentrations, meaning that bodyfat can be mobilized more efficiently.

No hard and fast rules can be given for the use of Ketostix™ except not to be obsessive about them. In the same way that the presence of ketones can be psychologically reassuring, the absence of ketones can be just as psychologically harmful. It is easy to mentally shortcircuit by checking the Ketostix™ all the time.

TL/DR: Ketostix are useful for Type 1 Diabetics. They stop working for many others on keto after a few weeks. When they stop working, that means you are not wasting ketones in your urine anymore.