nothingbetter
They test for ketones in urine - a sign of a ketosis metabolism. There is really no point in using them at all during a fast - you will definitely go into ketosis after 2-4 days. They are only really of interest to diabetics and people on Atkins diets (the later of which need to confirm they are in ketosis since unlike with fasting its not guaranteed - the ketosis in Atkins does not have the same benefits as in fasting however because Atkins dieters process tons of fat and protein).