Last month (March ’17) I did two extended fasts. I separated the two fasts by four days. The fasts were both ten days.
I tracked a bunch of numbers during the last fast including my Blood Ketone levels. I bought a Precision Xtra meter to measure Blood Ketones. The meter cost $25 on Ebay.
The meter comes with no strips. The strips are really expensive, at about $3 a strip. They are more accurate than any other home method of measuring Blood Ketones.
Blood Ketone numbers greater 0.5 indicate that you are in Ketosis. I saw numbers as high as 6.8 while doing extended fasting. Most non-fasting days I get values around 2.0.