The best way to monitor Ketones is with a meter the CareSens Dual and various other meters can measure Ketones with Ketone test strips though they do cost more the libre reader can also use ketone test strips to test Ketone levels.
"How to measure ketone levels
The most reliable way to measure your ketone levels is to use a blood ketone meter.
These devices work similarly to blood sugar meters, as you use a lancet and a blood testing strip to collect a small blood sample from your fingertip, which the device then reads.
Although your body produces three types of ketones, most blood ketone meters check for beta-hydroxybutyrate, which is the most prevalent type (6Trusted Source).
There’s some disagreement among researchers about the best time of day to test ketone levels. You should pick a time and consistently test at that time to get the most accurate readings.
When you last ate appears to be a major contributing factor to your ketone levels, so you may want to do your ketone testing 3 hours after eating a meal (7Trusted Source).
Urine and breath testing
You can also check to see whether you’re in ketosis by using urine test strips or a breath test. These methods are less accurate but easier than using a blood ketone meter.
Urine tests, which are test strips that you dip in your urine, change color based on the presence of the ketone acetoacetate. The darkness of the strip is an indicator of the extent to which you’re in ketosis. The results may be inaccurate if you’re dehydrated (8Trusted Source).
Urine strips are inexpensive but also the least accurate testing method (8Trusted Source).
On the other hand, breath tests measure the amount of the ketone acetone in your breath in parts per million (ppm).
Reliable breath testing devices are fairly expensive and slightly less accurate than blood testing, but they’re painless and do not require the purchase of extra supplies like lancets or strips (9Trusted Source).
Breath acetone levels ranging from 2–40 ppm, and even higher in some instances, may indicate nutritional ketosis (10Trusted Source)."
https://www.healthline.com/nutrition/ideal-ketosis-level-for-weight-loss#measuring
From this site
"The sweet spot for weight loss is 1.5 to 3.0 mmol/l. This level of nutritional ketosis is recommended by researchers Stephen Phinney and Jeff Volek."
https://www.diabetes.co.uk/keto/measuring-ketosis-on-a-keto-diet.html
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https://ketodietapp.com/Blog/lchf/Ketosis-Measuring-Ketones