This morning, after water fasting for 19 hours, my ketones were 3.9. Typically, I’m in the range of 2.5-3.1. Phinney & Volek say that optimal is between 1-3. I wouldn’t say that I’m concerned- more curious if others register this high from time to time.
(edited) Okay- I just saw in the fasting section that people sometimes have ketones much higher than 3.9. Sorry- I thought I searched before posting but I guess I missed that one 
Don’t worry about the actual measurement - look at it as more binary of In/Out of ketone in your body.
I’m on day 5 of an extended fast and my blood Ketones are at 4.8 mmol. I have the same concern. I’ll check the fasting section
My max was 5.7 mmol/L which occurred at the end of a one day fast. That’s definitely an atypical reading for me. Average over all of the readings I’ve taken is 1.4, though lately (over last month or so), my average is probably only around 0.8.
Anyway… on that 5.7 day I was feeling fine and had no idea ahead of time that I’d get that reading.