I read somewhere that for diabetics low blood sugar is below 70 and healthy people it is below 55. Double check that and maybe talk to a doctor about extended fasting if you are diabetic. (If you find one that actually knows anything about it, post their contact info.)
Not sure how ketones play into that mix or how you’d get your blood sugar to 55 or 70 without being in ketosis.
Know the symptoms of hypoglycemia and have some glucose pills handy.
Unless you have type 1 diabetes you will be fine. When you are fasting your body uses stored energy to make ketones which will provide cells with the energy they need. You might experience some brain fog and lethargy from low blood sugar/not eating but it is not hazardous.
Why are you testing your blood sugar? If you’re having symptoms of hypoglycemia that’s a different thing. In the emergency world we generally treat symptoms below 70 mg/dl or 3.9 mmol/L … I’ve had in the 56-60 mg/dl range while on a three week fast with no symptoms.