When I was diagnosed in the USA, I learned all about blood sugar in mg/dl. I was taught that a low blood sugar was anything under 70 mg/dl (a nice round number).
I moved to Hungary and after spending hours converting my blood sugar logs from mg/dl to mmol/L before each doctor's appointment, I finally decided to switch to using mmol/L in my daily life last year. I got used to the new measurements pretty quickly (though I still do sometimes convert the numbers). I was treating lows as anything below 3.9 mmol/L (since 70/18.05=3.87).
At one point my new CDE told me that lows are anything below 3.5 mmol/L (which is 63.17 mg/dl). Magically after that I started not feeling low at 3.7 :-) So I stopped treating the lows unless they are below 3.5. My endo still tries to keep my blood sugars above 4, but considers below 3.5 as the real lows.
But I was wondering about the rest of you who measure in mmol/L -- what is your cut-off for a low blood sugar?
I was taught that '4 is the floor' but that the actual threshold was 3.9. In the first weeks after dx, because my BG had been so high walking around undiagnosed, I would feel low even at 4-6. I then went through a period when I could go as low as 1.2 and not even feel it. That was followed by a period of illness and stress when my BG went out the window and that seemed to re-set my hypo awareness.
My advice would be to keep testing and find out how a set amount of carb affects your BG. In the beginning I over-corrected like mad. Later, through rigorous testing, I found out that each 10g of carb raises by BG by 3 mmol/L, and that this is fairly consistent. I keep hypo fixes in pre-measured 10g carb portions but if it's 3.5-3.9 I know I only need about 3g. I only take the whole 10g when it's under 3. I also take into account how much insulin is on board and whether it's trending up or down. This is what works for me, obviously your mileage may vary.
PS Meters have an error margin so if it's 4-ish but you are feeling weird, then I'd get a 'second opinion' using another finger.
I try to keep my awareness for lows by treating them early with glucose tabs - the tasty ones you already know - not the chalk ;-) By not getting used to low levels I start to feel values below 80 mg/dl. A kind of nervousness sets in and this is amplified when the descent rate is higher than normal. I think I would not have preserved this awareness for more than 20 years without treating lows early. As you now my A1c is in the 5ths and this is a permanent dance on the edge of going to low - the target corridor for normal is just very small. So for me it is not so much the exact number that counts but the feeling associated with it. Todays metering technology is too prone to errors to draw the line at 70mg/dl or 3.87mmol/l. I am questioning if the 85mg/dl I have measured might be something like 75mg/dl or lower - depending on my feelings. Too often I have shaken my hands and retested and with the new blood in my fingers I was lower - maybe 15 or more.
I think when a doctor sets a lower margin for lows he is assuming that the patient will overtreat the low leading to higher numbers later. I can understand this reasoning but I think it would be better to recommend the use of glucose tabs only. One tab will bring you from 70 to 85 or from 60 to 75 and that is an important step away from the low. With juice this rise will set in later and some of that juice will come much later because of the fructose. For me even short term activity gets a little glucose tab on top. If I need to ride my bicycle for 20min and I have not prepared for that a starting number of 90 would lead to 70. Here again one tab does the trick and prevents the low. This kind of proactive counterreaction gets more and more important the more normal your A1c.
Decision about low treatment = value + feeling + planned activity
I'd change the equation, in my case to value + recent IOB, activity, meal history + planned activity.
I still have low symptoms--but they tend to be less the sweaty, shaky symptoms of my youth and more the "I feel tired," which can also be attributed to my job, my family, my cats, etc., etc.
Some of this is due to age, some is due to the glucose levels at which I have lived for 37 years, and some is due to tighter control--I can slip just a few points to a low now that I live in a more narrow range, and it can be more difficult to sense a gradual decrease than a rapid one.
On a course we did they mentioned being at four ur near the floor so if I am 4.3 I consider it as a hypo.