What is a LOW -- 3.5 or 3.9? -- for members measuring in mmol/L

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?

Hi there. My advice is to ALWAYS treat your hypos if you ‘feel’ low, also if you are 3.9mmol or below. Normal BG range is between 4-9mmol so anything below 4 is a hypo. Sometimes you may not feel them at 3.9mmol because after having a few they aren’t so recognisable. Your system will send out a warning to you whenever a hypo is occuring but sometimes if you have regular hypos, are drunk or sleepy this warning system can take a little time to kick in and the hypo isn’t so recognisable. I’m not sure I agree entirely with your new CDE’s advice to be honest. I always try to keep my sugars between 4-9mmol before meals (70-162mg/l).

Yesterday i was at 1.97 mmol/L (37 mg/dl). The scarry part is that i was feeling just normal.... I guess anything below 70 is a low, since its beyond the normal blood range....

Happy New Year :)

Well, I remember a Canadian on another list saying “Four is the floor”. So, since I think level BGs are probably best, especially during pregnancy, I see no reason why you shouldn’t treat at 3.9. Plus doing that would prevent you from going even lower. Of course, the challenge is not to overtreat! Good luck!

I am verysituational in my approach to ‘lows’. If I’m in bed in the middle of the night and check and it’s 70, I don’t worry about it but, if I am doing something that will lower my BG-- running, shopping, running up and down the stairs to load the car up for a vacation-- I will treat it with more? Really, I would treat up to 100 if I am going running because I want to be up a shade for that sort of thing. I am trying to work with 10G as a ‘treatment’ instead of the ADA recommended 15 as I think that this helps avoid spiking on the other side of it? I am trying to keep my BG ‘stable’ rather than ‘avoid’ lows? Of course, accidents happen!

The difference detween 3.5 and 3.9 or 63 and 70 is really insignificant. The thing that is probably m,ore important is in which direction are the numbers moving? Thats where the CGM comes in. Istopped using mine because of insurance coverage but recently decided its only monewy and I cant take it with me so I ordered one out of pocket. I’m hoping that the directional arrows and trends on the Dexcom will provide the proper guidance for this situation!!

I think one has to be careful in letting the definition of a low drift. If you start to spend time at lower blood sugars, they will feel natural and you will lose hypoawareness and your natural counterregulatory response. It is important to protect that boundary, and I guess my thinking is that it should be 70mg/dl, not 63 ml/dl. You should treat anything under 70 mg/dl and your observation that you no longer feel low in the 60s is reason enough to hold that line. If you choose to only treat lows below 63 mg/dl, then you will likely start to not feel lows in the 50s, and that is too close for comfort. At that level you have no margin before adverse consequences on your thinking set in and you can fall victim to lows that require outside help. I never want to require outside help.

My recommendation, continue to treat anything below 70 mg/dl as a low. You can consider below 63 mg/dl as a "real" low, but hypoawareness should be "coveted" and protected.


Here in Holland everything below 4 is a hypo.

We're told by our endo's to stay between 4 - 7, not higher.

Any value above 7 is considered a hyper.

Good luck with this!

I can imagine that it is difficult to switch!

Thanks for all for your responses!

This may be unique to the fact that I was preparing for pregnancy. Non-diabetic pregnant women have lower blood sugar (often in the 60's). I am currently 14 weeks pregnant -- so it is probably OK to treat 3.5 as a low. But after pregnancy, I will go back to 4, especially while taking care of an infant!

Before pregnancy I felt my lows under 4, but during pregnancy I started only feeling them below 3.5.

Good point BSC. I would definitely like to keep my hypo awareness, which has proved very accurate so far.

Yes, this is a good point! I always correct a 70 before bed.

I don’t have a CGM, but it is true that knowing the direction does help. If I am borderline low (but NOT low), I will wait 15 minutes and test again to see if I am trending up or down.

Yes, that is one of my biggest fear – being low and not feeling it. Glad that you caught the low and that you are OK! Happy new year to you too!

I don't measure in mmol/L, but I only treat lows below 60. There seem to be 60 people and 70 people, so your 3.5 which is 63 sounds like a good compromise!

I do treat 70 and even in the 70s if it is bedtime. I'm also pretty sensitive to carbs so only need two glucose tabs unless I'm really low when I'll need 3.

Who knows for sure since the modern meters are inaccurate. I definitely feel it at 3.5 assuming the meter is correct. I have felt low with 5 using accucheck I am sure it was a bum reading. It is important to wash hands if you have eaten any fruit something we may forget to do.


It has been quite a while since I've been on tudiabetes and I've only just seen your prenancy!!! Congratulations! I'm delighted for you! I'm just over 33 weeks myself, hard to believe!

To answer your original question, I was always told that 4.0 was low until I got pregnant and then was told not to treat them unless under 3.5 since I have been pregnant. Of couse, if I feel low I treat them anyway!

I do not recall reading this ...maybe I missed it ...but for me important to know , how much insulin on board , when I test and show 3.9 ( lower or higher ) ....this will let me treat or not treat . And it depends , what I will do following the test ...just sit or move the body .

I am somewhat concerned , if I am at 3.5 and worry about re-occurring unawareness .As we age and for my security ( and I am over 70 ) , I believe that I need a higher number than 3.5 .

If I'm drifting down, I treat below 70 but only take 1 glucose tab (4 carbs). That will bring me into the 80s - I don't see any point of taking a 65 and bumping to 125 and then correcting back down. All those swings only increase the chance of ending up either way high or way low.

I'm with AcidRock in trying to maintain stable blood sugars rather than trying to avoid lows.


I think that it’s also important to have a flexible definition of ‘treat’? If I’m in bed in the middle of the night and hear the bleeping bleeping, I will check and, if it’s like 60, I’ll just have maybe 6 or 7 jelly beans to avoid a big spike? If it’s lower in the middle of the night, I don’t eat more but turn my basal rate down. Sometimes it will bleep for a while before I catch up but if I eat 15G of carbs, my BG seems to really go up afterwards? This is kind of true a lot of the time? If your insulin doses are small and you don’t have a lot on board, I think that you can get away with 10G of carbs, rather than the 15/15 recommendation? If I have 10G and wait 20 minutes, it seems to work pretty well most of the time without the big peaks afterwards. If I have a load of insulin on board and just whiffed and missed the carbs or something, I’ll eat more but I think that there may be a useful advantage to controlling carbs, even when you are lower?

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.