What constitutes a hypo?

I don't think there is any official cut-off where a hypo begins. If a normal fasting blood-sugar level is 5.0, with some people going as low as 4.5, then it seems to me that anything that is 3.5 or lower could be considered a hypo. Of course, you are not going to pass out at 3.5, but anything lower than a 2.0 should be considered dangerous in my opinion. The threshold at which a person passes out probably changes from one person to the next.

One other thing you should know: The symptoms you get when you are having a hypo (heart pounding, sweating) are caused by a release of epinephrine (adrenaline), which is your body's way of alerting you to the dangerous situation. Now, if you are coming down from very high BS levels (say, 11.0 or so), your body may secrete epinephrine if you go to only 4.5. But if your BS levels are near normal, it will take a lower level for the epinephrine to be released. If your body is experiencing frequent hypos, at a certain point it says, "This is normal", and it stops releasing the epinephrine. In other words, the epinephrine is your warning system that something is wrong. So it is really important that you learn to control your hypos so that you don't lose that important warning system.

Hypo's are generally considered to be present at a reading below 4.0 mmol/L but you may experience hypo symptoms at slightly higher readings as well.

Bear in mind that there is a certain amount of error permitted in blood glucose meters as well, which could play some part.

Swimmer,

Some useful information here from Diabetes UK on driving and hypoglycemia:

http://www.diabetes.org.uk/Guide-to-dia ... glycaemia/

Ah you guys are so helpful I came in here to ask a similar question

I already have to make sure I have something to eat before I do my morning body pump class as I was getting what I thought was low blood sugar. Anyway now I've had it a couple of times in the day, hard to concentrate, feeling a bit wobbly, sweaty and all that, and I have tested the last two times and found I was on like 4.5. I normally run around 5.5 to 6.5 on my meter.

I wouldn't say I'm a big eater normally, I have my moments but today I'd only had a small Bacon and sausage roll, and a slice of veggie fritata till 5pm. A bit of yummy beef jerky has sorted me right out.

I've been trying to learn to use my senses to decide if my blood sugar is ok or not, and I would say at 7.5 I start to get like a thirsty feeling kinda, where when I get to 4.5 I definately start to feel a little wibbly. Its clearly not a worry situation, but I've been doing complex work all day and suddenly I found it impossible to concentrate till I'd eaten something. So I feel lie you're right, at about 4.5 it can start to affect you in mild ways.