My experience thus far has been similar to yours. I was diagnosed pre-diabetic 2 years ago and didn't do much about it. ~5 weeks ago I went on low carb after my A1C shot up over 10. My fasting BG was 250 and spiking up to 325 after a meal. Right now I my BG is averaging about 120.

I've been doing a lot of reading. Google some of the articles that talk about LC diets inducing insulin resistance. As I understand it, just as it takes your body time to become "fat adapted" it would also take time for your body to adjust to carbs when you end a ketogenic diet. I don't think that suddenly ending your diet, running right out and scarfing down a bunch of carbs is going to give you an accurate picture, if indeed the LC diet INDUCES a form of insulin resistance. I plan to do it gradually, giving my body a few weeks to adjust before doing anything like a glucose tolerance test.