--but those "one shot" blood glucose levels are notoriously unreliable; they are affected greatly by what you've eaten, when you've eaten it, how much exercise you've gotten lately, stress, meds . . .

I second (or is it third?) the recommendation to get a long-term glucose tolerance test, with fasting glucose/insulin levels drawwn before you drink the sugary concoction and then at half-hour intervals afterwards. Not only are the gross readings important, but the insulin/glucose patterns, the rise and fall over time, are also illuminating as to just what may be going on.