Mel,
Glenn should probably answer the question, but as I understand it (and I've had the 3 hour test-not the 5 hour one) each reading gives a different rate that the body uses (or misuses) the glucose. By comparing those figures, a good Dx'ng doc can determine at what rate and how the body is either intolerant, or is marginally creeping up to the levels where the pre-diabetic stage occurs. My doc at Hopkins- Dr. Griffin, is a proponent of the theory that PN is onset by a pre-diabetic condition when there is no other obvious causation. (and then there are obvious markers that he believes contribute to the pre-diabetic condition).
We joke about the fact that I could have PN for 50 years and not develop diabetes until year 49 and he would say
"AHA !! See, I told you!".
A pre-diabetic condition, as he explains it- does not occur, then go away.
If you are pre -diabetic, you stay pre-diabetic until you develop a case of full blown diabetes. If you don't develop diabetes- you're still considered to be pre-daibetic with the propensity towards developing it. The percentiles may change and the probabilities may alter, but he still considers a pre-diabetic to stay that way until the diabetic condition occurs. (Which may be never, but the odds of developing it at later stages of advanced age, are pretty much in favor of getting a type 2 before the eulogy is read.)