I am going to try and answer this question, because I have some experience and training in the subject.

There is a separate system in the brain, that involves insulin.
That area cannot be tested yet at this time, with a blood test.

A blood test will show the rest of the body however.
Sometimes fasting insulins are elevated (as well as fasting glucose), but the person is not severely diabetic yet. A better test is the HbA1C which shows trends of 3 months or so. If you HbA1C is below six, you are in a good place. At 7 and above that is worrisome. In between you would be considered "borderline" and need to watch diet more closely.

The failure of the cells in the brain to produce insulin (yes they are separate from the pancreas), can happen. This is called Type 3 diabetes,now, and is thought to be connected to PD, Alzheimer's and other neurodegenerative diseases.

This is an article about it:
http://www.diabetesincontrol.com/ind...rticle&id=2582

Extremely low blood sugars are also problematic because the brain really needs glucose to function properly.
Low blood sugars can precede diabetes, and they are called insulin resistance. Very low blood sugars are serious where the brain is concerned.

Today we still don't have much knowledge about the brain--and the internal production of insulin, however. It is still a new finding.