When I went to see a consultant (don't shoot the messenger here he really annoyed me to) he batted back everything I asked for. But we did have the best part of 40mins together and even though he didn't do what I wanted he was a professor so he wasn't stupid. He said testing insulin was not very helpful. He said it would not change my course of treatment. He said it fluctuates all of the time far more than BG and not so easy to attribute. It is used when fat needs storing, when BG is high and for other reasons (which he didn't go in to). His view was HBA1C and FBG much better descriptors.
I put it to him that surely high insulin suggested IR and low insulin suggested broken Beta cells and again after telling me it wouldn't change the basic advice to lose a bit of weight and look at your diet and eventually take pills then injections (cheery soul). He said you could draw very little conclusions from an insulin test unless they were at the far ends of the scales.
He did concede but was not prepared to entertain one for me, that the clamp test (is that it?) was a good test. Where ever you can be in charge of the variables he said it was a good test. But too expensive, too time consuming and actually to disruptive to the patient. And he said - what are you going to do differently?
Now I personally do not prescribe to any of that above - I like to be armed with knowledge and data but that is what he said.
Please don't bomb me these are not the opinions of the poster I am simply sharing what cost the best part of 600ukp.