My fingers, and the rest of my body are much colder when my BG is low. The lower the BG, the colder I am. I can put on a sweater on a hot day sometimes when I drop very low. Then when I run high I get very warm and i want a cold drink and i take off some clothes.
I don't have circulation problems, but the blood is harder to squeeze out of my fingers when my BG is low and I am cold. Are you sure it is due to poor circulation when you are having this problem? Maybe your BG is low. If you always have cold fingers and always have trouble drawing blood, then that is different, but I thought it might be low BG.
I have tried my forearms for testing and there was too much discrepancy when compared to my fingers. I do use my fingers between the first and second joints, as well as the finger tips. That works out well for me. I have to adjust the depth when testing between the joints.