From the many blood draws I have done, standard practice is to fill a tosser vial before collecting the real samples in order to remove these contaminants.
Now, it's possible I'm mis-remembering this, although the protocol includes showing me the vial after the draw so I can confirm it's labeled correctly and I tend to remember additional vials, since that manipulation often hurts more than anything else. I'll pay attention, heck, ask the phlebotomist in a few months during my next annual checkup.
There's been some research into using piezoelectric elements coated with antigen or other reactive surfaces.
A positive result (meaning antibodies are attaching themselves to the substrate) would change the resonant frequency of the surface.
http://www.sciencedirect.com/science/article/pii/S0003267000...
http://www.cbc.ca/news/canada/british-columbia/uvic-blood-te...
Whereas a fingerstick produces a few drops of blood that flow to the surface moving past the tissue and cells that were cut getting to the blood vessels, and they'll contribute unrelated proteins, lipids from cell membranes, etc. etc. On its face, many many people have said this is so problematic real evidence that it worked was needed; heck, even a theory would have been nice, but neither were forthcoming.