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.
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...
I was at one of those firms 25 years ago and they were working on it back then. Either they abandoned the effort, or are still trying, but they haven't launched a single microfluidic immunoassay.
These assays are performed regularly on large samples no problem, but the sensitivity and accuracy of the assay becomes erratic when you only have a couple micro liters of blood.
Either they couldn't develop proper assays (an engineering problem) or microliters of blood don't contain enough substrates for their assays (a major lapse in knowledge)