Judging from what I read in comments on Derek Lowe's blog, the problem is not so much being able to do blood tests on µL of blood (which I suspect we might have the technology to do; whether it's cost-effective is a different question) but rather that the way you get only a little bit of blood screws up the results--you're extracting more from capillaries than veins, and you get more contamination from skin and other tissue cells.