[forgotpwagain]

Oops, missed this comment. If anyone is reading still reading this...

To be honest, I don't know. I think it is possible to get a lot of information out of a single drop of blood. But there are issues with sampling--blood from the capillaries in my finger is very different from blood coming back from my intestine.

Maybe more fundamentally: a lot of "interesting" things are specific proteins. These can be at extremely low concentration--on the order of nanomolar all the way down to femtomolar. It's very difficult to amplify such a signal -- DNA is easy to amplify but proteins are not. At such low levels, the wrong drop of blood may have a dramatically different meaning, due to stochastic effects.