system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Scientifically, why can't blood testing using a few drops of blood (i.e. Theranos' purported and testing regime) work? What can we actually test for using single drop blood testing?

Completely putting aside the fact that the Edison machine didn't work, what is and isn't possible for pathologists to do in terms of testing using a few drops of blood?

It's currently most commonly used for blood sugar, and this particular video I watched said that it isn't necessarily reliable for some tests, and some require separation or the addition of chemicals. Another article I remember reading said it can make your potassium levels look like you're dying.

That can't really be it to why single drop blood testing can't work, right? As far as I've seen, no one really has seemed to clearly explain in layman's terms as to why it couldn't or can't work. And likewise, what can actually be done accurately using such a test?

Edit: Thanks for all the really thoughtful and clear answers. Even from the answers alone some people don't seem to agree with one another - hopefully Reddit will help parse out which of these answers are more correct than others!
gpt
In a single drop of blood, you can test things that are present in the blood in large quantities.  That's great for cholesterol or red cell count.  However, most blood tests look for molecules which are not that dense in the blood.  If you want to measure the level of PSA, a biomarker for prostate cancer, a traditional test measures in nanograms/milliliter; 1-4 is ok, >6 is a warning. A typical drop in 0.05ml, so you're looking for something in the range of 50-200 picograms.  That's only thousands of PSA molecules.  To get a good reading, your machine would have to work at an atomic level.