Again, no expert, but glucose, insulin levels and blood oxygen have been measured in a drop of blood forever, so I guess they are sufficiently homogeneous and also available in sufficient amounts even in small drops of blood for that to work. But the promise of Theranos was that basically every serological test could be performed on a drop of blood (even many tests with the same drop).
I don't see it impossible if you account for expected changes. It probably needs a lot of more research and better technologies, so we are not there yet.
