[NoPie]

The counterpoint is Freestyle libre sensor. It measures glucose levels not in the blood but intercellular liquid and while not as precise as blood test, it is working.

Even if blood in capillaries is different, the measurements can still be meaningful if those differences are systematic and can be accounted for. We just need better science and technology to make use of it.

[dr_faustus]

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).

[NoPie]

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.