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 a

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).