Serum VEGF measurements are higher than plasma VEGF measurements because platelets release VEGF into the serum during clotting.

“Serum vs plasma: make sure you check whether your VEGF is being measured in serum or plasma; serum values are much higher, in short because the platelets are the major reservoir for VEGF in circulation. This is because serum is taken after the blood is allowed to clot, in the process platelets release their high content of vegf into the serum, that does not happen in plasma in which clotting is inhibited and then the sample is centrifuged to separate the cell from liquid compartments.”