OK, I,ll bite. It has to be something that would make the plasma more dense than the red cells or the red cells less dense than the plasma. If the former, it would have to be in solution, because anything in suspension would spin out below the red cells. I am betting on a plasma expander or maybe the cardiac cath dye. What's the half-life of these anyway? Regardless of the cause, is the sample okay for testing? Did it do weird things in tube, in gel??? Would you get the same results with serum? Did the patient survive?