Medimoon.com explains that plasma is the liquid portion of the blood that has had the red and white blood cells removed, whereas serum is plasma that has also had the clotting factors removed. Although both plasma and serum are used in a variety of medical tests, the Health Sciences eTraining Foundation explains that plasma is preferred to serum for most modern diagnostic tests.
The first step to study a person’s blood requires a technician or nurse to fill a vial with a sample of whole blood. This blood may come from an artery, vein or capillary, according to the Health Sciences eTraining Foundation. Whole blood contains plasma, white blood cells, red blood cells and platelets, which are also known as clotting factors. Whole blood is used for some tests, but if plasma or serum is desired, additional steps are necessary. Often, the sample is mixed with an anticoagulant, which prevents the blood from forming a clot.
The Regional Medical Laboratory explains that to separate the various components of the blood, the sample is put into a machine known as a centrifuge, which spins the sample at a high speed. This spinning causes the components of the blood to clump together into various layers. By removing the layers that contain red and white blood cells, medical technicians can separate the plasma from the sample. To remove the clotting factors, a gel is inserted into the sample, which causes the blood to form a clot. This clotting process draws the platelets out of the plasma, which yields serum.