Depending upon the tests that have been ordered, your blood sample may be processed before it is analyzed. Most routine laboratory tests are performed on either plasma or serum. Plasma is the liquid portion of blood. It is separated from the cellular portion of blood by rapidly spinning the specimen in a centrifuge for several minutes. The plasma, which has a light yellow color, appears at the top of the tube, while the blood cells are at the bottom.
Serum is plasma that has been allowed to clot. It is prepared in the same way as plasma - however, the blood is collected into a tube with no anticoagulant. While spinning in the centrifuge, the clot moves with the cells to the bottom of the blood collection tube, leaving the serum on top.
If the test requires whole blood (e.g., a complete blood count), the sample can be analyzed directly without further processing.