As was mentioned in previous posts (here and here), whole blood can go into a test tube and then be separated into layers with cells in the bottom portion of the tube and plasma above. It turns out that although some lab tests require whole blood or plasma, others need only blood serum (pronouced how rare steaks can be cooked – just sear ’em).
Serum is the result when clotting proteins are removed from plasma.
When a clot activator is added to the test tube in which blood is drawn, the sample can be centrifuged, leaving only serum at the top of the tube. This serum can then be poured off and tested.
Still trying to break information down into bite-sized chunks.