Plasma: Whole blood minus the cells–obtained by centrifuging the blood.
Serum: Whole blood minus the clotting elements–obtained after blood is allowed to stand and clot. It is the clearer liquid above the clotted red cellular material.
An important inquiry, particularly where the blood alcohol analysis was done by a hospital rather that a forensic lab is whether whole blood or plasma/serum was tested. Forensic labs typically test whole blood. This is important because plasma or serum values are higher than whole blood values. The variance can be anywhere from 16-20 percent. For that reason, when you are looking at the hospital blood results, if they tested plasma or serum, be prepared to convert it to its whole blood value. There is a formula to do that.