Apheresis is a medical procedure that involves a blood donor receiving their blood back once the platelets or plasma have been extracted. The other blood components, including red blood cells, are still intact, but either the platelets or plasma have been removed by apheresis. When a person's blood has a high platelet count, it can lead to clotting. Clotting occurs when the blood comes together to form a clump, which stops blood flowing from cuts. If there are not enough platelets in a person's blood, then clotting may not occur as the blood may be too thin.

The word apheresis comes from the Greek for, "to separate" or "to take away". Apheresis is pronounced ay-fur-ee-sis. Platelets extracted by this method have a shelf life of five days.

Anyone over 18 years of age can donate blood for an apheresis. An apheresis donor must weigh at least 50 kg (110 lb) and must have donated blood in the last two years. The previous blood donation must have had a good blood flow, and the donor must have had no adverse reactions. The blood group criteria for plasmapheresis is A, B or AB (Rh positive or negative), and for plateletpheresis it is A or O (Rh positive or negative).

The procedure for apheresis donation is much the same as a normal blood donation. The blood will be extracted from the arm through a sterile, disposable kit that is housed in a special machine. The machine is called a cell-separator. The cell-separator removes the required components from the blood by using another machine called a centrifuge. The remaining blood is returned to the donor using the same needle.

During the apheresis procedure, citrate is added to the blood to prevent the blood from clotting while it is outside of the body. A small amount of the citrate returns to the donor. This may cause a sensation of coldness and tingling around the mouth area during the donation, but this will quickly subside. The citrate is broken down very quickly once it enters the bloodstream.

Apheresis is performed when the target constituents in a patient's blood cause severe symptoms of disease, such as bleeding problems, or diseases such as cancer. Apheresis has to be performed quite often. As it is an invasive procedure, it is only performed if all other means of controlling the disease have failed. It is also performed if the symptoms of a disease are so severe that there is a risk of complications or suffering while waiting for medication to work.