Apheresis (ἀφαίρεσις (aphairesis, "a taking away")) is a medical technology in which the blood of a person is passed through an apparatus that separates out one particular constituent and returns the remainder to the circulation.  It is thus an extracorporeal therapy.
 One of the uses of apheresis is for collecting stem cells.
 Depending on the substance that is being removed, different processes are employed in apheresis. If separation by density is required, centrifugation is the most common method. Other methods involve absorption onto beads coated with an absorbent material and filtration.[citation needed]
 The centrifugation method can be divided into two basic categories:[citation needed]
 Continuous flow centrifugation (CFC) historically required two venipunctures as "continuous" means the blood is collected, spun, and returned simultaneously. Newer systems can use a single venipuncture by pooling blood in a vessel and cycling through drawing and returning blood though the needle while the centrifuge continuously processes blood remaining in the vessel. The main advantage of this system is the low extracorporeal volume (calculated by volume of the apheresis chamber, the donor's hematocrit, and total blood volume of the donor) used in the procedure, which may be advantageous in the elderly and for children.[citation needed]
 Intermittent flow centrifugation (IFC) works in cycles, taking blood, spinning/processing it and then giving back the unused parts to the donor in a bolus. The main advantage is a single venipuncture site. It does require a larger extracorporeal volume, and takes significantly longer to perform the procedure via IFC. As such, it is less likely to be used for therapeutic reasons, and is often seen in Donation Center settings. To stop the blood from coagulating, anticoagulant is automatically mixed with the blood as it is pumped from the body into the apheresis machine.[citation needed]
 The centrifugation process itself has four variables that can be controlled to selectively remove desired components. The first is spin speed and bowl diameter, the second is "sit time" in centrifuge, the third is solutes added, and the fourth is not as easily controllable: plasma volume and cellular content of the donor. The end product in most cases is the classic sedimented blood sample with the RBCs at the bottom, the buffy coat of platelets and WBCs (lymphocytes/granulocytes, PMNs, basophils, eosinophils/monocytes) in the middle and the plasma on top.[citation needed]
 There are numerous types of apheresis.
 Blood taken from a healthy donor can be separated into its component parts during blood donation, where the needed component is collected and the unharvested components are returned to the donor. Fluid replacement is usually not needed in this type of collection.  In many countries, apheresis donors can donate blood more often than those donating whole blood.[citation needed]There are several categories of component collections:
 Two apheresis kit recalls were:
 People who do not use a drug that may prevent blood donation, who do not have the risk of the carrier of a disease, and who have suitable vascular structure may be apheresis donors. For apheresis platelet donation the donor's pre platelet count should be above 150 x 10^9/L. For apheresis plasma donation, the donor's total protein level should be greater than 60 g/L. For double red cell apheresis, donors of either gender require a minimum hemoglobin level of 14.0 g/dl.
 Apheresis uses plastics and tubing, which come into contact with the blood. The plastics are made of PVC in addition to additives such as a plasticizer, often DEHP. DEHP leaches from the plastic into the blood, and people have begun to study the possible effects of this leached DEHP on donors as well as transfusion recipients.[citation needed]
 The various apheresis techniques may be used whenever the removed constituent is causing severe symptoms of disease. Generally, apheresis has to be performed fairly often, and is an invasive process. It is therefore only employed if other means to control a particular disease have failed, or the symptoms are of such a nature that waiting for medication to become effective would cause suffering or risk of complications.[citation needed]
 In 2010, the American Society for Apheresis published the 5th Special Edition(1) of evidence based guidelines for the practice of Apheresis Medicine.  These guidelines are based upon a systematic review of available scientific literature.  Clinical utility for a given disease is denoted by assignment of an ASFA Category (I – IV).  The quality and strength of evidence are denoted by standard GRADE recommendations. ASFA Categories are defined as follows:[citation needed]
 Only diseases (or mentioned special conditions thereof) with ASFA category I or II are displayed in bold, with category I being underlined in addition.
 When an apheresis system is used for therapy, the system is removing relatively small amounts of fluid (not more than 10.5 mL/kg body weight). That fluid must be replaced to keep correct intravascular volume. The fluid replaced is different at different institutions. If a crystalloid like normal saline (NS) is used, the infusion amount should be triple what is removed as the 3:1 ratio of normal saline for plasma is needed to keep up oncotic pressure. Some institutions use human serum albumin, but it is costly and can be difficult to find. Some advocate using fresh frozen plasma (FFP) or a similar blood product, but there are dangers including citrate toxicity (from the anticoagulant), ABO incompatibility, infection, and cellular antigen[citation needed]