Therapeutic plasma exchange, often called plasmapheresis or PEX, is a medical procedure designed to filter the blood outside of the body. A specialized machine removes a patient’s whole blood, separates the plasma, and then returns the remaining blood components with a replacement fluid. The main goal of this therapy is to remove large, disease-causing substances that circulate in the plasma, such as abnormal antibodies, immune complexes, or toxins. By removing this liquid component of the blood and replacing it with a clean substitute, doctors decrease the concentration of these harmful elements.
How Plasma is Separated
The successful removal of plasma from whole blood relies on technology that separates the liquid and cellular components based on their physical properties. This separation is achieved using one of two primary methods: centrifugation or membrane filtration. The selection of the method depends on the patient’s specific condition and the technical capabilities available at the treatment center.
Centrifugation, the most common method in the United States, works by spinning the whole blood at high speeds within a specialized chamber. This centrifugal force separates the blood components based on their different densities. The heavier cellular components—red blood cells, white blood cells, and platelets—settle to the outside, while the lighter plasma remains in the center, allowing for its collection and removal.
Alternatively, membrane filtration separates the plasma using a system of semipermeable filters, similar to the process used in kidney dialysis. The whole blood is pumped across a membrane containing microscopic pores that are large enough to allow the plasma and the smaller dissolved molecules to pass through. However, the pores are too small for the larger cellular components to pass, effectively sieving the plasma from the blood cells. This method is often favored by physicians experienced in dialysis-type equipment.
The separation process targets large pathogenic molecules that are too big to be removed by conventional methods like standard dialysis. These large molecules include autoantibodies that mistakenly attack the body’s own tissues, immune complexes formed by antigens and antibodies, and certain toxic proteins.
Treatment Session Logistics and Replacement Fluids
The practical experience of therapeutic plasma exchange requires temporary access to the patient’s bloodstream. Vascular access is typically secured by placing large-bore needles into a vein in each arm, with one needle drawing the blood out and the other returning the treated blood. For patients with small or fragile veins, a central venous catheter may be placed in a larger vein, such as one in the neck or chest, to ensure a stable and consistent blood flow.
Once access is established, blood is continuously circulated through the apheresis machine, which separates and removes the plasma. To prevent the blood from clotting as it flows through the external circuit, an anticoagulant is added, usually a solution containing citrate. A typical session lasts between two to four hours, depending on the patient’s total plasma volume and the amount that needs to be exchanged.
The removed plasma volume must be immediately replaced with an appropriate fluid before the blood is returned to the patient. This replacement is done to maintain overall blood volume and prevent a drop in blood pressure.
The most common replacement fluid is a five percent human albumin solution, a protein-rich fluid that helps maintain the necessary pressure within the blood vessels. In certain cases, such as when a patient has a clotting disorder or specific needs for coagulation proteins, fresh frozen plasma (FFP) from a donor may be used as the replacement fluid. Because albumin does not contain clotting factors, FFP is necessary to replenish these components.
A temporary dip in blood calcium levels can also occur as a side effect due to the citrate anticoagulant binding with the patient’s calcium. This is managed by adjusting the flow rate or administering calcium supplements.
Primary Uses in Medical Treatment
Plasma exchange is a treatment modality used across several medical specialties, particularly for diseases where the plasma contains harmful components that require rapid removal. The American Society for Apheresis categorizes the conditions for which this therapy is indicated, ranging from first-line to second-line treatments. The therapy is most commonly utilized for autoimmune and hematological disorders.
In the field of neurology, plasma exchange is a standard treatment for conditions like Guillain-Barré Syndrome, where the immune system attacks the peripheral nerves, and myasthenia gravis, a disorder causing muscle weakness. It is also used to treat acute, severe relapses of multiple sclerosis when the patient has not responded adequately to corticosteroid therapy.
For hematological conditions, plasma exchange is a treatment for thrombotic thrombocytopenic purpura (TTP), a serious blood disorder where small blood clots form throughout the body. In this instance, the procedure removes large von Willebrand factor multimers and replaces them with healthy plasma containing the necessary enzyme to break down these factors. Plasma exchange is also sometimes employed in the treatment of certain renal diseases, such as Goodpasture syndrome, to remove the antibodies that attack the kidneys and lungs.