Overview of Therapeutic Apheresis

Principle

Successful use of TA as a treatment modality requires that a disease results from a substance found in plasma (e.g., antibody to acetylcholine receptor in myasthenia gravis) or by a blood component (e.g., hemoglobin S RBCs in sickle cell disease), and that the pathogenic substance or component can be removed efficiently enough to permit resolution of the disease.

McLeod’s criteria to determine TA success in disease treatment consist of three components: (1) disease pathogenesis suggests clear rationale for TA, (2) abnormality meaningfully corrected by TA, and (3) strong evidence TA confers meaningful clinical benefit. American Society for Apheresis (ASFA) publishes an evidence-based guideline for TA use, which is updated every 3 years. It provides an ASFA category ( Table 74.1 ) and grade of recommendation ( Table 74.2 ) for each disease entity. The newest edition (7th edition, 2016) describes 87 diseases and 179 clinical indications ( Table 74.3 ).

Methods

Apheresis devices separate whole blood into component fractions, allow removal of the desired fraction, and return the remaining components. This separation can be achieved either by centrifugation or membrane filtration. Centrifugation separates based on differential density; filtration devices separate based on size. In the United States, most TA procedures are performed using centrifugation.

Vascular Access

Apheresis procedures require high blood flow rates (up to 150 mL/minutes). Peripheral venous access or double-lumen dialysis/apheresis catheter for adults are typically utilized. For pediatric patients, access size depends on the weight of the patient. Central and femoral catheters each pose their own risks and benefits. Patients who require prolonged TA may need a tunneled catheter or other long-term access device (e.g., arteriovenous fistula, graft, or port).

Volume Exchanged and Frequency

Determination of exchanged volume is based on a model of an isolated one-compartment intravascular space ( Fig. 74.1 ). This model works best for components located predominantly in the intravascular compartment (e.g., IgM, RBCs), and less well for IgG (which is 45% extravascular). Frequency is determined by time for reequilibration into intravascular space and the need to minimize the risk of bleeding as a result of depletion of coagulation factors, especially fibrinogen. Reequilibration of the intravascular IgG with extravascular IgG typically occurs within 2 days. Five to six one-PV exchanges over 14 days when combined with immunosuppressive medications achieve a 70%–85% reduction in IgG. Typically, plasma exchange processes 1–1.5 total blood volume (TBV) every other day.

Calculations

Calculations commonly used in TA include calculation of TBV, plasma volume, and extracorporeal volume (ECV). All approaches to calculating TBV overestimate in obese patients and underestimate in muscular patients. Nonetheless, they provide reasonable approximation. TBV is ∼70 mL/kg. Plasma volume = TBV × (1 − Hct). Total RBC volume = TBV × Hct.

ECV is the amount of blood outside the patient filling the apheresis set and tubing. ECV varies by system, type of procedure, and ancillary equipment. ECV should not exceed 15% of TBV to avoid intraprocedural volume depletion and/or anemia. Typically, ECV is not a problem in adults. If ECV exceeds 10%–15% of TBV, then blood priming with RBCs or 5% albumin is performed.