Therapeutic plasma exchange (TPE, plasmapheresis) is an extracorporeal blood purification technique designed for the removal of large molecular weight substances from the plasma. This topic will review the basic elements of the plasma exchange prescription and technique . Indications for and complications of TPE are discussed elsewhere. (See "Indications for therapeutic plasma exchange" and "Complications of therapeutic plasma exchange".)
In general, large molecular weight compounds equilibrate slowly between the vascular space and the interstitium. Thus, calculations of the rate of removal by TPE can be simplified to first order kinetics. A single plasma volume exchange will lower plasma macromolecule levels by 60 percent and an exchange equal to 1.4 times the plasma volume will lower plasma levels by 75 percent [2,3].
The following formula can be used to estimate the plasma volume in an adult :
Estimated plasma volume (in liters) = 0.07 x weight (kg) x (1 - hematocrit)
Performing more than one plasma volume exchange increases procedure time, challenges patient tolerance, and increases the cost. As an example, currently available cell separators can perform one complete volume exchange in 1.5 to 2 hours; exchanges equaling two to three plasma exchanges will double or triple the time required to perform the procedure. For most conditions in which plasmapheresis is used, it is considered acceptable to perform 1 to 1.5 plasma volume exchanges per procedure.