Plasma donation, known as plasmapheresis, is a specialized procedure where whole blood is temporarily removed from a donor, separated into its components, and then most components are returned. The primary goal is to collect plasma, the liquid portion of the blood rich in proteins and antibodies, for therapeutic use. Although the procedure is highly automated and designed to return all blood cells, a minimal, unavoidable amount of red blood cell (RBC) loss occurs. Understanding the mechanisms behind this minor loss is central to donor safety protocols and the technical design of apheresis equipment. This slight diversion of RBCs is heavily regulated to ensure the donor’s health is not compromised.
How Plasmapheresis Works: The Separation and Return Process
The plasmapheresis process relies on automated apheresis technology to selectively harvest plasma while protecting the donor’s cellular blood components. Blood is drawn from the donor through a sterile needle and immediately mixed with an anticoagulant, most commonly a citrate solution, to prevent clotting in the equipment. This mixture then enters the apheresis machine, where the separation of components takes place.
The most common separation technique involves centrifugation, spinning the blood to separate components based on their density. Red blood cells, being the heaviest, settle at the bottom of the collection bowl, while the lighter plasma remains on top. Alternatively, some systems use a specialized membrane filter to physically separate the plasma from the larger cellular components.
Once the plasma is successfully harvested, the remaining blood cells, including the red blood cells, white blood cells, and platelets, are recombined with a replacement fluid, typically a sterile saline solution. This mixture is then immediately returned to the donor. This cyclical process of drawing, separating, and returning is repeated multiple times over the course of a donation session to collect the target volume of plasma.
Technical Mechanisms Leading to Minor RBC Loss
The minor loss of red blood cells during the efficient return process is attributed to two primary technical factors inherent to the apheresis system: residual volume and mechanical damage.
Residual Volume
Residual volume accounts for the small amount of whole blood that remains in the single-use collection system after the procedure is complete. This includes the tubing, the separation bowl, and the various filters used in the circuit.
Despite the machine’s programming to maximize the return of blood cells, a minute quantity remains trapped in the disposable kit, which is then discarded. Studies indicate that the red cell mass lost due to residual volume can be around 11 milliliters per donation if no special rinsing procedures are used. This represents a loss of cells drawn from the donor that could not be successfully reinfused.
Hemolysis and Separation Efficiency
Another mechanism of loss is hemolysis, which is the physical rupture of red blood cells within the extracorporeal circuit. The mechanical stress of the pumping action, rapid changes in flow rate, and passage through narrow tubing can cause shear stress on the fragile cell membranes. This stress damages the RBCs, causing them to break open and release their hemoglobin into the surrounding plasma.
When hemolysis occurs, the collected plasma may appear visibly pink or red due to the presence of free hemoglobin, rendering that portion unusable for therapeutic purposes. The efficiency of the cell separation system also contributes to minor loss, as minute amounts of cells may pass through the separation mechanism and be collected along with the plasma. The design of the centrifuge or the pore size of the filter membrane dictates the level of cellular contamination, meaning some cell loss is intrinsic to the technology.
Managing Donor Health and Replenishing Lost Cells
The red blood cell loss experienced during plasmapheresis is proactively managed through strict regulatory guidelines and donor monitoring to protect the health of frequent donors. Regulatory bodies establish stringent limits on the maximum volume of red blood cells that can be unintentionally diverted or lost during the procedure. For example, a donor who loses a significant volume of red blood cells, such as more than 200 milliliters in a single incident, must be temporarily deferred from donating.
This deferral period, typically eight weeks, allows the donor’s body sufficient time to naturally replace the lost red blood cells. The bone marrow constantly produces new red blood cells to maintain a healthy hematocrit level, and the deferral ensures this natural replenishment occurs fully before the next donation. Monitoring a donor’s hematocrit and hemoglobin levels is a mandatory safety check prior to each donation.
These pre-donation screenings ensure the donor’s baseline red blood cell count is within a safe range, confirming recovery from any previous losses. The body’s ability to compensate for blood loss also depends on having adequate iron stores, as iron is a fundamental component of hemoglobin. Donors are advised to monitor their iron intake to support the ongoing production of new red blood cells and minimize the long-term cumulative impact of procedural losses.