Plasmapheresis is a procedure designed to collect the liquid portion of blood (plasma) while returning the cellular components to the donor. The process involves removing whole blood, separating the plasma, and then reinfusing the remaining red blood cells (RBCs), white blood cells, and platelets. Since the procedure is engineered to return these cellular elements, any significant RBC loss indicates a complication or technical failure. The goal is to conserve the donor’s RBC mass, meaning any destruction or accidental removal of these oxygen-carrying cells is monitored closely. Understanding the sources of RBC loss, whether physical damage or procedural missteps, is important for donor safety.
The Mechanism of Plasmapheresis
The standard plasmapheresis procedure uses an automated machine for extracorporeal circulation of the donor’s blood. Whole blood is drawn through a sterile needle into a single-use tubing set connected to the apheresis device. Inside the machine, the blood is routed through a centrifuge, which spins the components at high speed, separating them based on density. Red blood cells settle first, followed by white blood cells and platelets, leaving the plasma on top.
The machine siphons off the plasma for collection. The remaining cellular components, including concentrated red blood cells, are mixed with a replacement fluid, such as saline, and returned to the donor. This cycle of drawing, separating, and returning occurs repeatedly until the required volume of plasma is collected. The system uses sterile, disposable tubing to ensure safety and prevent cross-contamination.
Mechanical Stress and Hemolysis
Physical rupture, known as hemolysis, is a common cause of red blood cell loss caused by mechanical forces within the apheresis circuit. High flow rates or rapid pump speeds generate excessive shear stress as blood is forced through the narrow tubing and machine components. This turbulence and friction physically tear the delicate RBC membranes, causing them to burst and release hemoglobin.
Equipment integrity is another factor. Kinks, partial occlusions, or tight turns in the disposable tubing set create localized areas of extreme pressure. These points of resistance significantly increase shear stress, leading to substantial damage. Even needle placement can cause mechanical trauma; using a needle too small for the flow rate, or one partially occluded by the vein wall, stresses the cells before they enter the machine.
Beyond direct rupture, a small, unavoidable volume of red blood cells is lost during every donation due to the circuit’s design. These cells remain trapped within the machine’s separation chamber and the disposable tubing set when the procedure concludes. Regulatory guidelines account for this residual loss, which centers can mitigate by flushing the tubing with saline and returning that rinse to the donor.
Chemical and Procedural Factors
RBC loss can also be triggered by non-mechanical issues related to the chemical environment or donation protocol errors. To prevent clotting within the extracorporeal circuit, an anticoagulant, typically sodium citrate, is continuously infused into the drawn blood. Citrate works by binding to calcium, necessary for the coagulation cascade, but an incorrect ratio or concentration can affect RBC structural integrity.
While citrate is safe, excessive levels may temporarily increase the fragility of RBC membranes, making them susceptible to mechanical damage. Furthermore, the fluid used to replace the collected plasma must be precisely isotonic, matching the osmotic pressure of the blood cells. If the replacement fluid is hypotonic, it causes red blood cells to swell and burst in a process known as osmotic hemolysis.
Procedural adherence is important for minimizing cell loss and damage. Errors like incorrect sequencing during reinfusion or residual sterilizing agents left in the tubing can chemically injure the cells. Additionally, if the system is not properly monitored, the accidental introduction of air into the circuit can contribute to cellular damage and pose a risk.
Monitoring and Mitigation of RBC Loss
Plasma donation centers employ several protocols to detect and minimize red blood cell loss. The most immediate sign of significant RBC destruction is the visual appearance of the plasma line. When hemolysis occurs, the released hemoglobin discolors the plasma, turning it a pink or reddish hue, which trained staff easily observe.
Apheresis machines are equipped with advanced sensors and alarms that continuously monitor pressure, flow rates, and component ratios. If these parameters deviate outside of safe limits, indicating a potential blockage or excessive mechanical stress, the machine automatically triggers a safety shutdown. Trained personnel are responsible for adhering to standard operating procedures, including proper needle selection and correct calibration of pump speeds, to prevent initial damage.
Regulatory bodies set strict limits on the total allowable red blood cell loss a donor can sustain annually. If the machine calculates a significant, acute loss—typically defined as more than 200 milliliters in a single incident—the donor must be temporarily deferred. This deferral period allows the donor’s body to naturally regenerate the lost cells before they can donate again.