The following characterizes the impact of physical properties of blood: percent hematocrit, plasma viscosity, and temperature on plasma yield.
As previously discussed, sedimentation rate is directly related to the force of acceleration and is influenced by density, diameter and size of the cells being separated, viscosity of the fluid, and fraction of dissolved solids.
Effect of Hematocrit
When considering Stokes Law and the elements that impact sedimentation, it is logical that donor differences contribute to the variability in plasma recovery. Factors like the size, density and shape of cells, as well as the density and viscosity of the fluid fraction all impact sedimentation. While donor to donor variation in cell size, shape, and density must be considered in component production processes, these variables are most relevant when manufacturing platelets from whole blood components. Centrifuge conditions for plasma yield optimization are intended to produce a cell-free supernatant and therefore the factors identified above are less significant.
Donor percent hematocrit is relevant to plasma yield optimization in two ways. Donated whole blood with a low percent hematocrit has more plasma available and the red cells are more easily sedimented. The converse is also true. Figure 6 shows a statistically significant (p<0.05) inverse linear correlation (R2=0.58) between donor percent hematocrit and percent plasma recovery.
Correlation between % Hematocrit and % Plasma Yield
*Units were selected for % Hct and each centrifuge load contained hematocrits ranging from 34 to 43%. TCF applied during centrifugation was 2.5 x 106 g•s.
Effect of Temperature
As temperature decreases, plasma viscosity increases and red cell deformability decreases. These two factors reduce sedimentation efficiency. This effect can be minimized by centrifugation at higher TCF as shown in Figure 7. The data was collected under controlled laboratory conditions. Field data shown in Figure 8 support the conclusion that the temperature of blood at the time of centrifugation affects plasma yield. Room temperature (RT) stored whole blood centrifugation produced an average of 5% more plasma than cold stored whole blood.
Impact of Temperature of Whole Blood at Centrifugation
No statistically significance between RT plasma yield with high or low TCF. Statistically significant difference in cold conditions (paired t-test: p<0.05).
Impact of Temperature of Blood at Processing
Optimal Centrifuge Conditions
The maximum plasma that is obtainable from a unit of whole blood is between 90 and 95% of the total plasma. This conclusion, demonstrated graphically in Figure 9 is evident by the way the upper portion of the TCF curve becomes flat between 90 and 95% plasma recovery at TCF’s greater than 2.4 x 106. Similarly, when the optimized TCF between 2.4 and 2.65 x 106 g•s is used with different temperature and % hematocrit parameters, the maximum plasma yield is between 90 and 95% as highlighted in tan in Figure 9.
TCF, Temperature, Percent Hematocrit, and Plasma Yield
*Units were selected for % Hct and each centrifuge load contained hematocrits ranging from 34 to 43%.
The maximum plasma yield is consistent with the physical principles of sedimentation where the percentage of dissolved solids can limit the sedimentation rate. Once the red cell pellet is sedimented to the point that 90-95% of the plasma is present in the supernatant layer, additional sedimentation is inhibited by the density of the layer of pelleted red cells. For this reason, the risk of compromising red cell and plasma quality by higher centrifugation forces is greater than the perceived gains in plasma recovery.
The optimal TCF minimizes the effect of temperature and percent hematocrit on plasma recovery for a broader range of processing conditions. In Figure 10, the green shaded area of the plasma optimization curve represents the target area for maximizing plasma yield. The yellow shaded area indicates good plasma yield but with greater variability as shown by the large standard deviation. The red area of the curve will produce similar plasma yields as the optimized area; however there are uncharacterized risks of compromising red cell and plasma quality. Increasing TCF into the red zone will not increase plasma yield above 95%.
Optimized TCF for % Plasma Recovery
|4150 rpm 3 min
||5.62 x 105
||71.0 ± 1.7
|4150 rpm 5 min
||1.11 x 106
||84.6 ± 1.3
|4150 rpm 7 min
||1.72 x 106
||91.2 ± 1.8
|4150 rpm 10 min
||2.65 x 106
||91.9 ± 0.6
|4150 rpm 15 min
||4.11 x 106
||93.3 ± 1.0
In conclusion, the optimal centrifugation TCF for maximum plasma yield is between 2.40-2.65 x 106 g•s for the broadest range of processing conditions. This optimized TCF range minimizes the variability of processing associated with % Hct, temperature, and brake settings.