Many countries have implemented universal leukocyte depletion (LD) of blood components, whereas in others leukocyte-depleted components may be issued for selected patient groups only. In the UK, a perceived benefit in terms of reduction in the risk of variant Creutzfeldt-Jakob disease (vCJD) transmission was a major contributory factor in the decision to introduce universal LD in 1998. Other benefits of LD, such as the potential for reduced immune complications and transfusion transmission of some cell-associated viruses (e.g. CMV), were considered more important by other countries.
Although in the past LD was performed at the bedside, the preference is now, because of quality reasons, for LD to be performed prior to component storage, usually within 48 hours of donation. For whole-blood donations, this is achieved by filtration, whereas an LD step by centrifugation/elutriation is integral to some apheresis technologies. Most whole-blood LD filters remove > 2 logs of platelets in addition to > 4 logs leukocytes. Therefore, only fresh-frozen plasma (FFP) and red cells can be produced from whole blood that has been leukocyte depleted. To produce platelet concentrates, each component (red cells, plasma or platelets) must be filtered after their separation from whole blood. However, a second generation of whole-blood LD filters is becoming available that permit platelets to pass through the filter, although these are not yet in widespread use. LD results in a 10-15% loss of volume of whole blood or processed component but has minimal adverse effects on the quality of blood components.
The specification for leukocyte-depleted blood components varies between countries ( Table 1 ), but all reflect the current capability of LD systems, the fact that only a fraction of components are tested for residual leukocytes and that the limit of sensitivity of current counting methods is around 0.3 × 106/U. Recent studies have demonstrated > 3.8 log reduction in all leukocyte subtypes by whole-blood filtration and > 3.1 log reduction by platelet filtration and one platelet-apheresis technology.
Despite advances in technology, LD systems occasionally fail. The risk that an LD system will result in blood components being issued that fail to meet the required specification for residual leukocytes is dependent upon a number of factors: the capability of the LD system, potential manufacturing defects in the LD filter or pack system, the proportion of components that are tested for residual leukocytes and donor-related causes. An estimation of the likelihood of components is issued that exceed certain levels of residual leukocytes are illustrated ( Table 2 ). Although most donor-related causes of filter failure are poorly understood, it is known that donors with sickle cell trait are more likely to either block LD filters or fail to leukocyte deplete; 100% of donations from such donors are therefore usually assessed for residual leukocytes.
Transfusion Alter Transfusion Med. 2008;10(3):92-101. © 2008 Blackwell Publishing
This article is an advanced publication of a chapter to be published in the revised edition of the NATA textbook, Alternatives to Blood Transfusion in Transfusion Medicine, currently in preparation.
Cite this: Allogeneic Blood Components - Medscape - Sep 01, 2008.