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The first part of this 2-part series focused on the manufacture of filters and the application of filtration technology to intravenous fluids and point-of-care hospital water. This second part describes an apparent emerging potential for final filtration defined as bedside filtration of blood and component blood products leukocyte-reduced at the blood center prior to storage. Final filtration serves to further reduce the leukocyte burden in a previously leukocyte-reduced blood product. Another target for final filtration includes putative soluble mediators of morbidity.Selected patients may be at greater risk for alloimmunization and refractory to the benefits afforded by transfusion of blood leukocyte reduced to the current established standards. Multiparous patients who subsequently find themselves in need of a transplanted organ are alloimmunized by exposure to fetal proteins and may be further alloimmunized by transfusion. Such effects can put them at risk for increased latency for donor organ availability and organ rejection. Kidney transplant patients find themselves the recipients of transfused blood products particularly during end-stage renal disease and recent data suggest such patients are not benefited by the levels of leukoreduction prescribed by current standards and may need more dramatic leukocyte removal. The process of blood production is described and affords a greater appreciation for the levels of white cells found in component blood products. The development of alloimmunization is reviewed and fosters greater appreciation for a discussion of the potential for therapeutic value of more dramatic leukocyte reduction and blood conditioning accomplished through the removal of soluble mediators of morbidity.

Filters often are viewed as screens with openings smaller than the particles intended to be removed by a process technically known as direct interception. However, filter manufacturing embraces far more advanced technological approaches, with an evolution toward selective removal of cells or soluble constituents from complex physiologic solutions. An appreciation of filtration development makes it easy to understand how differently manufactured filters with the same claims may not perform identically. This article focuses on the filtration of intravenous solutions and point-of-use hospital water.

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