Microfiltration of stored blood.

Microaggregates begin to develop within a few hours of storage of blood in plastic or glass containers, but their numbers increase mainly towards the end of the first week. They include degenerated platelets, leucocytes, fibrin strands, denatured proteins and fragmented red cells, and range in size from 10 to 40 micrometer or more in diameter. The rate of formation is related to the platelet and leucocyte concentrations prior to storage and the anticoagulant used. While clinical and experimental evidence of deleterious pulmonary effects of these unwanted particles has been limited and contradictory, recent studies have demonstrated that significant increases in pulmonary arteriovenous shunting and alveolar-arterial oxygen differences occur in patients transfused more than 20% of their blood volume throught the standard 170 micrometer filters. These changes are not seen when the blood is passed through a 20 micrometer Dacron wool filter. Other methods of reducing the microaggregate content of transfused blood include the use of fresh blood (less than 2 days), glycerol-frozen fresh blood correctly thawed, or saline-washed packed red cells. Since none of these is feasible for routine use at present, removal by microfiltration prior to rransfusion is employed. Of the filters currently available, the 40 micrometer screen filters appear to offer important practical advantages over the alternative depth filters. Routine filtration of all stored blood transfused is advocated.