In Reply.—Dr Wuller's points are all valid, and I agree with him. Certainly, any device that poses a risk of air embolism ought not to be used, and the value of filtering debris in low volume transfusions is dubious. We favor a policy of using filters only when transfusions of more than 4 units, ie, 2,000 ml, are anticipated.
I also share Dr Wuller's feeling that the evidence for the value of these filters is largely circumstantial. Direct proof that they have major effects on morbidity and mortality is very difficult to obtain in humans. It would be easy to design a clinical study wherein two groups of patients would be investigated. The only variable distinguishing one group from the other would be the use of blood filters. All other variables bearing on the chief target organ, the lung (eg, transfusion volume, type and extent of bacterial infection, type and volume of noncolloidal