FAQs
Is Leukocyte Reduction Equivalent to CMV-seronegative Products for Prevention of Transfusion-transmitted CMV?
This question is really a loaded one! The answer, for most people, is "Yes", but the long answer is a little more complicated.
Problem: Cytomegalovirus (CMV) is a white blood cell-based DNA virus that causes a mild, transient illness in people that have normal immune systems. Over 50% of the US population has been infected at some point. In patients with a compromised immune system, however, CMV infection can be devastating, often with lethal consequences. Traditionally, blood banks tested donors for antibodies to CMV to try to prevent transmission to immunocompromised patients, but blood bankers believe that CMV transmission can also be accomplished through removal of CMV's host: white blood cells.
In 1995, a group from Seattle published the first large study (Blood, Vol 86, No 9, 1995: pp 3598-3603) which directly compared CMV-seronegative products with filtered products for CMV prevention in bone marrow transplant recipients. The study (known commonly as the "Bowden Study") concluded that there was no significant difference in CMV transmission between the two CMV seronegative and the filtered groups. This study's methods have been called into question somewhat in the years since, but the disagreements seem to be mostly with the wording and the scope of what was said, rather than with the basic point that leukocyte reduction reduces the risk of CMV transmission.
The main thrust of the issue is this: I think that many clinicians may have a false sense of security when giving blood that is CMV-seronegative. Unfortunately, testing a donor for anti-CMV antibodies does not give us a real idea of whether or not he is currently infected! As a result, there is a fairly well-defined failure rate for CMV-seronegative blood products of between 1% (in most studies) and 3% (meaning that transfusion of a small number of CMV-seronegative products are associated with CMV infection). So, unfortunately, when comparing leukocyte reduction to antibody testing, it is important to recognize that neither method is fail-safe. The failure rate for leukocyte reduction is similar, about 1% in most studies.
At this point in the discussion, someone usually raises the obvious question: "Why not do both? If leukocyte reduction and CMV antibody testing both reduce the risk, should doing both reduce it even more?" While that is certainly a logical argument, honestly, it doesn't seem that the combination of the two strategies offers additional protection. As it turns out, in modern blood banking practice where most every product is leukocyte reduced, giving someone CMV-seronegative blood will, by default, use both strategies.
These discussions led to a 2002 AABB Association Bulletin (Bulletin #02-4, member access only) that was a little bit noncommittal, but concluded in general that if leukocyte reduction was done properly (and prior to storage of the product, so-called "prestorage leukocyte reduction"), it yielded a product which is considered equivalent to a CMV-seronegative product for preventing CMV transmission, a so-called "CMV-safe" product.
The policy I use is simply this: If a clinician specifically requests CMV-seronegative blood, I will try to accomodate that request. However, if I have to give an untested, leukocyte reduced unit due to supply problems, I won't hesitate to do so. This is a situation, however, that is best discussed thoroughly with clinicians prior to the time of the transfusion, so that both the blood bank and the clinician know what to expect when a request for "CMV-safe" blood is made.