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Leucocyte Depletion of Blood Components

Leucocytes in blood components are responsible for a number of adverse effects associated with blood transfusion. The pathogenesis has not been precisely elucidated in many instances but it is likely that it is immunologically mediated. Potential mechanisms include clonal deletion or anergy, induction of suppressor cells, production of anti-idiotypic antibody and suppression of NK cell activity among others.

Accordingly, filters capable of removing leucocytes by several orders of magnitude have been developed and can effectively reduce the number of white cells in, for example, a red cell concentrate to < 1 x10⁶. A less efficient but much more economical process for depleting components of leucocytes involves removing the buffy coat from red cell components and also using the buffy coats to prepare random donor platelet concentrates. This results in red cell and platelet components with residual leucocytes intermediate in number between filtered components and those with the buffy coat retained. The standard red cell concentrate prepared in South Africa is buffy coat reduced.

A number of countries have adopted a policy of universal pre-storage leucocyte depletion (ULR) while others have adopted a policy of selective leucocyte depletion of components. The costs associated with ULR are considerable e.g. in the USA it would amount to > $400m and in South Africa (based on 2002/2003 figures), the costs would amount to ± 24% of the total annual turnover of all the services. Given the competing health priorities in South Africa, there should therefore be convincing evidence that such an intervention is clinically beneficial and cost effective.

The transfusion services in South Africa have reviewed the medical literature and conclude the following:

• There is good evidence to support the avoidance of febrile non-haemolytic transfusion reactions (FNHTR's) by leucocyte depletion.
• Leucocyte depletion of platelet concentrates will reduce the incidence of platelet refractoriness to platelet transfusions.
• Leucocyte depletion significantly reduces the risk of transfusion-transmitted CMV infection in susceptible individuals.
• The evidence for reduction in post-operative infection is not consistent.
• The evidence for reduction in cancer recurrence is not consistent.
• Although meta-analyses do not provide convincing evidence of a reduction in post-operative mortality for leucocyte depleted products, sub group analyses suggest a benefit for seriously ill and cardiac surgery patients.
• An association with reactivation of viral infections (HIV and CMV) and non-¬leucocyte depleted components has not been demonstrated.
• Sensitisation to transplant antigens can be ameliorated by leucocyte depletion where HLA allo-immunisation is important.
• Leucocyte depletion may reduce prions in blood components but there is as yet no evidence that leucocyte depletion will avoid transmission of vCJD by blood components.

A policy of selective leucocyte depletion of blood components is therefore recommended as follows:

• All standard red cell concentrates are buffy coat depleted.
• Random donor platelet concentrates are prepared from buffy coats.
• Single donor platelet concentrates collected by apheresis incorporate a leucocyte depletion process.
• Patients on chronic transfusion regimens should receive leucocyte depleted products.
• Patients at risk for CMV infection should receive leucocyte depleted products.
• Organ and stem cell transplant patients should receive leucocyte depleted products.
• Infants < 1 year old should receive leucocyte depleted products.
• Critically ill patients and patients undergoing cardiac surgery should receive leucocyte depleted products.
• Pre-storage (< 48 hours after donation) leucocyte depletion in blood processing laboratories is recommended. If this product is unobtainable it is recommended that freshest units available be filtered in the blood bank for immediate use.