Is it time to focus on preserving the functionality of red blood cells during storage? (2006)

Over the past few decades, developmental efforts in red blood cell (RBC) preservation focused primarily on improving RBC viability during storage and extending their shelf life. Little attention was given to the ability of RBCs to function shortly after transfusion to meet patient’s tissue oxygenation needs. Current standards focus on RBC preservation during storage with a standard for hemolysis and on RBC viability with requirements for posttransfusion in vivo recovery. There are no standards that address RBC function. The assumption is that if RBCs survive, they function in an adequate manner to meet the patient’s oxygenation needs. Emerging data indicate that patients who receive RBC transfusion may have worse clinical outcomes than those who do not receive transfusion. Other studies suggest that in patients who receive transfusions, the longer the storage of RBCs the worse the outcome.1,2 Although transfused RBCs meet the current standards of in vitro and in vivo quality, they are not fully functional immediately after transfusion because they take some time to recover their normal oxygen delivery function. RBCs that are stored in conventional anticoagulant and preservation solutions undergo many changes including decrease in the pH, adenosine triphosphate (ATP), and 2,3-diphosphoglycerate acid (2,3-DPG); an increase in free hemoglobin (Hb); and membrane changes as well as formation of microvesicles. After transfusion, damaged or nonviable RBCs are removed rapidly from circulation within the first 24 hours. Viable RBCs recover from storage-induced changes and continue to have a normal life span. Historically, efforts to preserve RBCs after collection have been ongoing since early last century and continue today. In 1916, Rous and Turner3 found that glucose retarded in vitro hemolysis of stored RBCs. Solutions containing glucose could not be sterilized by autoclaving because glucose caramelized in the presence of alkaline citrate. In 1943, Loutit and coworkers4,5 overcame this problem by introducing an acidified citrate medium, acid citrate dextrose (ACD), which could be autoclaved as a single solution. This advance was followed by the introduction of citrate phosphate dextrose (CPD) in 1957 by Gibson and associates.6 Both ACD and CPD or some modifications of either are widely used today. Although 2,3-DPG was known to decrease rapidly during storage of RBCs in both ACD and CPD, its importance was not appreciated because it appeared to have no effect on stored RBC viability. When the effect of 2,3-DPG on Hb affinity for oxygen was recognized, it became clear that lower levels of 2,3-DPG reduced the ability of RBCs to deliver oxygen.7 Shortly after, studies showed that stored RBCs recovered their normal 2,3-DPG levels within 24 hours of transfusion.8,9 It is important to recognize that these studies were performed in normal volunteers with normal blood volume and no circulatory problems. Patients differ from healthy volunteers because of the effects of trauma, surgery, anesthesia, or massive blood loss. It is not known whether RBCs transfused to patients with severe blood loss or in shock with poor circulation can recover their 2,3-DPG quickly or recover it at all. The inability of transfused RBCs to transport oxygen in the early critical time after blood loss may have an impact on patients’ clinical outcomes. Although the clinical significance of low levels of 2,3-DPG has been debated, efforts to develop media that can preserve 2,3-DPG levels during storage continued. In the RBC glycolytic pathway, 1,3-diphosphoglycerate serves as a substrate for either of two competing reactions that lead to the production of ATP and 2,3-DPG. Acidic pH favors production of ATP whereas higher pH favors production of 2,3-DPG. Armed with this information, Beutler and coworkers10,11 developed a storage medium containing bicarbonate, adenine, glucose, phosphate, and mannitol (BAGPM). This medium maintained RBC viability and normal 2,3-DPG levels for more than 6 weeks. Unfortunately this medium could not be manufactured because glucose caramelized in the bicarbonate-containing solution due to high pH during sterilization. It is conceivable that a separate glucose solution could be kept in an additional bag to allow sterilization of all solutions without problems, but concerns over adding more bags and complicating the process as well as the possibility of error during preparation make this proposal unattractive. In this issue of TRANSFUSION, Hogman and associates12 report on their efforts to develop a solution to address the decline of 2,3-DPG during RBC storage and to circumvent the technical difficulties preventing the availability of previously proposed preservation solutions. They describe a method that uses a new additive solution to raise the intracellular pH and maintain 2,3-DPG levels for 4 weeks. The method uses CP2D as a source of glucose to avoid technical problems related to sterilization of glucose solutions with high pH. Although the study shows that the use of the new preservation solution and CP2D as a source of glucose maintained viability and higher levels of 2,3-DPG for 4 weeks, it requires additional processing steps before the storage of RBCs. Additional effort and investment will be required to work out the details of this process and bring it to a practical level. The need for storage solutions that prolong RBC survival and maintain their functional properties is clear. Artificial preservatives currently in use fall short of the ideal, because they do not maintain 2,3-DPG levels or prevent changes in the RBC membrane. Despite hints that blood transfusion is associated with altered clinical outcomes in patients, only a few studies have attempted to correlate RBC quality and length of storage to clinical outcomes. The picture is further clouded by recent randomized controlled trials, which show that age of transfused RBCs has no effect on clinical outcomes in certain clinical situations.13–15 Effects of storage have only been rarely studied in animal models that are inadequate surrogate models of transfusion to critically ill patients. Additional studies to identify clinically relevant changes during storage may help guide developmental efforts to prevent storage lesion. Advances in manufacturing techniques are needed to allow the production of better solutions such as BAGPM. The development of systems that allow selective addition of supplementary components that may not be compatible with currently available storage media before transfusion would be a significant step toward making RBC transfusion safer and more effective.