The preservatives within the bags, or 'units', of red blood cells have made a tremendous difference when it comes to extending shelf life—and new research suggests that one new mixture called SOLX could perhaps expand this to as long as 56 days.

Red cells in plastic bags, called blood units, are stored on refrigerated shelves until they're needed for transfusion or until they've reached their approved shelf life (42 days in the US and 35 days in many European countries). Additives such as sodium citrate, which binds calcium, keep red cells from clumping together. The sugar dextrose gives the red cells energy. Phosphate, meanwhile, preserves levels of ATP, the red cell's energy molecule. Adenine and mannitol help stop red cells from rupturing.

According to Haemonetics, the Massachusetts-based blood management company that's currently developing the preservative, SOLX (also known as AS-7) buffers pH levels, thus maintaining red cell metabolism. Two studies of SOLX are slated to be published online in the journal Transfusion this fall. One trial demonstrated that the percentage of transfused red cells recovered at 24 hours after the red cells were stored at 42 and 56 days and then transfused into healthy volunteers met FDA criteria. Red cells stored for 42 days in SOLX had less hemolysis and higher levels of ATP than blood stored in a standard storage solution.

The second study tested whether SOLX could extend the hold time of whole blood to 24 hours at room temperature before processing. (In the US, red cells have to be stored in the refrigerator within eight hours of collection.) The cell recovery after transfusion at 42 days showed low amounts of hemolysis and met FDA requirements. Although the recovery and hemolysis rates did not meet FDA requirements at 56 days for the cells that were held 18–24 hours before processing, a lower percentage of red cells stored in SOLX during that hold period ruptured compared with those that had been held in a standard solution.

The current US hold time of eight hours creates logistical problems for some blood centers because donated blood may often be collected hundreds of miles away from the handling labs. “People don't donate blood at midnight, so collection centers need second and third shifts to process the blood,” Dumont says. Haemonetics is seeking approval from the FDA so that SOLX can be used to increase the hold time of collected blood to 24 hours. “A 24-hour hold could smooth out work flow,” Dumont notes, “and reduce cost and waste in some places.