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As red blood cells are stored, changes occur in the hydrogen ion concentration and potassium levels. Stored red blood cells contain an increased concentration of hydrogen ions leading to a lower pH. In addition, the potassium level increases in stored red blood cells over time. PRBCs may contain greater than 20-30 meq/L depending on the length of storage time. Studies in trauma patients have demonstrated increased potassium levels after transfusions and a high incidence of hyperkalemia in patient’s receiving greater than seven units of PRBCs. The increased hydrogen ion concentration and increased potassium levels are the result of biochemical reactions that occur during storage of PRBCs. During storage, glucose is metabolized to lactate, the concentration of hydrogen ions increase, and the plasma becomes acidotic. PRBCs are stored at 1-6 oC and at this temperature, activity of the sodium-potassium pump increases leading to increased plasma potassium levels.