Lay summary
Red blood cell transfusion is a common and potentially life-saving treatment in intensive care units. However, blood stored for a prolonged time deteriorates and previous studies have suggested transfusing older blood may worsen outcome in critically ill patients. This study will randomly assign 5000 patients in intensive care in Australia and New Zealand requiring transfusion to receive either the freshest available blood or current standard-issue older blood, to determine if the age of blood transfused affects the duration of support treatment required for failure of lungs, circulation and kidneys, new infections, and survival after 3 months. If there are such adverse effects of older blood, then the national standards for the age of blood to be provided for critically ill patients will be revised.