This uncertainty is important for patients, clinicians and the blood services. For example, patients who require longer stays in ICU have long recovery times, use significant hospital resource, and suffer symptoms such as fatigue, breathlessness and reduced Health Related Quality of Life (HRQoL) for many months. It is possible that blood transfusions could alleviate some of these problems. In addition, donated blood is in short supply and is increasingly expensive to produce. As 40% of ICU patients receive transfusions, and use 8% of the Scottish blood stocks, a stronger evidence base could optimise the use of this precious resource.