 Transfusion of red blood cells RBCs is often effective at preventing morbidity and mortality in anemic patients. In contrast, recent studies indicate that some RBC components may have functional defects RBC storage lesions that actually cause morbidity and mortality when transfused. For example, patients transfused with RBCs stored 14 days have statistically worse outcomes than those receiving fresher RBC units. In addition to the age of stored RBCs, the volume transfused may be important. The Transfusion Requirements in Critical Care TRICC study showed that specific patients whose transfusions were limited by a restrictive trigger RBCs transfusions only when hemoglobin Hb 7 gdL had significantly better outcomes than those transfused with a more liberal trigger Hb 10 gdL Hb. This finding has been particularly difficult to understand since conventional wisdom suggests that an elevated Hb should be beneficial because it supports increased oxygen O2 delivery. Recipient-specific factors may also contribute to the occurrence of these adverse events. Unfortunately, these events have been difficult to investigate because up to now they have existed only as statistical occurrences of increased morbidity and mortality in large data sets. There are currently no clinical or laboratory methods to detect or study them in individual patients. 