Background: The hemostatic property of "fresh" whole blood (WB) has been observed in military application and cardiac surgery and is associated with reduced blood loss, transfusion requirements, and donor exposures. The time from donation to transfusion defining "fresh" has not been systematically studied. We undertook an in vitro study of coagulation properties of refrigerated WB stored for 31 days. Study design and methods Twenty-one WB units were obtained from healthy volunteer donors and stored under standard AABB refrigerated conditions. Samples were obtained on the day after donation and again on Days 2, 4, 7, 11, 14, 17, 21, 24, and 31. Tests included complete blood count, pH, pO2, pCO2, glucose, lactate, thromboelastography (TEG), and platelet function by light transmission aggregometry (LTA). Results: There was progressive decline in pH, pO 2, glucose, and sodium, but progressive increase in potassium, pCO2, and lactate. TEG variables in all units were normal through Day 11; abnormal values in some variables in some units began on Day 14. Final aggregation levels exhibited no change from Day 1 to Day 21 with adenosine diphosphate and epinephrine, but a decline with collagen (Day 7) and ristocetin (Day 17). Cconclusions: This in vitro study of coagulation properties demonstrates preservation of normal integrated coagulation function to a minimum of 11 days under standard conditions of refrigerated storage of WB for transfusion. These observations strongly suggest that the hemostatic quality of WB may extend beyond current transfusion practices. If confirmed clinically, this would increase availability and extend benefits of reduced donor exposure and transfusion requirements. © 2010 American Association of Blood Banks.