Blood is an opaque red fluid consisting of the pale yellow plasma (called serum when the fibrinogen is removed) and the cells suspended in it - the red corpuscles (erythrocytes), the white corpuscles (leukocytes) and the platelets (thrombocytes). Blood has an important role in clinical diagnosis, because it is easy to collect and there are many diseases in which the blood composition and properties of the components are characteristically altered. (CP2D) or citrate phosphate dextrose adenine (CPDA-1) is an FDA-approved product when it is collected, stored and tested for transfusion-transmitted disease (TTD) by a licensed blood donor center. It can be stored for 21 das at 1–6°C in CPD and CP2D, or for 35 days at 1–6°C in CPDA-1 and is designated SWB in this CPG. Currently, CPD and CPDA-1 SWB are provided by the Armed Services Blood Program (ASBP). The shelf life of SWB is determined by the capacity of the anticoagulant solution to sustain red blood cell (RBC) integrity. SWB retains in vitro hemostatic parameters to an acceptable level during the maximum approved storage duration (i.e. up to 35 days in CPDA-1); however, after the first 2 weeks of storage, the hemostatic function of WB may vary and supplementation with fresher whole blood units or blood components, pecially platelets (PLTs), may be necessary to promote hemostasis. FWB refers to WB collected on an emergency basis from a “walking blood bank” (WBB). FWB can either be stored at room temperature and used within 24 hours of collection (current practice is to destroy it if not used, though available data suggest that it could be used for up to 72 hours following collection and storage at room temperature) or it can be refrigerated within 8 hours of collection, after which point it becomes SWB. FWB is considered to have full hemostatic function. FWB is collected from pre-screened donors when possible, but does not undergo complete TTD testing prior to transfusion; this fact makes it not approvable by the FDA in the civilian setting. Because FWB presents a higher risk of disease transmission, it is reserved for situations in which tested blood products are unavailable and the need for transfusion is urgent. The most important safety consideration in transfusing WB is that donor RBCs be compatible with the recipient’s preformed anti-A and/or anti-B antibodies to avoid acute hemolytic transfusion reactions (a.k.a., major mismatch). WB from group O donors contains RBCs that are compatible with all recipients, but the plasma in group O WB contains anti-A and anti-B antibodies that could cause hemolysis in a non-group O recipient (a.k.a., minor mismatch). There are two approaches to mitigating this risk: (1) transfuse only group-specific WB (i.e. A to A, B to B, AB to AB and O to O) or (2) anti-A and anti-B antibody titers can be measured in group O WB and only units containing a low titer of antibody (e.g., titer <256 saline dilution, immediate spin method) are designated “low titer O WB” (LTOWB) and these are used as “universal WB.” LTOWB has been used extensively to resuscitate combat casualties and was the standard of care in WWII, and the conflicts in Korea and Vietnam.Note that LTOWB may be either SWB or may be collected from pre-screened O donors in a WBB protocol and thus be considered FWB (e.g., the Ranger O Low titer or ROLO protocol)