Volume of One Unit of Blood
One unit of blood equals approximately 450-500 milliliters (cc) of whole blood. 1
Understanding Blood Unit Volumes
The standard volume of one unit of whole blood is typically collected into a blood collection bag containing anticoagulant. According to transfusion guidelines:
- A standard blood collection pack is designed to collect approximately 450 ml of blood into 63 ml of anticoagulant (CPDA-1), creating a final anticoagulant:blood ratio of approximately 1:7 2
- The total volume of one unit including anticoagulant is therefore approximately 450-500 ml
Blood Component Volumes
When whole blood is separated into components, the volumes differ:
- One unit of packed red blood cells (PRBCs) is approximately 300 ml 3
- One unit of fresh frozen plasma is approximately 250-300 ml
- One unit of platelets is approximately 50-70 ml
- One unit of cryoprecipitate is approximately 10-15 ml
Clinical Impact of Blood Unit Volume
Understanding the volume of blood units is critical for proper dosing in transfusion medicine:
- In adults, one unit of PRBCs typically raises the hemoglobin by approximately 10 g/L (1 g/dL)
- In children, transfusion is calculated by volume rather than units: 10 ml/kg of RBCs should increase hemoglobin by approximately 20 g/L 1
Pediatric Considerations
For pediatric patients, blood components are dosed by weight rather than by units:
- Red blood cells: 10 ml/kg (increases Hb by ~20 g/L)
- Platelets: 10-20 ml/kg
- Fresh frozen plasma: 10-15 ml/kg
- Cryoprecipitate: 5-10 ml/kg 1
Blood Volume Calculations
When calculating blood loss or replacement needs:
- Total blood volume in adults is approximately 70 ml/kg
- In newborns, blood volume may be as high as 100 ml/kg 1
- A 6.4% increase in hematocrit can be expected per liter of packed red blood cells transfused (approximately 1.9% per unit of 300 ml PRBCs) 3
Understanding these volumes is essential for proper blood component administration and for estimating blood loss during procedures, which directly impacts patient morbidity and mortality outcomes.