When discussing blood transfusions, the term “1 unit of blood” is commonly used. But what exactly does this mean? A unit of blood refers to a specific volume of blood or blood components that is transfused into a patient. Understanding what comprises a unit of blood is important for medical professionals who prescribe and administer blood transfusions. This article will provide a comprehensive overview of what constitutes 1 unit of blood, including the volume, components, and clinical applications.
Volume of 1 Unit of Blood
In most countries, 1 unit of blood is defined as approximately 450-500 milliliters of volume (mL). The standard blood collection bag holds this amount. In the United States, a unit of whole blood is defined as 450-500 mL, with each unit separated into components as follows:
- Red blood cells – typically 200-250 mL
- Plasma – typically 200-250 mL
- Platelets from 4-6 whole blood donations combined to make 1 unit
The volume of 1 unit may vary slightly depending on the collection system, anticoagulant used, and patient blood volume. Collections may range from 350-700 mL. However, the standard unit is still considered to be in the 450-500 mL range.
Components of 1 Unit of Blood
Whole blood is composed of several components that can be transfused individually or in combination for different clinical needs. The key components in 1 unit of blood are:
Red Blood Cells
The red blood cells (RBCs) transport oxygen from the lungs to tissues and organs throughout the body. One unit of packed red blood cells contains a high concentration of RBCs with the plasma and platelets removed. The typical volume is around 200-250 mL per unit.
Plasma
Plasma is the liquid portion of blood that contains clotting factors, proteins, and minerals. It makes up approximately 55% of total blood volume. One unit of plasma is around 200-250 mL from a single blood donation.
Platelets
Platelets are small cells that help blood clot and stop bleeding. Since an individual donation does not contain enough platelets for transfusion, platelets from 4-6 whole blood donations are pooled together to make one therapeutic unit. The volume of one unit is 200-300 mL.
Cryoprecipitate
Cryoprecipitate is the portion of plasma that remains after the blood is frozen and thawed. It is high in clotting factors needed to treat bleeding disorders. One unit of cryoprecipitate is approximately 15-20 mL.
Clinical Applications of 1 Unit of Blood
The choice of blood component to transfuse depends on the patient’s clinical situation and needs:
Red Blood Cells
The most common component transfused is packed red blood cells, given to restore oxygen delivery in anemia or hemorrhage. One unit of RBCs will increase the hemoglobin level by approximately 1 g/dL in an average adult. Severe anemia often requires transfusion of multiple units.
Plasma
Plasma transfusions help replace volume and clotting factors in trauma, liver disease, and bleeding disorders. Patients with severe clotting deficiencies may need multiple plasma units to control bleeding.
Platelets
Platelet transfusions are critical for patients with thrombocytopenia or platelet dysfunction to prevent hemorrhage. Complex procedures like surgery, chemotherapy, or organ transplant may require platelet support. The general goal is to maintain a platelet count above 10,000/microliter.
Cryoprecipitate
Cryoprecipitate provides concentrated levels of von Willebrand factor and factor VIII needed to treat hemophilia and von Willebrand disease. Bleeding episodes may require multiple cryoprecipitate units.
Considerations for Transfusing 1 Unit of Blood
Several key factors go into deciding how much blood to give a patient:
- Severity of anemia or thrombocytopenia
- Rate and amount of blood loss
- Patient blood volume
- Presence of active bleeding or clotting
- Patient tolerance and fluid status
The benefits of transfusion must be balanced with the risks of fluid overload, transfusion reactions, and infectious transmission. Using a restrictive threshold of 7-8 g/dL for red cell transfusion can limit exposure in stable, non-bleeding patients.
For platelets and plasma, the focus is on maintaining adequate levels for clotting function rather than normalizing the count. Giving the minimal effective dose reduces the risk of complications. Ongoing assessment determines if additional units are required.
Common Doses of Blood Products
Some common dosage examples of blood components include:
- 1 unit RBCs for acute blood loss up to 10% of blood volume
- 1 unit plasma and platelets for minor procedures in stable patients with mild bleeding risks
- 2 units RBCs for symptomatic, chronic anemia with hemoglobin less than 7 g/dL
- 4-6 units platelets to prevent bleeding if platelet count is less than 10,000/microliter
- 10-15 units cryoprecipitate for major bleeding episodes in hemophilia
The required transfusion dose varies based on the clinical scenario. Ongoing monitoring and testing determine if additional units are needed.
Storage and Expiration of Blood Products
Blood components have a limited shelf life and storage requirements:
- Red blood cells last up to 42 days stored at 1-6°C
- Platelets last up to 5 days stored at 20-24°C
- Plasma can be frozen for up to 12 months at -18°C or below
- Cryoprecipitate lasts 1 year frozen at -18°C or below
Outdating rates tend to be higher for platelets since they have the shortest expiration. Inventory management aims to minimize product wastage while ensuring availability. Proper storage and handling preserves efficacy and safety.
Cost of 1 Unit of Blood
The cost of 1 unit of blood varies globally based on testing, processing, and overhead costs of the collecting organization. In the U.S., estimates range from $201-$300 per unit of red blood cells and $500-$800 per unit of platelets. Plasma costs $30-$50 per unit.
Factors impacting pricing include:
- Donor recruitment and blood drives
- Staff wages and facility expenses
- Testing and processing
- Storage and transportation
- Hospital acquisition costs
While blood centers aim to cover costs, pricing also needs to stay affordable for hospitals that transfuse the products.
Alternatives to 1 Unit of Blood Transfusion
Due to risks and costs, alternatives to decrease transfusion are often considered:
Erythropoietin
Erythropoietin drugs stimulate red blood cell production and can treat anemia, reducing transfusion needs.
Intraoperative Blood Salvage
Surgical blood loss can be collected, processed, and reinfused back into the patient.
Acute Normovolemic Hemodilution
Blood is collected from the patient pre-surgery, replacing it with fluids to dilute blood loss during surgery. The blood is then transfused back post-operatively.
Restrictive Transfusion Thresholds
Evidence supports using hemoglobin thresholds of 7-8 g/dL for most hospitalized, stable patients.
Pharmacologic Agents
Medications like tranexamic acid and desmopressin help stabilize clotting in certain bleeding disorders, reducing transfusion needs.
Conclusion
In summary, 1 unit of blood refers to a standardized volume of approximately 450-500 mL, with whole blood separated into its component parts during processing. The key components transfused are red cells, plasma, platelets, and cryoprecipitate, each with specific clinical indications based on the patient’s condition and needs. Appropriate dosing, storage, and monitoring are essential to maximize the benefits while minimizing risks. Efforts to conserve blood products and avoid unnecessary transfusions remain a priority. An understanding of what constitutes a unit of blood is imperative for all members of the transfusion medicine team.
