Packed red blood cells, also known as PRBCs, are a blood product used in transfusions to treat low red blood cell counts (anemia). PRBCs contain concentrated red blood cells that have been separated from other blood components such as plasma and platelets. Knowing how many mL are in 1 unit of PRBCs is important for medical professionals who prescribe and administer blood transfusions.
What are packed red blood cells?
Packed red blood cells are composed primarily of red blood cells that have been concentrated and separated from other blood components. During the preparation of PRBCs:
- Plasma is removed, leaving only red blood cells and a small amount of residual plasma.
- A preservative solution is added to help maintain red blood cell viability during storage.
- The red blood cells are concentrated to a hematocrit of around 80% (normal hematocrit is around 45%).
This process yields a blood product with a high concentration of viable red blood cells but lower amounts of plasma and platelets. PRBCs can be stored for up to 42 days at refrigerated temperatures before use.
Why are PRBC transfusions used?
There are several clinical situations in which PRBC transfusions may be used:
- Anemia – PRBCs can increase red blood cell counts in patients with anemia stemming from causes such as chronic illness, bleeding, or chemotherapy.
- Acute blood loss – Rapid blood loss from trauma or surgery can be treated with PRBC transfusions to replace lost red blood cells.
- Exchange transfusions – PRBCs may be used to exchange blood in procedures to treat sickle cell disease or remove toxins.
- Transfusion-dependent conditions – Diseases like myelodysplastic syndrome may necessitate regular PRBC transfusions.
The goal of PRBC transfusions is to increase the number of circulating red blood cells and in turn improve the oxygen-carrying capacity of blood in anemic patients.
What is the standard unit size of PRBCs?
In most countries, including the United States, the standard unit size of packed red blood cells is:
240 mL
This unit size contains a concentrated amount of red blood cells but a lower volume of plasma and preservative solution compared to whole blood units. Smaller PRBC units may be available for pediatric patients who require smaller transfusion volumes. For adults, most PRBC transfusions will consist of standard 240 mL units.
Other key facts about standard PRBC units:
- Hematocrit of 55-65%
- Typically contains around 180-240 mL of red blood cells
- Also contains about 30-60 mL of preservative solution
- May contain a small residual amount of plasma
While the exact composition can vary, a standard adult PRBC unit will contain a substantial concentration of red blood cells in a 240 mL volume.
How many red blood cells are in 1 unit of PRBCs?
Looking deeper into the contents of a standard PRBC unit:
- 1 unit = 240 mL volume
- Of this, around 200 mL is red blood cells
- 1 mL of packed red blood cells contains around 6.7 x 10^9 red blood cells
Doing the math:
- 200 mL RBCs x (6.7 billion RBCs / 1 mL) = 1,340 billion RBCs
This means one standard unit of packed red blood cells contains approximately 1.3-1.4 trillion red blood cells.
The number of red blood cells transfused helps determine how much a PRBC transfusion will increase the patient’s hematocrit and improve their oxygen delivery.
How long do transfused PRBCs survive?
When infused intravenously into a patient, the lifespan of transfused PRBCs can vary:
- Average lifespan around 17-28 days
- Some older RBCs destroyed within first 24 hours
- Younger RBCs may survive up to 90 days
The wide range relates to the normal age range of stored RBCs, with older cells removed first and younger ones surviving longer after transfusion.
The lifespan of transfused PRBCs is shorter than the normal 120 day lifespan of native red blood cells. But the majority will still survive for several weeks after transfusion to contribute to improved oxygen delivery.
How much does a unit of PRBCs increase the hematocrit?
As a very general guideline, one unit of PRBCs can be expected to increase the hematocrit by about 3 percentage points. However, the exact increase depends on several factors:
- Patient’s blood volume
- Baseline hematocrit
- Rate of PRBC destruction
- Ongoing blood losses
For example, infusion of 1 PRBC unit in a patient with a very low starting hematocrit of 15% could increase it to around 18%. The same unit infused into a patient with a baseline hematocrit of 30% may only increase it to around 33%.
The patient’s clinical response guides whether additional units are required to reach the target hemoglobin or hematocrit.
How many units of PRBCs are typically transfused?
The number of PRBC units transfused depends on the clinical situation and goals of therapy. Some general transfusion guidelines:
- 1-2 units PRBCs for mild anemia
- 2-4 units PRBCs for moderate anemia
- 4-6 units PRBCs for severe symptomatic anemia
However, these are just typical ranges. The treating physician will determine the number of units to be transfused based on:
- Severity of anemia symptoms
- Target hemoglobin or hematocrit
- Patient’s cardiopulmonary status
- Ongoing blood losses
Additional units may be prescribed if the patient remains significantly anemic after the initial transfusion.
What are the risks of PRBC transfusions?
While often lifesaving, PRBC transfusions do come with potential risks, including:
- Infections – viral, bacterial, or parasitic infections can rarely be transmitted by blood products.
- Hemolytic reactions – recipient’s antibodies attack donor RBCs, causing red cell rupture.
- Transfusion-associated circulatory overload (TACO) – fluid overload leading to pulmonary edema.
- Allergic reactions – urticaria, anaphylaxis in patients with anti-IgA antibodies.
- Iron overload – each unit provides 200-250mg iron, which can accumulate.
Doctors weigh these risks against the often substantial benefits PRBC transfusions provide to anemic patients. Using leukoreduced products that remove white cells decreases infection risks.
When are PRBC transfusions contraindicated?
There are some situations where PRBC transfusions may be contraindicated or used with caution, including:
- Active bleeding or hemorrhagic disorders
- Heart failure
- Known hemolytic transfusion reactions
- Autoantibodies to multiple RBC antigens
- IgA-deficient patients with anti-IgA antibodies
Transfusions may worsen active bleeding or circulatory overload. Evaluation of risks versus benefits is required in these complex cases.
Conclusion
To summarize key points on unit sizes of packed red blood cells:
- The standard adult PRBC unit size is 240 mL
- This contains around 200 mL red blood cells
- Equivalent to approximately 1.3-1.4 trillion RBCs
- In general, raises hematocrit by about 3 percentage points
- 1-6 units are typically transfused based on clinical needs
Understanding the composition and effects of transfusion with standard PRBC units allows medical professionals to appropriately prescribe and administer these lifesaving blood products.
