Single-unit transfusion refers to a standard dosing scheme of one unit of red blood cells (RBC), platelets or plasma in the non-bleeding patient, followed by an assessment of the patient. For transfusion therapy in patients with active or critical bleeding, please visit also the Clinical Guidelines available on the Academy ePortal.
For RBC transfusion, single-unit dosing has been proven safe when applied together with a restrictive transfusion trigger (pre-transfusion haemoglobin level or symptoms of anaemia) in a number of randomized controlled trials (RCTs). Transfusion of one unit should be followed by a clinical or laboratory assessment of the patient to review if transfusion therapy is adequate or more units are needed.
Transfusion of two or more RBC units in succession is associated with an increased risk of pulmonary oedema or transfusion-associated circulatory overload (TACO), especially in the elderly, females and patients with heart or kidney failure, or a positive fluid balance.
A single unit of RBC will often relieve acute symptoms of anaemia and/or raise haemoglobin level in a stable patient to a level above the guideline trigger, in which case transfusion may be unnecessary according to the guideline, or can be postponed or replaced by other anaemia treatment. If one unit of RBC is given to an anaemic and otherwise stable patient, this is a single-unit transfusion provided that no further units are given until seeing a post-transfusion haemoglobin, or stating a new clinical transfusion indication, or within 24 hours after transfusing the first RBC unit. RBC transfusion is inappropriate therapy for iron deficiency anaemia unless there is acute bleeding or symptoms of acute physiological stress due to compromised oxygen delivery (for more information on anaemia diagnosis and treatment, please visit the Pre-operative Anaemia page).
For patients who are chronically transfused with RBC, there are no randomized studies to guide threshold or dosing scheme. Transfusion treatment should be tailored individually, based on a clinical decision and the patient’s response to previous transfusions.
The principle of avoiding over-transfusion through routinely ordering more than one unit in the non-bleeding patient also applies to platelets and plasma. However there is limited evidence to support a fixed number of units, as platelets and coagulation factors may be removed or consumed in various non-bleeding clinical conditions, e.g. in the presence of platelet antibodies, sepsis or disseminated intravascular coagulation.
- Platelet transfusion should be guided by the clinical indication, platelet count and functional platelet tests, as recommended by guidelines for specific patient populations.
- Plasma transfusion should be guided by clinical indication and pre- and post-transfusion coagulation tests and/or point-of-care whole blood functional assays according to evidence and guidelines for the specific patient population.
- Australian National Blood Authority (NBA) - Patient Blood Management [17,18] (GRADE-equivalent methodology)
- The management of anaemia and red cell transfusion in adult critically ill patients (GRADE-based) 
- Carson et al. AABB  (based on a Cochrane-review)
- Danish Health and Medicines Authority  (GRADE-based in Danish; expected as an English publication in the course of 2015)
- The Prevention of Transfusion-Associated Circulatory Overload 
Single-unit RBC transfusion was standard care in the majority of RCTs comparing liberal to restrictive transfusion in circulatory stable patients with anaemia [1-6]. These studies form the evidence base for RBC transfusion in these patient populations, and therefore when translating these findings into daily practice, both haemoglobin trigger and RBC dose should be followed.
Moreover, some of the RCT studies [2,3] and meta-analysis of RCT data  have shown that even with a single-unit dose, liberal transfusion is still associated with higher frequency of pulmonary oedema or TACO, compared with the restrictive regime. This indicates that either the higher number of units transfused to patients in the liberal arm or the higher haemoglobin level reached in this way increases the risk for TACO.
Finally, observational studies have found that the risk of TACO increases with every RBC unit transfused , that passive reporting greatly underestimates the frequency of TACO [10,11], and that TACO may increase morbidity and mortality. Accordingly the introduction of a single-unit regime is generally encouraged to avoid TACO, and in patients with cancer or haematological malignancy a single-unit regime has not been associated with adverse effects (non-randomized studies) [12,13]. Transfusion therapy in severely bleeding patients should follow separate guidelines/evidence for this patient population [14-16].
Clinical assessment of the patient’s haemoglobin after transfusion of a single-unit is recommended to guide the evidence-based transfusion treatment of RBC . Education, engagement and training may lead to an increase in single-unit transfusion, as may computerized physician order entry (CPOE) and/or regular audits. All of these have proven effective in some institutions but not in others, and no specific single type of intervention has proven superior in all settings.
- A dose of 4 ml red cells/kg raises haemoglobin concentration by 10g/L, which corresponds to the increment of one unit of red cells in a 70 kg patient.
- One adult therapeutic dose of platelets (from a pool of four units derived from whole blood donations or single-donor apheresis) typically raises the platelet count by 20-40x109/L.
- The typical dose of fresh frozen plasma is 12-15 ml/kg [Reference].
- Audit (whether electronic or manual) should clearly define the patient population
- The target population audited should correspond to the target population of the guideline (for a GRADE-based guideline, this corresponds to the patient population of each “PICO-question” leading the comparison)
- Diagnoses and surgical procedures should be registered
- Patients with bleeding emergencies (or massive transfusion) should be excluded
- The Joint Commission has proposed a definition of patient populations for the purpose of performance measurement 
- The audit will often address both pre-transfusion trigger as well as single-unit transfusion, as the single-unit transfusion is the dosage for each transfusion trigger
The content of this resource has been developed and reviewed by members of the ISBT Clinical Transfusion Working Party and should be used at the discretion of healthcare professionals utilising this clinical resource. The authors or the International Society of Blood Transfusion cannot accept legal responsibility for the content of this resource.