Blood Components and Transfusion Practice
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Direct answer
Blood components are four separate products, packed red cells, fresh frozen plasma, platelets and cryoprecipitate, and each is prescribed to correct one documented deficit rather than to top up volume. Packed red cells treat symptomatic anaemia, with a restrictive trigger of about 7 g/dL in stable adults and a higher trigger for pre-existing cardiac disease. Plasma corrects multiple factor deficiency with bleeding, platelets treat thrombocytopenia or platelet dysfunction, and cryoprecipitate supplies fibrinogen, factor VIII and von Willebrand factor.
What you must remember
- Red cell threshold: About 7 g/dL in stable hospitalised adults, with symptoms guiding the decision; around 8 g/dL is commonly used for pre-existing cardiovascular disease or cardiac surgery, per current guidance.
- Plasma dose and use: About 10 to 15 mL/kg for bleeding with a documented factor deficiency, massive transfusion or plasma exchange. A raised INR in a patient who is not bleeding is not an indication.
- Platelet thresholds: About 10,000/µL for prophylaxis in marrow failure, about 50,000/µL for active bleeding or surgery, and about 100,000/µL for bleeding into the central nervous system.
- Cryoprecipitate: Source of fibrinogen, factor VIII, von Willebrand factor and factor XIII; the usual choice when fibrinogen falls below roughly 150 mg/dL in massive obstetric haemorrhage or disseminated intravascular coagulation.
- Massive transfusion: Loss of about one blood volume within 24 hours, or half the volume within three hours; red cells, plasma and platelets are given in roughly equal proportions, with ionised calcium checked as citrate accumulates.
- Compatibility: Group O red cells are universal red cell donors and group AB plasma is universal plasma; ABO and Rh typing, antibody screening and cross-matching precede elective transfusion, while uncrossmatched O-negative blood is reserved for emergencies.
- Modifications: Leucoreduction lowers febrile reactions and cytomegalovirus transmission; irradiation is given to immunocompromised patients and to recipients of intrauterine transfusion.
Worked scenario: a bleeding patient on the ward
A 58-year-old man is admitted with haematemesis, a pulse of 118 per minute, a blood pressure of 88/50 mmHg and a haemoglobin of 6.2 g/dL. Each component is chosen for one documented deficit rather than for the volume it adds.
- Resuscitate and sample: Crystalloid restores perfusion, two large-bore cannulas are placed, and a cross-match sample goes to the laboratory at once, not after the haemoglobin returns.
- Start red cells: Packed red cells come first because the falling number is haemoglobin, so the aim is oxygen-carrying capacity.
- Wait for the coagulation screen: Plasma is considered only once the screen shows a deficit and bleeding continues, not on a raised number alone.
- Add platelets by threshold: Platelets follow only when the count is below the bleeding threshold.
- Check the chemistry: Every few units the team checks ionised calcium, potassium and temperature, because stored blood and cold infusions cause hypocalcaemia, hyperkalaemia and hypothermia.
Where the examiner tests the reflex
The examiner tests whether the candidate corrects a patient, not a number.
- Trap: correcting a raised INR, a platelet count of 40,000 or a haemoglobin of 7.5 g/dL in a patient who is not bleeding Correct: none of these justifies a component on its own; the decision rests on active bleeding or a documented deficit.
- Trap: applying red cell compatibility rules to plasma Correct: group AB plasma is safe for every recipient, while group O plasma carries anti-A and anti-B and suits only group O recipients.
- Trap: asking why cryoprecipitate is not simply plasma in a smaller volume Correct: its concentrated fibrinogen, factor VIII and von Willebrand factor are the viva answer.
Frequently asked questions
Which component corrects a raised INR in a bleeding patient?
Fresh frozen plasma is the usual component for a raised INR with active bleeding, given at about 10 to 15 mL/kg. Guidance often favours prothrombin complex concentrate for urgent warfarin reversal. Plasma is not given to correct an INR in a patient who is not bleeding.
What is the restrictive haemoglobin threshold for red cell transfusion in stable adults?
Stable hospitalised adults are transfused when haemoglobin falls to about 7 g/dL, with symptoms guiding the decision. Patients with pre-existing cardiovascular disease or those undergoing cardiac surgery are usually managed to a higher threshold, around 8 g/dL.
Why is group AB plasma called universal plasma?
Group AB plasma contains neither anti-A nor anti-B antibodies, so it suits recipients of any ABO group. Group O red cells are the universal red cell donor because they carry no A or B antigen.
When is cryoprecipitate preferred over fresh frozen plasma?
Cryoprecipitate is preferred when the problem is low fibrinogen, since it delivers concentrated fibrinogen, factor VIII, von Willebrand factor and factor XIII. It is the usual choice for fibrinogen below roughly 150 mg/dL in massive obstetric haemorrhage or disseminated intravascular coagulation.
Why must ionised calcium be checked during massive transfusion?
Citrate used as an anticoagulant in stored blood binds ionised calcium, and rapid infusion of several units can cause hypocalcaemia with arrhythmias and reduced cardiac output. In massive transfusion it is measured repeatedly and replaced as needed.